Chapter 4

TECHNOLOGY

Photo credit: Federa/ Aviation Administration 1

Contents

The Airport and its Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Airport and Airspace Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Guidance, Surveillance, and Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Microwave Landing System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Surveillance Radar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Traffic Management Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Supporting Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airspace Use Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reduced Lateral Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reduced Longitudinial Separation on Final Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Separate Short Runways for Small Aircraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Weather and Atmospheric Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Wake Vortex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Wind Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Noise Control and Abatement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Aircraft Noise.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Aircraft Operating Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airport Surface Uilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Surveillance and Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Taxiways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Apron and Gate Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Terminal Facilities and Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Terminal Building Design.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Terminal Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Landside Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Terminal Curbfront . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Airport Ground Access.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Applications of Technology to Airport Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6060626266687 0

71717476

767 77 7

798 081

8 38 38 48 68 6889 0

9 39 59 6

9 7Capacity and Delay Problems at Selected Airports . ..............................100Prospective Technological Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .,. ...........101

List of TablesTable Page13. Technology To Increase Airport Capacity and Reduce Delay . . . . . . . . . . . . . . . . . . . . . 6314. Summary of NAS Plan Benefits and Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69IS. Airport Capacity Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..............10216. Airport Technology Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................104

List of FiguresFigure Page

y. Airport Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 618. Comparison of Microwave Landing System and Instrument Landing System . . . . . . . 659. Arrival and Departure Separations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

10, Effects of Low-Altitude Wind Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7811. Airport Landside Functional Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8712. Airport Terminal Design Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893.3. Federal Capital Funding of Airports and Related Facilities . . . . . . . . . . . . . . . . . . . . . . . . 9514. Zurich Airport and Rail Terminal Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9815. Off-Site passenger Terminal Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

Chapter 4TECHNOLOGY

The airport system in place in the United Statestoday is extensive and highly developed; in gen-eral, it serves the Nation well. Still, there areproblems of congestion and delay at the busiestairports, where facilities are not adequate to ac-commodate demand at all times and in all condi-tions of weather and visibility. The Federal Avia-tion Administration (FAA) forecasts that growthof commercial and private aviation could be con-strained by lack of airport capacity, which it con-siders to be the most serious problem facing civilaviation through the remainder of this century. z

Recent policy statements by FAA acknowledgethat, with a few exceptions, the direct solutionof building new airports and expanding existingones may not be practical due to lack of suitablenew airport sites, physical limitations of presentfacilities, and concerns about environmental im-pacts of aviation on surrounding communities.3

Similar views have been expressed in two recentstudies of airport capacity,4 5 and there is a widelyheld opinion that, while the airport system is ex-pandable in the broad sense, there is little hopeof creating major new facilities in those key metro-politan areas where air travel demand and avia-tion activity continue to outstrip available airportcapacity unless airport planners can persuade sur-rounding communities that airports can be goodneighbors.

For this reason, the aviation community andFAA have sought technological solutions that will

IThis chapter is based on material prepared for OTA by Landrum& Brown, Inc.

National Airspace System Plan, revised edition (Washington, DC:Federal Aviation Administration, April 1983), p. 11-10.

31bid., p. I-5.4Report of the Industry Task Force on Airport Capacity Improve-

rnent and Delay Reduction (Washington, DC: Airport OperatorsCouncil International, September 1982).

5Report and Recommendations of the Airport Access Task Force(Washington, DC: Civil Aeronautics Board, March 1983).

ease congestion by allowing fuller and more effi-cient use of the airports we already have. Thistechnology includes new equipment for surveil-lance, navigation, and communication and revisedprocedures for using the airspace and airport fa-cilities. In this way, it is hoped that additional de-mand can be absorbed within the infrastructurenow in place, without adversely affecting sur-rounding communities.

This chapter examines technological measures,either currently available or under development,that could be employed to relieve congestion anddelay. It consists of a survey of possible improve-ments in airport technology, with emphasis on thecircumstances in which this technology would beapplicable, the extent to which it could increasethe amount of traffic handled, and the prospectsfor development and deployment over the com-ing years.

In aviation, the term technology typically bringsto mind sophisticated electronic and mechanicaldevices used for navigation, surveillance, com-munication, and flight control. Such devices areclearly of interest, but for the purposes of this re-port, technology is interpreted in a broader sense.As used here, technology refers not only to newdevices and equipment but also to new opera-tional concepts and procedures that they makepossible. Also, many in the aviation communitydraw a distinction between technology (meaningequipment and sometimes procedures) and civilengineering (referring to the design and construc-tion of physical components of the airporttheconcrete, so to speak). While recognizing that dif-ferent engineering disciplines and techniques areinvolved, this report does not make such a ciistinc-tion and considers the design and construction ofimproved physical components such as runways,taxiways, and terminal buildings as simply onemore form of technology that will add to airportcapacity or permit more effective and economi-cal use of the airport as a whole.

59

.

60 . Airport System Development .

THE AIRPORT AND ITS COMPONENTSThe airport is a complex transportation hub

serving aircraft, passengers, cargo, and surfacevehicles. It is customary to classify the severalcomponents of an airport in three major catego-ries: airside facilities; landside facilities; and theterminal building, which serves as the interchangebetween the two (see fig. 7).

Airside components, sometimes called the aero-nautical surfaces, or more simply the airfield, arethose on which aircraft operate. Principally, theyare the runways where aircraft take off and land,the taxiways used for movement between the run-way and the terminal, and the apron and gateareas where passengers embark and debark andwhere aircraft are parked. Because the airspacecontaining the approach and departure paths forthe airfield has an important effect on runway uti-lization, it is also customary to include terminalarea airspace as part of the airside.

The terminal consists primarily of the buildingsserving passengers and is made up of passengerloading and waiting areas, ticket counters, bag-

6 Some experts do not employ this tripartite classification. For ex-ample, R. Horonjeff and F. X. McKelvey, Planning and Design ofAirports (New York: McGraw Hill, 3d cd., 1983), distinguish onlybetween the airside and the landside, making the division at thepassenger loading gates and including the terminal as part of thelandside.

gage handling facilities, restaurants, shops, carrental facilities, and the like. Loading, handling,and storage areas for air cargo and mail, oftenseparately located, are also part of the terminalcomplex.

The landside is essentially that part of the air-port devoted to surface transportation. It beginsat the curbside of the terminal building and in-cludes roadways, parking facilities, andin somecasesrail rapid transit lines and stations that arepart of a larger urban mass transit system. Cus-tomarily, only roadways and transportation fa-cilities on the airport property are considered partof the landside, even though they are actually ex-tensions of, and integral with, the urban and re-gional transportation network.

In the discussion that follows, attention is fo-cused initially on those airside components wherecapacity and delay problems tend to be severe.The landside and terminal areas are not trouble-free, however, and congestion of these facilitiescan have an important effect on the overall ca-pacity of the airport. An examination of possi-ble technological improvements in terminals andlandside access is included at the end of thischapter.

AIRPORT AND AIRSPACE TECHNOLOGYTechnological approaches to expanding airport

capacity or reducing delay fall into three broadcategories. First, there are improved devices andprocedures that will expedite the flow of air traf-fic into and out of the airporti.e., techniquesthat will augment airside capacity or mitigate air-craft delay by increasing the runway operationrate. The second category includes techniques tofacilitate movement of aircraft on the airport sur-face. The purpose of these technologies is to moveaircraft from the runway to the passenger loadinggates and back again as expeditiously as possi-ble, thereby shortening the taxi-in and taxi-outcomponents of delay and easing congestion ontaxiways, aprons, and loading ramps. The thirdcategory embraces techniques that can be used toaid the transit of passengers through the terminal

building and the flow of vehicles on airport cir-culation and access roads. In contrast with the firsttwo categories, where the aim is to alleviate air-craft delay, the third category is intended to fa-cilitate the movement of people and to reduce thatpart of delay incurred in getting to and fromaircraft.

Thus, the survey that follows addresses thebroad question of airport capacity, not just air-side capacity or aircraft delay. The intent is toexamine ways to improve the overall adequacyand efficiency of the airport as a transportationhub. The underlying proposition is that delayany form of delayultimately affects the passen-ger through loss of time and increased cost of airtransportation service. In this sense, it is parochial

Ch. 4Techno/ogy l 61

///////////t

Figure 7.Airport Components

SOURCE Federal Avlatlon Admlnlstratlon

62 Airport System Development

to speak only of aircraft delay since the basic pur-pose of the air transportation system is to movepeople from origin to destination, in safety, withminimum expenditure of time and money. Allmeasures taken at airports to shorten travel time,to lower travel cost, or to lessen inconvenienceare of equal importance, regardless of whetherthey apply to the airside, the landside, or passagethrough the terminal.

The scheme of organization for this survey isoutlined in table 13, which lists various forms of

technological improvements and identifies thearea of the airport where they could be appliedand the purpose they could achieve. Discussionof specific technologies listed in table 13 is pre-sented in the sections that follow, which make upthe bulk of this chapter. In the concluding partof the chapter is a survey of the capacity and de-lay problems at a representative sample of airportsand a tabulation of possible forms of technologi-cal relief.

GUIDANCE, SURVEILLANCE, AND CONTROLThe position and spacing of aircraft in the air-

borne traffic stream is a key factor in determin-ing airfield capacity. For the pilot, it is vital toknow where the aircraft is in relation to the run-way and the airspace corridors around the air-port. This is accomplished by ground-based navi-gation equipment and airborne receivers. The airtraffic controller uses surveillance radar to mon-itor the position of the aircraft on approach anddeparture paths and in relation to other aircraftusing the airport. The success of these activitiesnavigation by the pilot and surveillance by thecontrolleris affected by the inherent accuracyof the equipment used. (Is the aircraft in factwhere the pilot and controller think it is?) Thedata update rate is also important. (How recentis this information and what may have happenedsince the last position reading?)7

In conditions of good visibility, when visualcues can be used by the pilot to confirm the posi-tion of the aircraft and to supplement guidancesystems, the spacing between aircraft can be re-duced to the minimum permitted by safe operat-ing procedures. When visibility is lessened bydarkness, rain, or fog, the pilot must rely on in-struments and the controller on radar. In such cir-cumstances, a margin of safety must be added tothe interval between aircraft, in effect increasingthe time that must be allowed for each to use . .

To appreciate the magnitude of this uncertainty, consider thatat typical jet approach speeds, an aircraft can travel almost 1,000ft horizontally and descend 50 to 60 ft in the 4 seconds between suc-cessive scans of the radar presently used for air traffic control atairports.

an assigned portion of the airspace or to occupythe runway, and correspondingly lowering thethroughput rate. If the accuracy of navigation andsurveillance devices could be improved, the ca-pacity of the airfield under Instrument Meteoro-logical Conditions (IMC) could be closer to thatattainable under Visual Meteorological Condi-tions (VMC).

Three technologies that could improve aircraftguidance, surveillance, and control are plannedfor deployment in the next few years. They arethe Microwave Landing System, improved sur-veillance radar, and automated traffic-manage-ment systems for the air traffic controller.

Microwave Landing SystemThe guidance system for approach and land-

ing now in use is the Instrument Landing System(ILS), which has been the standard system in thiscountry since 1941 and is widely used by civilaviation throughout the world. ILS provides guid-ance by radio beams that define a straight-linepath to the runway at a fixed slope of approx-imately 30 and extending 5 to 7 miles from therunway threshold. All aircraft approaching theairport under ILS guidance must follow this pathin single file, spaced at intervals dictated by stand-ards for safe longitudinal separation and the needto avoid wake vortex. This long, straight-in ap-proach is a bottleneck that reduces the runwayutilization rate, especially when fast and slow air-craft are mixed in the approach stream or whenarrivals from different directions must be merged

Ch. 4Technology . 63

Table 13.Technology To Increase Airport Capacity and Reduce Delay

Area ofapplicationTechnology

Aircraft guidance, surveillance, and control:Microwave Landing System . . . . . . . . . . . . . . . .

Purpose Benefit

Improve precision ofnavigation; make moreflexible use of airspace

Improve surveillance; reduceseparation

Improve traffic flow

Increased capacity; reduceddelay; less noise impact

Airspace

Surveillance radar . . . . . . . . . . . . . . . . . . . . . . . . Airspace

Airspace

Improved safety; increasedcapacity

Reduced delayTraffic management techniques . . . . . . . . . . . .

Airspace use procedures:Reduced lateral separation for parallel and

converging runways Airspace

AirspaceAirspace

Increase utilization of multiplerunways in IMC

Reduce in-trail separationSegregate air traffic by size

and speed

Increased capacity

Reduced longitudinal separation . . . . . . . . . . .Separate short runways for small aircraft . . . .

Increased capacityIncreased capacity; reduced

delayWeather and atmospheric effects:Wake vortex detection . . . . . . . . . . . . . . . . . . . .Wind shear detection . . . . . . . . . . . . . . . . . . . . .

AirspaceAirspace

Reduce in-trail separationAlert pilots to wind shear

Increased capacityImproved safety; reduced

delayNoise control and abatement:Control of aircraft noise . . . . . . . . . . . . . . . . . . . Reduce aircraft noise Increased capacity; reduced

delayIncreased capacity: reduced

delay

Airspace

AirspaceAircraft operating procedures . . . . . . . . . . . . . . Lessen or distribute noiseimpacts

Airport surface utilization:Surveillance and control . . . . . . . . . . . . . . . . . . . Taxi ways Improve surveillance, control,

and guidance of aircrafton ground

Reduce runway occupancytime

Increase efficiency of taxiwayuse

Improve docking at gate;improve aircraftmaintenance and servicing

Increased capacity; reduceddelav; improved safety

High-speed turnoffs and improved taxiways. .

Taxiway marking and lighting . . . . . . . . . . . . . .

Apron and gate facilities . . . . . . . . . . . . . . . . . .

Runway

Taxi ways

Ramps andaprons

Increased capacity

Reduced delay

Increased capacity; reduceddelay

Terminal facilities and services:Terminal building design . . . . . . . . . . . . . . . . . . Terminal

Terminal

Increase utility and efficiencyof terminal building

Improve circulation interminal; reduce walkingdistance

Expedite ticket purchase andpassenger check-in

Expedite baggage check-in,transfer, and pickup

Make screening faster andmore reliable

Expedite customs andimmigration clearance

Increased capacity; reduceddelay

Reduced delay; greaterpassenger convenience

Passenger movers . . . . . . . . . . . . . . . . . . . . . . . .

Ticketing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Terminal

Terminal

Terminal

Terminal

Reduced delay

Reduced delay

Reduced delay;security

Reduced delay

Reduced delay

Reduced delay

Reduced delay

Baggage handling . . . . . . . . . . . . . . . . . . . . . . . .

Passenger security screening . . . . . . . . . . . . . . improved

Federal Inspection Service . . . . . . . . . . . . . . . .

Airport access:Terminal curbfront design . . . . . . . . . . . . . . . . . Terminal;

landsideLandside

Land side

Facilitate airport entranceand exit

Facilitate automobile trafficflow

Reduce access time; lessen

Airport circulation roads . . . . . . . . . . . . . . . . . .

Airport ground access . . . . . . . . . . . . . . . . . . . .road congestion

SOURCE Office of Technology Assessment

64 l Airport System Development . . . .

on the common final approach path. As a result,the capacity of the airfield under IMC, when thelong ILS common approach path must be used,is usually less than under VMC.

The runway utilization rate under IMC couldcome closer to that attainable under VMC if air-craft could follow multiple approach paths, de-scend at different approach angles, or aim at dif-ferent touchdown points on the runwaynoneof which is practical with ILS. If this flexibilitywere possible, as it is under VMC, airfield capa-city would be less affected by weather conditions,and throughput would be governed almost ex-clusively by runway geometry and aircraft per-formance characteristics.

The Microwave Landing System (MLS), whichhas been under development by FAA for over adecade, would overcome some of the disadvan-tages inherent in the ILS. Because MLS uses abeam that scans a wide volume of airspace, ratherthan the pencil beam of ILS, it permits aircraftto fly any of several approach angles (includingtwo-step glide slopes) and, in the horizontal plane,to approach along curving paths that intersect theextension of the runway centerline at any chosenpoint. In effect, MLS offers a degree of freedomin using the airspace that is closer to that enjoyedunder conditions of good visibility (see fig. 8).

The chief motive for FAA in seeking to developand deploy the MLS is not the potential capacitybenefits, however, but its operational advantagesmore precise guidance, ease of installation, im-proved reliability, less susceptibility to electro-magnetic interference, and greater number oftransmission channels. The capacity benefits aresecondary but still of great importance at someairports where the present ILS acts to constraincapacity in adverse weather conditions. In termsof its effect on capacity, the chief advantage ofMLS is that, in IMC, it allows pilots and con-trollers greater flexibility in selecting an approachpath so as to shorten the approach time, to avoidair turbulence generated in the wake of precedingaircraft, or to avoid noise-sensitive areas. Anotheradvantage is that MLS can provide guidance forthe aircraft during missed approach, allowing asafe exit from the terminal airspace and smoothreentry into the approach pattern. The availabil-

ity of missed approach guidance could have a sig-nificant capacity benefit at those airports with par-allel or converging runways that cannot now beused in IMC. A third advantage is that MLS canbe installed on runways where ILS is not possi-ble due to siting problems and on short auxiliaryrunways reserved for commuter and small gen-eral aviation (GA) aircraft.8 On some runways,MLS can increase capacity during IMC by pro-viding lower landing minimums than ILS andthereby allowing the airport to remain open inmarginal weather conditions. A fourth advantageof MLS is its capability to provide nonconflictingroutes into closely situated airports, where ap-proach or departure paths may mutually interfereand limit capacity utilization.

The capacity benefits of MLS are highly site-specificdepending on the runway configuration,the prevalence of adverse weather, the mix of air-craft using the airport, and the extent to whichthese aircraft are equipped with MLS receivers.Estimates by FAA indicate that the benefits couldrange up to 10 or 15 percent greater capacity atsome airports under IMC. The overall effects oncapacity at these airports would be somewhatsmaller since they depend on how often Instru-ment Meteorological Conditions occur. The neteconomic benefits are estimated by FAA to be$500 million over a 20-year period (1976 dollars),principally to air carriers and commuter airlinesin the form of reduced delay costs and savingsof passenger time.9

This estimate has been challenged in a recentreport by the Industry Task Force on Airport Ca-pacity Improvement and Delay Reduction. TheTask Force found that the chief advantages wereat small or remote airports served by helicoptersand commuter airlines and in high-density traf-fic areas where MLS could permit commuter air-craft to approach and land on separate short run-

6For a further discussion of MLS technology and its benefits, seeAirport and Air Traffic Control System (Washington, DC: U.S. Con-gress, Office of Technology Assessment, OTA-STI-175, January1982), pp. 92-96, 117; and Improving the Air Traffic Control System:An Assessment of the National Airspace System Plan (Washington,DC: Congressional Budget Office, August 1983), pp. 9-18.

An Analysis of the Requirements for, and the Benefits and Costsof the National Microwave Landing System (MLS), FAA-EM-80-7(Washington, DC: Federal Aviation Administration, June 1980).

Ch. 4Technology l 6 5

Figure 8.Comparison of Microwave Landing System and Instrument Landing System

4 0- wide t W (

Instrument landing system

No continuous distancemeasurement two discrete range Aindications provided by fan markers A

\* .

Outer

k ~

marker

Middle

Extended

I

\

SOURCE: Federal Aviation Administration

ways. 10 The direct benefits to major air carriers procedures that have not yet been tested andare much less clear, according to the Task Force, proven in an operational environment .11because they depend on use of curved or seg- - FAA is now proceeding with MLS implemen-mented approaches and multiple glide paths tation. A contract for production and installation

of 172 units was let in late 1983, with follow-on

66 . Airport System Development

procurements planned for 1985-95 (900 units) and1996-2000 (350 units), making a total of approx-imately 1,425 installations by the beginning of thenext century. Priority will be given to large andmedium hub airports and to those airports nowlacking ILS because of siting restrictions or lackof available transmission channels.12 FAA esti-mates the total cost of ground equipment to be$1.33 billion. User costs for MLS receivers are esti-mated to be an additional $1.63 billion, bringingthe total cost for full deployment of MLS to nearly$3 billion over the coming 20 years.13 14

Replacement of the existing ILS poses two prob-lems that may complicate the transition to MLSand delay realization of the full benefits. Thereare at present about 650 ILS units in commissionat some 460 airports and another 150 or so unitsin various stages of procurementsome as re-placements for existing units, others as new in-stallations. The MLS transition plan calls for theseILS units to remain in service for many years tocome, until at least 60 percent of the aircraftroutinely using the ILS/MLS runway are equippedwith MLS. While ILS and MLS can be colocatedand operated simultaneously without signal in-terference, there may be procedural difficulties inblending aircraft equipped with ILS (and there-fore capable of only straight-in approaches) intoa traffic stream with MLS-equipped aircraft fly-ing curved or segmented approaches. Thus, thefull capacity benefits of MLS may not be attain-able at a given airport until all or nearly all air-craft are MLS-equipped and the ILS can be decom-missioned.

A second factor that may delay taking fulladvantage of MLS at specific sites is the agree-ment with the International Civil Aviation Orga-nization whereby the United States is committedto retaining ILS service at international gatewayairports until 1995. There are 75 such airports, .

12 The aviation industry has voiced strong opposition to the Pro-posal for installing MLS at large and medium airports first, and inMay 1984 FAA agreed to a complete review of the deploymentstrategy. Depending on the outcome of this review, the early stagesof the MLS program schedule might be set back a year or more.

13 Microwave Landing System Transition Plan, APO-81-1 (Wash-ington, DC: Federal Aviation Administration, May 1981).

14 Preliminary Analysis of the Benefits and Costs TO Implementthe National Airspace System Plan, DO~/~AAIEM-82-2?2(Washington, DC: Federal Aviation Administration, June 1982).

generally the busiest U.S. airports and those mostprone to capacity and delay problems. RetainingILS service at these airports may influence someusers to defer purchasing MLS equipment foranother 10 years or more.

While the capacity gains attributable to MLSmay be rather small for the airport system as awhole, MLS does appear to offer promise at thoseairports where it could be used to create a moreflexible traffic pattern or to provide commuter andsmall GA aircraft access to an alternate runwayin IMC, thereby relieving pressure on the mainrunway used by large air carrier aircraft. Beyondthese direct benefits, moreover, MLS may permitprocedural changes that could also increase ca-pacity or reduce delay. These potential benefitsof MLS are discussed in a later section on airspaceuse procedures.

Surveillance RadarSurveillance is accomplished by radar and asso-

ciated electronic and computer systems that locate,identify, and display the position of aircraft in theairspace. In terminal areas, two types of radar arepresently used for this purpose: search radar(technically termed primary radar) and the ra-dar beacon system (sometimes called secondaryradar). Search radar emits signals and displaysthe returns reflected from the body of the aircraft,objects on the ground, and precipitation or weatherfronts, thereby providing a basic two-dimensionalmap of the airspace. The beacon system, knownas the Air Traffic Control Radar Beacon Systemor ATCRBS, displays only replies from aircraftequipped with electronic devices, called trans-ponders, that send out a coded signal when in-terrogated by the radar beacon. This signal in-dicates not only the position of the aircraft butalso its identity (flight number) and altitude (ifthe aircraft is equipped with an altitude-encodingtransponder). The beacon system is presently themain source of surveillance information for airtraffic control (ATC).

This radar-derived information is correlatedand presented to the air traffic controller on oneof four different types of display systems: TPX-42, ARTS II, ARTS III, or ARTS 111A. The TPX-42 is the least sophisticated equipment. It is a non-

Ch. 4Technology l 6 7

programmable device that correlates and displayssearch radar data and beacon returns on each suc-cessive sweep of the antenna. The TPX-42 is usedat airports with little traffic. The Automated Ra-dar Terminal System (ARTS II) is a program-mable data processor that displays primary andsecondary radar data on the controllers scope butdoes not track aircraft or predict their position.It is used at airports with low to medium levelsof activity.

ARTS III detects, tracks, and predicts the posi-tion of aircraft. This information is presented onthe controllers display as computer-generatedsymbols (denoting altitude, ground speed, andidentity) positioned alongside the secondary ra-dar return. ARTS 111 also incorporates featuresthat alert the controller when aircraft descendbelow minimum safe altitude or when two air-craft are approaching too closely and require ac-tion to assure safe separationa feature knownas conflict alert. ARTS 111A is a refinement ofARTS 111 that is capable of tracking aircraft de-tected by search radar alonei.e., aircraft notequipped with an ATCRBS transponder. ARTS111 and ARTS 111A equipment is installed at the62 busiest air traffic hubs.

FAA is now in the process of replacing muchof the primary radar and display equipment. Theexisting primary surveillance radars used at air-ports (ASR-4, ASR-5, and ASR-6) are based onvacuum tube technology that suffers from relia-bility problems and maintenance difficulties.Newer solid-state equipment (ASR-7 and ASR-8) has been installed at some locations, but theseradars, like earlier versions of ASR, are adverselyaffected by ground clutter, false targets generatedby flocks of birds, propagation anomalies, andmasking of aircraft returns by weather. Of theseshortcomings, weather masking is perhaps themost severe operational problem. The strong re-turn from storms conceals the weaker return fromaircraft detected on primary radar alone. To com-pensate, controllers alter the polarization of theradar to reduce weather echoes and make the air-craft return stand out more clearly, but this lessensthe apparent severity of weather fronts and pre-cipitation.

Between 1986 and 1990, FAA plans to installa new primary radar system (ASR-9) which willhave a separate weather channel allowing the con-troller to assess the severity of storms while re-taining the ability to detect small aircraft with-out transponders. The ASR-9 will also incorporatean improvement called Moving Target Detectionto overcome the problems of ground clutter andspurious targets. These improvements in primaryradar information, when coupled with the pres-ent radar beacon display, will provide the con-troller with a clearer and more accurate pictureof the airspacethereby lessening workload andcreating a better basis for decisionmaking aboutaircraft movement around the airport. The esti-mated cost of installing 105 ASR-9 systems is $480million, with the option of adding 35 more in the1990s at a cost of roughly $125 million.ls

As radar systems are being upgraded, FAA alsoplans to improve the data processing and displayequipment used by air traffic controllers. Initially,the TP)(-42 system will be replaced by a new ver-sion of ARTS II, designated ARTS 11A, which willincorporate minimum safe altitude warning andconflict alert features like the present ARTS III.The ARTS III equipment will also be enhancedwith greater memory to handle heavier trafficloads and improved software that will reduce thenumber of false conflict alerts. In the period 1990-95, ARTS II and III will be replaced by new dataprocessing and display consoles, called sectorsuites, that will provide improved presentationof surveillance and weather data, display of traf-fic management and planning information, andautomated assistance to the controller in sep-arating and routing traffic in terminal airspace.lb

The immediate capacity benefit of the ASR-9radars will be surveillance information of im-proved reliability and accuracy, which will pro-vide the controller with a better picture of theairspace situation. Of even greater importance,the improved ASR-9 radar, the upgraded ARTSII and III, and the eventual installation of new sec-tor suites will support changes in traffic manage-ment techniques that will help the controller make

68 . Airport System Development

more efficient use of the airspace. These prospectsare discussed next.

Traffic Management TechniquesA major task of the air traffic controller is man-

agement of traffic so as to maintain a smooth flowof aircraft to and from the airport with minimumdelay. This is done by the techniques of meter-ing, sequencing, and spacing.17 With current tech-nology, these are largely matters of controller artthat depend heavily on the individuals skill andexperience. On a typical day, the controller mustmake literally hundreds of related decisions aboutthe order and timing of aircraft movements in thetraffic pattern under the prevailing conditions ofwind and weather. The chief problems that thecontroller must deal with in performing theseactivities are randomness in the arrival and depar-ture streams and differences in the speed and flightcharacteristics of successive aircraft using theairspace. The extent to which the controller is suc-cessful in applying the techniques of traffic man-agement has a significant influence on delay andefficient use of airport capacity.

It has long been recognized by ATC experts thatthe key to more effective traffic management,especially in circumstances of heavy demand, isto involve computers in the decisionmaking proc-ess. In some instances, this means providing thecontroller with computerized aids to decision-makingdevices to collect, integrate, and displayinformation that will give a better picture of thetraffic situation and help in executing a controlstrategy. In other instancesparticularly wheredecisionmaking is routine, repetitive, and reduc-ible to unambiguous rulesthe approach is tosubstitute the computer for the human operator,thus relieving him of workload and guardingagainst human error and inconsistency.

As part of the planned modernization of theATC system, FAA is developing new softwarepackages that will assist in traffic management

17 Metering is regulating the arrival time of aircraft in the terminalarea so as not to exceed a given acceptance rate. Sequencing entailsspecifying the exact order in which aircraft will take off or land.Spacing involves establishing and maintaining the appropriate in-terval between successive aircraft, as dictated by considerations ofsafety, uniformity of traffic flow, and efficiency of runway use.

at and around airports. Known under the collec-tive designation of Traffic Management System(TMS),18 this new software will perform severalimportant functions to increase the efficiency ofairport and airspace utilization: airspace con-figuration management, dynamic planning andcomputation of acceptance rate, tactical execu-tion of control strategy, runway configurationmanagement, and departure flow metering.

For incoming flights, TMS will establish anacceptance rate and order of landing based on esti-mated arrival time and predetermined flight paths.As aircraft progress toward the runway, TMS willadjust landing time and spacing between aircraftas necessary to eliminate gaps or surges in the traf-fic stream and to make efficient use of airspaceand runways. In the earlier stages of implemen-tation, the computer will generate recommendedinstructions and command messages for the con-troller to relay to pilots by voice radio. In laterstages, the computer will transmit commandsdirectly to individual aircraft by the Mode S datalink.19

Photo credit: Federal Aviation Administration

Traffic management can smooth the flow

Ch. 4Techno/ogy l 6 9

Other components of TMS will contribute tomore efficient traffic management in other ways.Runway configuration management, a softwareprogram that has been under development at Chi-cago OHare since 1980, will assist controllers inestablishing the most efficient combination of ar-rival and departure runways for given conditionsof weather and demand. Departure flow meter-ing will help assure an appropriate blend of take-offs and landings and will feed aircraft out of theterminal area and into en route airspace.

FAA plans do not call for implementation ofTMS all at once, nor at all airports. The compo-nents are being developed separately and will betested and put in place as ready and where needed.The overall timetable is contingent on the devel-opment and installation of new computers andsector suites in terminal area control centers andon the development of companion software pack-ages for the en route ATC systemthe AdvancedEn Route Automation (AERA) program. Full im-plementation of TMS, AERA, and related tech-nological changes will not occur until 1995 orlater.

TMS and AERA are tied together because FAAslong-term response to air traffic growth involvesa general application of the flow management con-cept so as to provide strategic and tactical plan-ning, continuous performance monitoring, andflexible and adaptive exercise of control for theairspace as a whole. For example, en route meter-ingwhich is a feature of AERA-will contrib-ute to efficient runway use by treating all arrivalsalong all routes as a single traffic pattern and ad-justing in-trail separation so as to achieve a steady

rate of delivery into the terminal area. The pres-ent method of flow management, which uses uni-form, preestablished in-trail separation, can re-sult in inefficient runway utilization (surges andgaps in the traffic flow) because it cannot adaptreadily when flow along arrival routes does notexactly match the nominal rate used as the basisfor selecting in-trail spacing.

The capacity benefits of TMS are difficult toestimate on a systemwide basis. The anticipatedbenefits are highly specific to conditions at the air-port site and particular patterns of demand. Fur-ther, it is not always possible to distinguish be-tween the benefits of TMS and those that wouldresult from other planned improvements in theATC system. Estimates published by FAA as partof an analysis of overall benefits and costs of theNational Airspace System Plan (NAS Plan) sug-gest that the benefits arising from improved traf-fic management and flow planning in terminalareas could be fuel savings on the order of 0.75to 1.25 percent. FAA calculates the value of thesesavings to be between $165 million and $280 mil-lion per year (1982 dollars) for the period 1993-2005. Of these savings, about 60 percent wouldaccrue to air carriers, with the remainder aboutequally distributed between business and privategeneral aviation.20

The FAA report does not provide a projectedcost for TMS alone, but lumps these costs withthose of AERA and other airport and airspaceprograms in the NAS Plan (see table 14). The total

Table 14.-Summary of NAS Plan Benefits and Costs (billions, 1982 dollars)20-year totals Present (discounted) valuesa

Benefits costs Net Benefits costs Net

70 Airport System Development

costs are estimated to be $12 billion ($4 billionto aviation users and $8 billion to FAA) over thenext 20 years; the associated 20-year benefits arecalculated to be $24.7 billion to users, primarilyin fuel savings attributable to AERA and $24.3billion to FAA in operating cost savings. (All esti-mates in 1982 dollars. )

Supporting TechnologiesIn addition to programs aimed specifically at

reducing delay and increasing the throughput ofmajor airports, FAA is pursuing other technologi-cal developments that will either facilitate theATC process or provide greater assurance ofsafety. Three particularly important developmentsof this sort are the Mode S data link, the CockpitDisplay of Terminal Information (CDTI), and theTraffic Alert and Collision Avoidance System(TCAS). These technologies will not, by them-selves, provide relief to the problems of conges-tion and delay in terminal areas, but they couldmake possible other technological improvementsor procedural changes to improve the flow oftraffic.21

The addition of Mode S to the present ATCRBStransponder has perhaps the most far-reaching im-plications for air traffic control. Mode S will allowthe air traffic controller to interrogate aircraft in-dividually and will make possible direct and selec-tive two-way digital communication between airand ground. Mode S thus will form the basis forthe more automated forms of air traffic controlenvisioned in the TMS and AERA programs.Equally important, Mode S will open up a new,high-capacity channel of communication that willprovide more complete and rapid exchange of in-formation and greatly reduce controller and air-crew workload by relieving them of the time-consuming process of transmitting, receiving, andacknowledging messages by voice radio. A thirdbenefit of Mode S is that it can enhance the sur-veillance function by reducing interference amongtransponder replies of aircraft operating close to-gether in terminal airspace.

An important potential application of the ModeS data link is that it could be used to improve the .

See Airport and Air Traffic Control System, op. cit., for moredetailed discussion of these technologies.

quantity and quality of information available inthe cockpit by providing a display of traffic inthe surrounding airspace. This display, CDTI, hasbeen under development for several years and hasbeen recommended by pilots and ATC experts asa valuable new tool to enhance safety and to aidmaneuver in terminal airspace. The CDTI, byshowing the location and path of nearby aircraft,could give the pilot an overall view of the trafficpattern and could provide an additional sourceof information under conditions of reduced visi-bility.

The CDTI is not envisioned as a substitute forground-based air traffic control nor as the basisfor independent maneuver to avoid collision orto assure safe separation. Rather, it is intendedas a supplemental display that will allow the pilotto read the air traffic pattern and to cooperatemore effectively and confidently with the ground-based controller in congested airspace. FAA, incooperation with the National Aeronautics andSpace Administration (NASA), is currently ex-ploring roles for a CDTI. The focus of this effortis to develop CDTI system requirements and todetermine the compatibility of these requirementswith Mode S and TCAS data sources.

The overriding concern in seeking ways to in-crease airport throughput and runway acceptancerates is maintaining safe separation among air-craft. Basic separation assurance is provided intwo ways: by application of the see-and-avoidprinciple in Visual Flight Rules (VFR) and by ATCprocedures and ground-based surveillance in In-strument Flight Rules (IFR). Pilots and others con-cerned with aviation safety have long advocatedadditional assurance in the form of an airborne(i.e., ground-independent) collision avoidancesystem. The system currently proposed by F&lTraffic Alert and Collision Avoidance Systemisan independent airborne device designed to useATCRBS (or Mode S) transponder informationfor generating a warning to the pilot that an ap-proaching aircraft is a threat and that evasive ma-neuver may be called for.

TCAS is in the development stage at presentand may not be ready for operational use untilthe late 1980s. The availability of TCAS, or anequivalent system of airborne collision avoidance,

Ch. 4Techno/ogy . 71

will be an important factor in the decision toadopt revised procedures for increasing the effi-ciency of airspace use. Without assurance that safeseparation can be maintained and that there is a

backup to ground-based air traffic control, nei-ther airspace users nor FAA are likely to have theconfidence to proceed with revision of presentlongitudinal and horizontal separation standards.

AIRSPACE USE PROCEDURESProcedures governing the use of terminal air-

space and airport runways, which are designedprimarily to assure safety, sometimes slow ordisrupt the flow of traffic. In general, these pro-cedures consist of rules and standards pertainingto the permissible distances between aircraft invarious weather conditions and approach pat-terns. Actually, there are two sets of procedures:one for use in Visual Meteorological Conditions(VMC) and another, more stringent, set for usein Instrument Meteorological Conditions (IMC).Instrument Flight Rules which are largely deter-mined by available navigation, communication,and surveillance technologyoften cause delaysat busy airports because of the increased separa-tion standards and special safeguards that mustbe applied in restricted visibility.

There is a widely held, but not unanimous,view among airspace users that revisions of theexisting instrument flight procedures are practicaland that they would be warranted in the interestof reducing delay. While these revisions are some-times spoken of as capacity improvements, theywould not in most cases actually increase the ca-pacity of airports. Instead, they would allow ex-isting capacity to be used more fully or withgreater efficiency and would bring the through-put attainable under IMC closer to that whichprevails under VMC.

In response to urging from airspace users, FAAinstituted a comprehensive examination of air-space use procedures in October 1981. This ef-fort, known as the National Airspace Review(NAR) is a 42-month joint undertaking by FAAand the aviation industry to identify and imple-ment changes which will promote greater effi-ciency for all airspace users and simplify [theATC] system. Additionally, the NAR will matchairspace allocations and air traffic procedures totechnological improvements and fuel efficiency

programs.2 2 The portion of NAR concerned spe-cifically with terminal area ATC procedures wascompleted in July 1984.

Many of the procedural changes sought byairspace users and under study by FAA in NARwere also examined by a special aviation indus-try task force convened at the request of FAAunder the auspices of the Airport Operators Coun-cil International. The task force report, issued inSeptember 1982, strongly urged FAA to revisepresent airspace use procedures, especially thosepertaining to the use of multiple runways underInstrument Meteorological Conditions.23

Reduced Lateral SeparationSeveral of the proposed revisions would per-

mit changes in the standards for lateral separa-tion of aircraft under instrument flight conditions.The present standards often severely restrictthroughput because they preclude use of all theavailable runways when visibility is reduced. Ifthe airport could continue to operate these run-ways, the disparity between IMC and VMC ac-ceptance rates could be substantially narrowed.The following are the major capacity-relatedchanges under consideration.

Converging RunwaysConverging runways are those whose extended

centerlines meet at a point beyond the runwaysthemselves. Simultaneous approaches to con-verging runways are presently authorized onlyduring VMC. The proposed procedure would ex-

25-420 0 - 84 _ 6

-

72 . Airport System Development

tend this authorization to IMC in certain circum-stances. The major problem to be overcome inusing converging runways under instrument con-ditions is development of procedures to assureseparation in the event of a blunder by one of theaircraft during the approach or in case both air-craft must execute a missed approach at the sametime. These procedures, in turn, depend on theavailability of improved surveillance radar, MLSto provide missed approach guidance, and per-haps automated aids for the controller to coordi-nate simultaneous approaches to two runways.

In time, it maybe possible to extend these pro-cedures to the case of intersecting runways-thosewhose surfaces actually cross at some point. Inaddition to the problems of blunder protectionand separation assurance during missed approaches,this configuration poses the risk of collision be-tween two aircraft on the ground, and there mustbe adequate safeguards that aircraft on both run-ways can stop or turn off before reaching the in-tersection. Because of the inherent safety prob-lems, most observers are skeptical about thefeasibility of using this type of runway layout forinstrument operations.

Dependent Parallel RunwaysAt present, instrument approaches maybe con-

ducted on parallel runways that are as close as3000 ft apart so long as a diagonal separation of2 nautical miles (nmi) is maintained between ad-jacent aircraft. For parallel runways separated by2,500 ft, the diagonal spacing requirement is 2.5nmi. In addition, aircraft must be separated by1,000 ft vertically or 3 nmi horizontally as theyturn onto their parallel approach paths. Theserunways are termed dependent because the ap-proaches to each must be coordinated to main-tain the prescribed diagonal spacing. Hence, theoperational rate attainable on either is constrainedby the movement of aircraft on the other.

FAA studies suggest that the diagonal spacingrequirements for IFR operation on dependent par-allel runways could be reduced. For runwaysseparated by 2,500 ft, the standard could be re-duced from the present 2.5 nmi to 2 nmi with cur-rent technology and no other changes in existing

procedures. 24 Reducing the spacing requirementsfor approaches to parallel runways less than 2,500ft apart requires: 1) that the pilot be able to con-firm that he is, in fact, on approach to the properrunway since radar surveillance would no longerbe sufficient; and 2) that wake vortices from air-craft approaching one runway do not interferewith operations on the other. Because of wakevortex, current procedures require that aircraftapproaches to closely spaced parallel runways(less than 2,500 ft apart) be treated as approachesto a single runway and separated accordingly.

An operational solution to the wake vortexproblem on closely spaced parallel runways en-tails that the following additional conditions bemet:

l

l

l

l

l

there must be a steady crosswind to diminishthe effects of wake vortex, but the windvelocity must be less than maximum cross-wind limitation;small aircraft that are vulnerable to wakevortices must use the upwind runway of theclosely spaced pair;the threshold of the upwind runway mustbe displaced from that of the downwindrunway;the upwind runway must have a high-angleglide slope to allow for a steeper descent byvulnerable aircraft so that they can remainabove, and hence avoid, wake vortices; andwind monitors must be set up along the ap-proach path to ascertain that conditionsare favorable for the dissipation of wakevortices.

Satisfying these requirements may be difficultat airports that do not have runways with suitablystaggered thresholds and a sufficiently large num-ber of aircraft that can approach at a steeper thannormal glide slope to avoid wake turbulence. Inaddition, there are operational difficulties thatmay limit the applicability or the capacity benefitsof this procedure. First, the wake vortex gener-ated by a heavy aircraft carrying out a missed

Ch. 4Techno/ogy . 73

approach could interfere with operations on theother runway. One possible solution would be torequire that both the leading and trailing aircraftexecute missed approaches along diverging pathswhenever the leading heavy aircraft misses the ap-proach. Second, interference from departurescould limit capacity gains since it may be neces-sary to retain present longitudinal separationstandards between heavy aircraft departing onone runway and small aircraft landing on theother in order to avoid wake turbulence. Finally,as the distance between parallel approaches is re-duced, there will be a need for more accurate sur-veillance to verify that aircraft are on approachto the proper runway. The radar now in use,which has a 5-milliradian accuracy and a 4-secondupdate rate, is probably not adequate for this pur-pose and may have to be replaced with new ra-dar capable of l-milliradian accuracy and 1-second update .25 Such radar performance has beenachieved in the Precision Approach Radar systemformerly installed at some airports but now de-commissioned. Military radar also has this capa-bility but would have to be adapted and testedbefore use in civil aviation.

Independent Parallel RunwaysIndependent instrument approaches to paral-

lel runways separated by at least 4,300 ft arepresently authorized under the following condi-tions: 1) when aircraft are turned onto the ap-proach path, they must be separated vertically byat least 1,000 ft or laterally by 3 nmi from air-craft turning on approach to the other runway;and 2) a No Transgression Zone, at least 2,000ft wide, must be maintained between the ap-proaches, with a separate controller assigned tomonitor this zone. A study by FAA indicates that,as with dependent parallel runways, reducinglateral spacing for independent parallel runwaysfrom 4,300 to 3000 ft would require installationof more accurate radar but no other changes incurrent procedures. 26

251bid.b Ibid.

Triple Parallel RunwaysDemand at some of the busier airports, such

as OHare, Atlanta, Dallas/Fort Worth, Pitts-burgh, and Detroit, sometimes exceeds the capac-ity of the runway system in IMC, and additionof a third approach stream would be desirable.Current ATC procedures allow approaches to tri-ple parallel runways only during VMC. Revisionof separation standards to permit their use dur-ing IMC would significantly expand the time thatmaximum airfield capacity is available at thesefew very busy airports.

While the requirements for three parallel ap-proaches are similar to those for two parallel ap-proaches, the addition of a third runway com-plicates the approach procedures and limits possiblegains in capacity utilization. To be most effective,at least the outside pair of approaches should beindependent from each other, although both maybe dependent on the middle runway. If all threeparallel runways were dependent, there would beonly a minor increase in throughput compared tothat attainable with two dependent runways.Also, since a blunder on one of the outside ap-proaches could affect more than one other air-craft, establishment of triple independent paral-lel approaches necessitates two No TransgressionZones, with a separate controller assigned tomonitor each. Because the l, 000-ft vertical separa-tion rule for aircraft turning onto parallel ap-proach paths still apply, final approach courses,particularly for the center runway, would belongerthereby diminishing somewhat the through-put gain attainable with the triple parallel con-figuration.

A few airports have runway layouts that allowa converging approach to be added to two existingparallel approaches. This third approach is usedduring VMC, but in IMC the converging runwaymust be closed because separation between air-craft executing missed approaches cannot beassured visually.

The requirements for three approaches, one ofwhich is converging, are similar to those for twoconverging approaches. However, establishing thethird converging approach for use with a paral-lel pair involves additional safeguards because ablunder by an aircraft on one of the outside ap-

preaches affects more than one other aircraft. Themissed approach path for the converging runwaymust be coordinated with those of the other tworunwaysa procedure that is quite complex andcannot be implemented without further researchand evaluation. In particular, FAA is studyingwhether MLS will be required to provide non-conflicting missed approach paths .27

Reduced Longitudinal Separationon Final Approach

Current procedures require longitudinal (in-trail) separation of 3 nmi between aircraft con-ducting instrument approaches to the same run-way In VMC, in-trail separations of 2.5 nmi

Ch. 4Technology l 7 5

For those airports where runway occupancy timeaverages so seconds or less, FAA studies indicatethat minimum in-trail separation of 2.5 nmi couldbe allowed in circumstances where wake vortexand ATC workload permit. Flight tests conductedby the U.S. Air Force have demonstrated the fea-sibility of 2.5-mile separation for military use.However, safety standards for commercial oper-ations are different than those for military oper-ations, and analysis of radar accuracy and updaterates, controller and pilot response times, and air-craft performance characteristics will be neededto determine whether 2.5-mile separation duringIMC is safe for civil aviation. Since there is a di-rect relationship between in-trail separation andthroughput, this procedural change would be avery effective method to reduce delay under in-strument flight conditions.

Present ATC procedures specify that the nom-inal longitudinal separation standards for VMCor IMC be adjusted to compensate for the possi-ble effects of wake turbulence. These separationstandards, shown in figure 9, are based on a three-way classification of aircraft according to grosstakeoff weight and attempt to account for thewake-turbulence characteristics of aircraft andtheir vulnerability to wake vortex encounters:

l

l

l

heavy ah-craftmaximum gross takeoffweight (GTW) in excess of 300,000 lb,large aircraftmaximum GTW between12,500 and 300,000 lb, andsmall aircraftmaximum GTW less than12,500 lb.

Definition of aircraft categories based on GTWalone is not an accurate index of of wake vortexgeneration for all aircraft, notably those aircraftwhose GTW is slightly over 300,000 lb such asthe DC-8 and B-767. As the number of B-767 air-craft in the fleet grows and as the re-engining pro-gram for DC-8S proceeds, aircraft whose GTWis roughly 300,000 lb will become an increasinglylarge proportion of the commercial aircraft fleet.If these aircraft continue to be classified as heavy,greater arrival separations will be required, withadverse effects on capacity and delay.

If aircraft were classified on the basis of moreprecise analytical or empirical data concerningtheir specific aerodynamic and wake-vortex char-acteristics, it might be possible to reduce the in-trail separation rules for some types. As a mini-mum, the use of approach weight rather thanmaximum GTW as the basis for separation cri-teria could be considered. To be even more pre-

Figure 9.Arrival and Departure Separations

utical MilesInstrument Flight Rules

Leads 3 3 3L 4 3 3H 6 5 4

Minimum Departure SeparationsSecondsVisual Flight Rules* Instrument Flight Rules

K ., I [ I f l - . - : l I I [

1 s I 35 I 45 [ 50H I 120 I 120 I 90

KEY: S = Small, L = Large, H = Heavy (see text.)

76 l Airport System Development

cise, wingspan, approach speed, and engine andflap configurations should also be taken into ac-count. A recommendation to this effect was madein the report of the Industry Task Force on Air-port Capacity Improvement and Delay Reductionand is now under consideration by FAA.31

Separate Short Runways forSmall Aircraft

The current practice in air traffic control is toorganize aircraft on approach according to timeof arrival, not type of aircraft. So long as the traf-fic mix is reasonably uniform, this practice hasa minor effect on throughput. At many airports,however, small aircraft represent a significant por-tion of traffic. To avoid wake turbulence gener-ated by the heavy and large classes of transports,these small aircraft are required to follow in trailat distances of 4 to 6 nmi from the larger aircraft.Since many of these small aircraft operate at slowspeeds, safety requires that larger and faster air-craft be spaced more than 3 nmi behind so thatthe leading small aircraft are not overtaken on ap-proach. One way to overcome these operationalpenalties would be to segregate small general avia-tion and some commuter aircraft into a separatetraffic stream using a different (short) runway. Atsome airports such a runway is already availablebut not usable for instrument approaches because

of inadequate instrumentation; at others, newrunways would have to be built and equippedwith MLS.

There is some disadvantage to separate shortrunways in that they do not provide as muchoperational flexibility as a full-length additionalair carrier runway. However, the separate shortrunway can be built at a fraction of the cost ofan air carrier runway, and runway siting prob-lems as well as local environmental issues maybe easier to resolve.

Ideally, the separate short runways for smallaircraft should be parallel to and operate inde-pendently from the main runway used by largeair carrier traffic. A short runway that is not par-allel to the main runway would not be availablefor use in IMC unless revised procedures for con-verging instrument approaches are also imple-mented; but even so, dependency on the mainrunway would limit the throughput gain becauseof the need to coordinate the two traffic streams.If the procedures described above to reduce spac-ing requirements for independent and dependentparallel approaches prove feasible, the siting ofthese short secondary runways could becomeeasier. Another development that would facilitatesiting of short runways and broaden the appli-cability of the concept would be installation ofMLS to allow curved approaches and steeper glideslopes by small aircraft, not only to alleviate waketurbulence problems but also to achieve a greaterrate of runway use.

WEATHER AND ATMOSPHERIC EFFECTSPerhaps the single greatest technological need scribed above is a better method to detect or to

in relieving delay at airports, aside from improved predict the occurrence of wake turbulence.radar to monitor aircraft more closely spaced interminal airspace, is development of techniques Beyond this, improvement in the ability to pre-to improve the detection and prediction of weather diet weather and atmospheric phenomena couldand atmospheric effects. Weather-related technol- lead to general reductions in delay. Present tech-ogies are typically viewed as safety improvements nology does not always permit sufficiently ac-rather than capacity improvements, but there are curate prediction of the time and magnitude ofsignificant exceptionsnotably methods to pro- adverse weather conditions, making it necessarytect from wake vortices. Current aircraft arrival to increase safety margins and thereby reduceand departure separations are predicated in large throughput. The ability to foresee disruptions duepart on avoidance of wake vortices, and the key to weather would permit planning to compensateto many of the revised approach procedures de- fer the impacts on traffic flow.

Ch. 4Technology 77

Wake VortexWake vortex is an aerodynamic disturbance

that originates at the wingtips and trails in cork-screw fashion behind the aircraft. Since the strengthof the turbulence increases with lift, the strongestvortices occur behind heavy aircraft. These vor-tices spread downward and outward in the wakeof the aircraft and may persist along the flight pathfor as long as 2 or 3 minutes in still air. Whenthe aircraft is within 300 ft of the ground, the vor-tices can bounce off terrain and rise back towardthe flight path, creating even more disturbance.Wake turbulence can be of such strength andduration that it poses a hazard to following air-craft (especially smaller aircraft), and present pro-cedures require separation of 3 to 6 nmi depend-ing on the size of the leading and following aircraftand the movement of the airmass.32

Alternatives to the present procedural methodof avoiding wake turbulence are being soughtboth in the interest of safety and for the capacitybenefits that could be realized through closer spac-ing of aircraft in the approach zone. Two avenuesare being taken. FAA has concentrated on devel-opment of techniques to detect wake vortex andto predict its movement and persistence. NASAhas focused on aerodynamic research to providebetter understanding of the mechanics and causesof wake vortex and to develop designs to alleviateit at the source. NASA research indicates that cer-tain combinations of flaps, spoilers, and protru-sions on wing surfaces can reduce turbulence orcause it to dissipate more quickly. Unfortunately,many of these techniques also tend to increasenoise and reduce energy efficiency. Work is con-tinuing on ways to minimize wake vortex at anacceptable price in terms of noise and fuel con-sumption, but no ready solution is in sight. Thisis an important area of research and developmentsince the alternativewake vortex detection andavoidancehas not been perfected to the pointthat pilots have confidence in its reliability.

FAA has sought to develop equipment and aconcept of operation that provide real-time vortex

32J. N. Barrer, Operational Concepts for Reducing Vortex Spac-ings on Closely Spaced Parallel IFR Approaches, The MITRE Corp.,WP-81W520, September 1981.

sensing capability and to devise a predictivealgorithm that will warn pilots and controllers.An experimental device, known as Vortex Advi-sory System (VAS), was installed and tested atOHare in 1978. VAS is made up of wind sensorsmounted on towers along the approach path, acentral computer to process wind data and pre-dict the strength and movement of wake tur-bulence, and a display to alert the controller whena hazardous condition exists. VAS has not yetproven operationally acceptable, and FAA plansfurther development and test.

The disadvantage of VAS is that it does notdetect wake vortices; it only measures wind direc-tion and velocity, from which an inference canbe made about the presence and strength of waketurbulence. This deficiency is particularly evidentfurther out on the approach path (beyond themiddle marker) and in crosswind conditionswhere turbulence on one approach path maymigrate to a parallel approach. To overcome theselimitations, FAA is also investigating other tech-nological approaches such as short-wave radar,lasers, and infrared devices that could provide bet-ter long-range sensing and wider coverage.

No practical solution is now in view, and itseems likely that procedural methods to avoidwake turbulence will continue to be employed.So long as wake vortices cannot be reliably de-tected and predicted, the present separation stand-ards (perhaps with some modification to accountfor the aerodynamic characteristics of specifictypes of aircraft) will remain in force and precludeany throughput gains that might be achievedthrough reduced in-trail spacing.

Wind ShearWind shear is any sudden change in wind veloc-

ity or direction. It may be associated with warmand cold fronts, low-level jet streams, or moun-tainous terrain. One of the most dangerous typesof wind shear is a downward surge of air strik-ing the ground and spreading out in all directions.This kind of wind shear is often associated withthunderstorms, but it may occur in other weatherconditions. These downdrafts, called microbursts,are difficult to predict because they are small and

78 . Airport System Development

localized, extending only 2 or 3 miles and oftenl a s t i n g l e s s t h a n 5 m i n u t e s .

For the pilot of an aircraft , wind shear is ex-

perienced as an abrupt increase or decrease of l ift( o r o f t e n o n e r a p i d l y f o l l o w e d b y t h e o t h e r )

caused by a sudden shift in the relative wind. I nthis condition, the aircraft may gain or lose al-titude unexpectedly and become difficult to con-trol in angle of attack and flight path (see fig. 10).If this occurs near the ground on takeoff or land-ing, there can be extreme hazard.33 While the pri-

mary concern is safety of flight, wind shear alsodisrupts airport activities and can cause suspen-sion of operations until the condition abates.

In 1982, The Federal Government undertooka project known as Joint Airport Weather Studies(JAWS) to provide a better understanding of windshear, thunderstorms, and related weather hazardsand to identify weather conditions that could bewarning signs to pilots. A multi-agency effort in-volving the National Science Foundation, the Na-tional Aeronautics and Space Administration, theNational Oceanic and Atmospheric Administra-tion, and the Federal Aviation Administration,JAWS collected data on downbursts at DenverStapleton airport during a 3-month period in thesummer of 1982. The knowledge of wind shear

Figure 10.Effects of Low=Aititude Wind Shear