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responsibility of the Committee for AviationEnvironmental Protection (CAEP), establishedin 1983, which published its first set of regula-tions (CAEP 1) in 1988.

After training and serving as a technician inthe Royal Air Force, followed by a number ofyears with a British regional aircraft manufac-turer, I was recruited by Snecma to join the CFMsales team with specific responsibility forScandinavia. It quickly became apparent thatenvironmental issues were a principle concernfor my new customers, one clue being that thepresident of the one-time Swedish domestic air-line Linjeflyg had his environmental advisorlocated in the office next to his! Meeting people

at airlines such as SAS, Braathens SAFE,Finnair and Icelandair, I came to understand thefragile nature of the Scandinavian environmentand how much it is under threat from externalsources of pollution such as large power sta-tions in other parts of Europe. While they couldnot do much about external sources they weredetermined to protect themselves from internalsources, developing the ‘bubble’ conceptwhereby any new industrial project was requiredto account for all emissions generated by itsactivity. It was as a result of this philosophy thatthe world’s first local emissions legislation wasapplied to Swedish airports in 1990, limitingNOx emissions emanating from all aspects of

The Engine Yearbook 2011

During the 1990s the aerospace industry was forced to confront the world’s growingenvironmental awareness, which manifested itself in tougher certification requirements foraircraft engines and the introduction of local environmental regulations, particularly in Europe.David Cook, President of ASM Consulting, was sales director at CFM International from 1989 to2001 with account responsibility for Northern Europe. Through sales campaigns at AustrianAirlines, Swissair, SAS and Finnair he saw firsthand how the aero-engine industry responded todemands for cleaner engines which, in turn, explains why the industry is so well-equipped tomeet the challenges facing a new generation of commercial aircraft engines.

The evolution of low-emissionscombustion chambers incommercial aircraft engines,1990 — 2010

As mass air travel started to develop inthe 1970s the public, particularly thoseliving around airports, became more and

more vocal in their concerns about aircraftnoise and atmospheric pollution. Images ofearlier generation four-engined jets heading fordistant destinations with engines bellowinglong plumes of smoke are emblematic of thisperiod. By the late 1970s the International CivilAviation Organisation (ICAO) decided to act bybringing in limits for aircraft engine noise andpolluting emissions, defining certification stan-dards for Nitrous Oxides (NOx), UnburnedHydrocarbons (UHCs) and Carbon Monoxide(CO). These standards were put under the

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the airport’s activities and initiating charges foraircraft movements dependent on the aircraftengine’s certified NOx emission levels. Whileground vehicle activity, in particular private cars,was clearly the main source of airport NOxemissions this concept was also aimed at cur-tailing, or at least penalising, movements of air-craft with high NOx emitting engines.

At around about the same time AustrianAirlines and Swissair launched an evaluation toreplace their ageing DC-9 fleets, finally select-ing the Airbus A320-family as their preferredairframe. This gave them an opportunity to eval-uate both CFM56-5B and IAE V2500-A1engines in a competitive selection process. AsSwitzerland had recently followed Sweden’sexample by introducing airport NOx emissionstaxes the message from the airlines was clear— all other things being equal, they wouldselect the engine with the lowest certified NOxemissions. The gauntlet had clearly beenthrown down and it was up to the aero-engineindustry to respond.

Controlling aircraft emissionsIt is perhaps worth reminding ourselves

what emissions are produced by an aircraftengine and how they may be controlled. As pre-viously mentioned, those emissions controlledby the engine’s certification process are NOx,UHCs, CO and smoke. Limits for these emis-sions are defined by ICAO dependant on theengine’s overall pressure ratio (OPR) and meas-ured through what is called the landing andtakeoff cycle (LTO) ie: the sequence of eventsbeginning with start-up and taxi out to the endof the runway, engine acceleration to takeoffthrust, through the takeoff run, up to 3,000feet on the climb-out, then from 3,000 feet onthe descent to touchdown on the runwaythreshold, the landing run, taxi in and shut-down. Visible smoke consists of small sootparticles in the jet exhaust and is created byinefficiencies in the combustion process.Similarly, UHCs and CO are produced by ineffi-cient combustion, particularly at low enginerpm. The most important pollutant in terms ofamounts produced and potential environmentalimpact is NOx. This is produced by an enginewhen air (consisting of oxygen and nitrogen) issubjected to high temperatures, particularlyduring the combustion process, and decom-poses. It plays many different roles in terms ofits environmental impact: it is a recognisedhuman health risk, promoting asthma and awide range of respiratory diseases and, in thepresence of sunlight, NOx creates ozone at lowaltitudes thus contributing to the greenhouseeffect. NOx is also very persistent, remaining inthe atmosphere for many years after other pol-lutants have either dispersed or decomposed.

As is often the case with anything to do withaircraft engines, the combustion chamberdesigner is faced with a multitude of conflictingpriorities. He must make sure that maximumthrust is produced at takeoff, that maximumfuel economy is achieved at cruise, that theflame does not go out at high altitude or inheavy rain ... and, of course, that the enginemeets its environmental certification require-ments. In order to minimise UHC and CO pro-duction the combustion chamber needs toavoid producing these compounds by a highlyefficient combustion process, or be able toburn off the by-products of inefficient, low-rpm,combustion. In order to minimise NOx produc-tion the combustion chamber needs to reduceas much as possible the amount of air sub-jected to the elevated temperatures of high-rpmoperation. This is achieved by either limitingthe volume of air subjected to high tempera-tures or by reducing combustion temperatures.The problem facing CFM and IAE back in theearly 1990s was how to resolve these two con-flicting design requirements to meet theAustrian/Swissair challenge.

Obviously, I was not privy to IAE’s proposalbut we at CFM were led to believe that theirsolution revolved around modifications to anexisting combustion chamber design.Conventional chambers function on the basisof what is called the Rich Quench Lean (RQL)process. The fuel mixture at the nozzle is rela-

The Engine Yearbook 2011

80

60

40

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10 20 30 40 50

Overall pressure ratio (OPR)

NO

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issi

on le

vels

(g/k

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rust

- L

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ycle

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Talon X?TAPS II?

Early turbofan enginesCFM56-5B4 familyCFM56-7B26 familyIAE V2527-A5 family

CAEP 4 limits

CAEP 6 limits

Conventional combustors

TAPS / TALON combustors

DAC combustors

tively rich, providing high power and good fueleconomy but generating large amounts of UHCand CO at low power settings. These pollutantswould be burned off as they moved into a rela-tively lean combustion environment when addi-tional air was introduced further down thecombustion chamber. At high power settingsthe NOx produced in this rich burn processwould be limited by quickly cooling, or ‘quench-ing’, the combustion gases by introducing largevolumes of cooling air just downstream of theburner flame. It is my understanding that thework done IAE/Pratt and Whitney at this stagemade a significant contribution to the develop-ment of their TALON (Technology for AdvancedLow NOx) combustion chambers which laterequipped the IAE V2500 and PW4000 seriesengines.

The CFM solution was to draw on a radicallynew combustion chamber design already in theprototype stage at GE (GE have design respon-sibility for the CFM56 engine core). Called thedouble annular combustor, or DAC, it effectivelysplit the combustion chamber in two, each sub-chamber having its own fuel nozzle. The outerchamber was relatively long and operated atthe lower thrust levels. This long chamber pro-vided the time in the combustion process toburn off UHCs and, together with a leanerfuel/air ratio, reduced CO. At high thrust levelsboth chambers were lit, providing the requiredlevels of thrust but with a relatively shorter

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ficult for them to agree on a replacement air-frame. As mentioned earlier, Sweden hadalready introduced emissions taxes at its air-ports and so SAS took a great deal of interestin the Austrian/Swissair engine selectionprocess. It was therefore inevitable that, onceSAS had decided to replace its DC-9 fleet withthe 737-600 aircraft, they too should specifythe DAC for their CFM56-7B engines.

Finnair was another airline which laterselected the DAC engine. While not subjectedto specific airport emissions taxes themselves,Sweden was an important market and they didnot want to be at a competitive disadvantagewith SAS. Having selected the Airbus A320-family to replace their DC-9s the decision was,to some extent easier for Finnair as theCFM56-5B DAC engine had already been inservice with its launch customers for a coupleof years. The first Finnair A321 with CFM56-5BDAC engines entered commercial service onFebruary 5, 1999.

Teething problemsWith such a complex combustion chamber

design, and the requirement to optimise thestaging of the two sets of fuel nozzles, it wasinevitable that there would be some teethingproblems. From the start of the DAC pro-gramme CFM recognised that this new cham-ber would generate additional maintenancecosts due to erosion of the centre body which

chamber compared to the equivalent conven-tional chamber, thus reducing the time at whichair was exposed to the high combustion cham-ber temperatures (residence time) and soreducing NOx. The design was complex andcould only be controlled by the use of an elec-tronic fuel control system, or FADEC, in order tocorrectly manage the staging of the two sets offuel nozzles throughout the flight regime.However, with its promise of over 30 per centNOx reduction compared to its equivalent sin-gle annular combustion design, and the factthat this programme had more credibility due toits advanced prototype testing, this was thesolution selected by Austrian and Swissair. InMarch 1995 the first CFM56-5B DAC-poweredA321 entered service with Swissair, the first ofa total of 375 DAC engines to go into airlineservice.

While Austrian and Swissair were the firstairlines to specify low-emission engines fortheir aircraft they were not the only ones inter-ested in the subject. During the 1990s therewere a number of attempts to create closealliances between Austrian, Swissair, SAS andLufthansa. While these early negotiations didnot reach a definitive conclusion they were tolead to what is now known as the Star Alliance.Much of the discussion focused on fleet com-monality but, despite the fact that these air-lines operated large numbers of Douglas DC-9and MD-80 series aircraft, it was extremely dif-

separated the two parts of the chamber. Totheir credit the airlines who selected the DACaccepted this additional maintenance cost bur-den, as well as a healthy supplement to theengine list price, believing it to be a fair price topay to demonstrate their environmental cre-dentials. In retrospect, it would probably be fairto say that the -5B engine entered service withrelatively few problems: some hot starts, someover-temping during taxiing, but no major diffi-culties as far as I recall. As the -7B used thesame core (HP compressor, combustion cham-ber, HP turbine) as the -5B CFM believed thatthey could confidently offer a -7B DAC to SASwhich would build on the Austrian/Swissairexperience and provide a ‘low risk’ entry intoservice. This was not the case.

Problems began even during initial enginetesting. Austrian and Swissair were using their-5B engines on A320 and A321 aircraft atthrusts ranging from 25,000lb up to 31,000lb.SAS had selected the 737-600 aircraft for theirdomestic and intra-Scandinavian routes and,as such, the aircraft were expected to operatewith very light fuel loads, little baggage and ina relatively cool operating environment. Theyonly required the -7B engine at its minimumcertified thrust of 18,000lb and, even then,expected to operate with a significant derate.Despite the fact that the -7B core was thesame as for the-5B, this lower thrust provedtroublesome for DAC development. No matter

The Engine Yearbook 2011

The author at the delivery of the first SAS 737-600 equipped with CFM56-7B20 DAC engines, September 1998. The green flower logo was used by SASto promote its environmental strategy.

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how they tried the GE engineers just could notget the -7B DAC to work correctly at 18,000lbthrust. Get the NOx right and the CO would gooff the scale. Get the NOx and the CO right thenthe smoke would be uncontrollable. In the endCFM were forced to accept that the -7B18 DACengine was not certifiable and agreed to pro-vide SAS with a -7B20 engine, 20,000lb thrustbeing the minimum thrust level at which theycould get the DAC to work effectively. Theengine was duly certified, the aircraft deliveredand the first scheduled flight of an SAS 737-600 equipped with CFM56-7B DAC technologytook place on October 31, 1988 - ironicallyenough, on an early morning rotation betweenStockholm and Paris Charles de Gaulle.

The -7B DAC quickly settled into service andseemed to bear out CFM’s claims of a reliable,derivative engine. However, during the long,dark winter of 2000, worrying stories began toemerge from the SAS flight line. An engine wasshowing signs of high vibration and borescopeinspection revealed that it had lost a low-pres-sure turbine (LPT) blade, sheared off cleanly atthe blade root. A few days later another engineexhibited the same symptoms and, within amatter of weeks, SAS had lost five engines.This was clearly a serious problem and the fullweight of CFM customer support swung behindthe effort to help this major customer. Engines

were replaced, unserviceable engines strippeddown, turbine blades and disks rushed into thelaboratories for analysis. The problem was obvi-ously related to LPT blade fatigue but what wasthe cause, and why had the other DAC opera-tors not experienced the same problem? Theanswer came from a careful analysis of SASflight data and an understanding of the waythey operated their 737 aircraft. DAC FADECsoftware was programmed to schedule a fairlyclear ‘switch’ from single, outer burner opera-tion at low rpm to double burner operation athigh rpm. What in fact was happening was that,in operating their aircraft into congestedEuropean airports, SAS were forced to fly longlanding approaches at intermediate altitudes,stepping down into the landing pattern. Thisforced the FADEC to keep switching the DACfrom single burner operation to double burneroperation during the landing approach, thusinducing a resonance in the LPT disk whichweakened the LPT blade root. After more than1,000 cycles or so blades started to break.

Alternative operating procedures wererushed in to avoid ‘long, low’ approaches, LPTdisks were re-designed and, over a period ofalmost two years, engines were modified. Thiswas a major challenge to both CFM and the air-line but again, their willingness to resolve theproblem and keep the DAC engine flying was a

The Engine Yearbook 2011

The combustion chamberdesigner is faced with amultitude of conflictingpriorities. He must make surethat maximum thrust isproduced at takeoff, thatmaximum fuel economy isachieved at cruise, that theflame does not go out at highaltitude or in heavy rain ... and,of course, that the enginemeets its environmentalcertification requirements.

The CFM56-5B entered service with relatively few problems.

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UHCs and CO at the burner nozzle, rather thanin the body of the combustion chamber, theywere able to revert to a more conventional RQLchamber design but with a highly sophisticatedstaged nozzle concept called TAPS — TwinAnnular Premixing Swirler. TAPS was initially con-ceived as part of the Tech56 technology acqui-sition programme aimed at developingtechnologies to be incorporated into a new gen-eration of engines to be introduced in the 2012-2013 timeframe when Boeing and Airbus wereoriginally expected to launch their next genera-tion single-aisle aircraft. However, as these newaircraft programmes began to slide further intothe decade CFM took the decision to commer-cialise the Tech56 technology by launching amajor product update of both its -5B and -7Bfamily of engines. The Tech Insertion (TI) pro-gramme provides for important improvements inthe engine core, with TAPS technology leading toa 28 per cent reduction in NOx compared to anengine with the original standard combustionchamber without the DAC complexity, as well asreductions in fuel burn and maintenance costs.While initially introduced as an upgrade kit theTI hardware is now standard build configurationfor all CFM engines.

CAEP continues to reduce NOxlevels

During this time the industry has continuedto come under both political and social pres-

sure from national governments and environ-mental groups. However, much to the frustra-tion of those airlines who had invested in DACtechnology, there was not, as feared, a wide-spread proliferation of airport environmentaltaxes. None the less CAEP continues to chal-lenge engine manufacturers to lower certifiedemission levels of new engines: CAEP 2, intro-duced in 1996, reduced NOx emission certifi-cation limits by 20 per cent compared to theoriginal levels. CAEP 4, in 2004, reduced NOxlevels by another 16 per cent and CAEP 6(2008) by a further 12 per cent. Emission lim-its have been tightened to such an extent thatno engine can be offered to the market todaywithout a low emissions combustor and allengine manufacturers have their low emis-sions technology programmes. IAE and Pratt &Whitney have continued to develop their TALONcombustor such that the new geared turbofanseries of engines, due to enter service in2013, will incorporate the latest TALON X tech-nology offering NOx emission levels 55 percent below current CAEP 6 certification limits.The Rolls-Royce Trent series of engines areequipped with the ‘Phase 5’ combustor, pro-viding margin relative to CAEP 6, but the‘Phase 6’ combustor programme currentlyunder development has even more ambitiousNOx objectives.

The aerospace industry, and in particularcommercial air transport, continues to comeunder close scrutiny and, as mass air traveldevelops, will continue to be asked to makeeven greater reductions to its environmentalimpact. The CAEP 8 meeting which took placein Montreal this February proposed a further 6-15 per cent reduction in certified NOx emis-sions for new aircraft engines from 2014, andalso ‘committed to a timetable for the devel-opment of a new Carbon Dioxide (CO2) stan-dard by the time of the next CAEP meeting(CAEP 9) in 2013’ (ICAO press release). Whilstcompetitive commercial pressure has broughtabout substantial reductions in fuel burn —and hence CO2 emissions — over the years ithas, until now, escaped the concept of certifi-able limits. Now, as a direct result of therecent Copenhagen Environmental Summit,the industry will be subjected to even furtherscrutiny.

I am convinced that, as a result of its expe-rience over the last 20 years, the aero enginemanufacturing industry is well placed to meetthose challenges and, while the DAC may soonbe consigned to the top shelf of history, Ibelieve it has played a vital role in helping theindustry to understand the infinite complexitiesof combustion chamber thermodynamics andthe operation of low emission combustors inscheduled airline service. �

The Engine Yearbook 2011

potent demonstration to their joint commitmentto the environmental challenges facing theindustry.

Learning from the DAC experienceAfter the initial problems the DAC has con-

tinued to operate well although anecdotal evi-dence suggests that the maintenance costpenalty of the split chamber design is ratherhigher than initially indicated. A number of air-lines who have ‘inherited’ DAC engines fromleasing companies following the break-up ofSwissair and Sabena in 2002 regret the highmaintenance costs of these engines and theproblems of intermix without necessarily under-standing (nor being sympathetic to) the impor-tant role they played in our industry’s battle withthe environmentalists. Let us not forget eitherthat the GE90 engine, powering over 500 777aircraft with various airlines around the world, isalso fitted with a DAC which has operatedimpeccably. However, GE maintains that theirchoice of this technology was aimed primarily attaking advantage of its short length to reduceengine weight rather than any specific environ-mental advantages. GE and CFM did, none theless, learn a lot from the DAC experience and,as their understanding of the complex issuesrelating to combustion chamber thermodynam-ics evolved, they were able to propose an alter-native to the split chamber design. Byeffectively resolving the contradictions of NOx,

CFM56-7B engines underwent extensive modification following a bumpy start with SAS.

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