Straight in on

GPs data cards now contain Non-Precision Approaches for a number of Australian aerodromes for training by day in VMC. Approval for use in IMC is expected later this year.

THE FIRST AUSTRALIAN GPS NON-Precision Approach procedures (GPS/NPA) were published in the Feb-

ruary 1997 Departure Approach Proce-dures (DAP) amendment. They should now be available on the GPS navigational data-base distributed to owners of TSO C129 GPS receivers.

The approaches - which may only be flown in VMC conditions - are published for practice and familiarisation, and to allow for the issue of GPS/NPA endorse-ments which will become possible when CAO 40.2.1 is amended for this purpose.

It is likely that the procedures will be approved for use in IMC in the third quarter of 1997.

The first approaches available are at Avalon, Brisbane, Cunderdin, Darwin, Goulburn and Wollongong. More approaches will become available as they are designed.

An essential step before the approaches can be used in IMC is the scheduled changeover to the use of the WGS 84 data for instrument approach charts in July this year. A flight inspection and validation policy for GPS/NPA must also be established and implemented.

To conduct GPS/NPA in IMC, pilots will be

required to hold a GPS/NPA instrument rating endorsement, for which a flight test will be required. However, because of the relative complexity of the procedures required to select and conduct GPS/NPA5, and because different makes and models of GPS receivers differ markedly in their operating procedures, pilots will also be required to conduct at least three GPS/NPA5 using the same type of receiver. This latter requirement is not a flight test but sim-ply a means of ensuring that pilots fully famil-iarise themselves with the operation of each type of receiver.

Recency for the conduct of a GPS/NPA will be of two kinds; a general requirement for one approach within the previous 90 days; and a receiver specific recency of one approach using the same type of receiver within the pre-vious six months. The methods by which pilots are able to maintain recency are the same as for current ground-based radio nav-igation aids.

The equipment required to fly a GPS/NPA is an approach capable TSO c129 GPS or equiv-alent receiver equipped with a current navi-gational database. Non-TSO receivers are not suitable because they lack the capability to retrieve an approach procedure from the

database or to provide the required accuracy and integrity during the approach. Note that not all TSO c129 receivers are approach capa-ble, so if in doubt, consult the operating man-ual supplied by the manufacturer. Note also that TSO C129 receivers will allow a GPS/NPA to be flown with an out of date database. This feature is incorporated for last ditch emer-gency use only and a current database is essential for GPS/NPA in the same way that current approach plates are for any instru-ment approach procedure.

Also of critical importance is the correct placement of the GPS receiver on the aircraft instrument panel. Although most types of equipment may be interfaced to an external CDI or HSI, vital information such as the active waypoint, distance to waypoint, and other messages and warnings are displayed on the receiver itself.

For this reason, the receiver must be mounted in a position that is easily included in the pilot's primary instrument scan. The centre radio stack is the logical position in most aircraft. Some receivers include external annunciators and warning lights - the same principle applies to the positioning of these items.

Flying the approach

The traditional method of flying non-preci-sion instrument approaches consists of estab-lishing overhead a radio navigation aid, track-ing outbound, and then conducting either a reversal or base turn and tracking inbound to the aid.

It is possible to fly this type of approach using GPS and there are many examples in the US and Canada where GPS approaches have been "overlaid" on existing VOR and NDB approaches.

However, most GPS TSO C129 receivers are not well suited to flying reversal procedures and in any case, there is no need to fly over a ground-based aid to establish position with GPS. Therefore, Aus-tralian GPS approaches are all designed as straight-in runway aligned approaches that do not require reversal turns. They are not only easier to fly but also eliminate the need for visual circling at minimum altitude.

Each GPS/NPA consists of a sequence of waypoints stored in the GPS navigational data-base which define the approach procedure the aircraft must fly. Waypoints for GPS/NPA can-not be entered manually into the receiver or modified in any way by the pilot. The design philosophy is to minimise pilot workload dur-ing the approach, and to eliminate the possi-bility of data entry errors.

When flying the approach, the GPS receiver automatically transitions through the way-point sequence, providing track and distance

FLIGHT SAFETY AUSTRALIA AUTUMN 1997 23

information to the next waypoint in the pro-cedure. This waypoint sequencing is auto-matic, provided each waypoint is crossed within the required parameters. If holding is necessary, the automatic sequencing can be suspended and GPS used to hold at a waypoint in a similar mariner to holding at a VOR station.

The waypoints used for Australian GPS/NPA are identified by five characters. The first three characters are the letters of the aero-drome identifier minus the leading Y used for Australian locations. The fourth character is N, S, E or W to indicate the direction from which the approach to the runway is made.

approach to be selected for each flight plan. After selecting the approach procedure, a con-fidence check of the waypoint sequencing and the tracks and distances between each way-point must be made. This is done by scrolling through the procedure and comparing the tracks and distances displayed by the GPS against those shown on the approach plate.

Once the approach has been selected, the equipment is capable of predicting whether RAIM will be available for the approach with-in ±15 minutes of the specified ETA. This pre-diction is based on the current satellite almanac, and does not account for scheduled

GPS/NPA design. Three waypoints allow for the approach to be joined from any direction without the need for holding or sector entry procedures. Choosing any other waypoint to join the approach may mean that obstacle protection is compromised if you are flying below the minimum sector altitude. The approach should not be continued past the initial waypoint unless RAIM is available.

Provided the aircraft crosses the waypoint within a sector 700 either side of the track to the next approach waypoint, the GPS will automatically sequence to provide track and distance to the next waypoint. If the aircraft does not cross the waypoint within the roni urn,1 cnrtnr hnlrlina mill h0 ,iitnmoutirul_

The last character identifies the position of the waypoint in the approach sequence: • A,B, and C Initial approach fix (usually three per approach). .1 Intermediate approach fix. •F Final approach fix. •M Missed approach fix. •T Missed approach turning point. •H Missed approach holding point.

As an example, the final approach fix for the runway 18 GPS/NPA at Avalon is identi-fied as MAVNF.

The approach procedure will be identified by the name of the aerodrome and the runway with which it is aligned. The approach can be selected directly from the database when required or it may be added to a flight plan.

Most equipment permits only one

or unscheduled satellite unserviceablity. After the approach is selected and the air-

craft is within 30nm of the aerodrome, you will be requested to activate the approach. If you choose to activate the approach at this point, the CDI display will automatically transition to terminal tracking accuracy of ±1 .Onm. You may delay activation until clos-er to the aerodrome if this CDI sensitivity is not desired. In any case, RAIM will automat-ically assume terminal mode of ±l.Onm within 30nm of the aerodrome, whether or not the approach is activated.

The GPS at this point should provide track-ing guidance to the waypoint selected as the transition waypoint. The transition waypoint should normally be one of the three initial approach fixes provided by the Australian

pended. The pilot must then position the air-craft to track over the waypoint within the correct sector and manually cancel the hold mode to continue the approach.

As mentioned previously, if holding is required for any reason, the automatic sequencing may be suspended by the pilot. This involves activating the 'hold' mode before crossing the waypoint on which the holding will be based. The desired inbound track to the waypoint is then selected and a holding pattern can be flown in the same way as a VOR. To resume automatic sequencing for the approach, the 'hold' mode must be can-celled by the pilot before crossing the holding waypoint.

In Australian GPS/NPA, the track from the initial to the intermediate approach fix allows

24 FLIGHT SAFETY AUSTRALIA AUTUMN 1997

the aircraft to be aligned with the final approach before reaching the Final Approach Fix. RAIM and CDI sensitivity will continue in the terminal mode of ±1 .Onm.

It is important to remember that the dis-tance displayed by GPS is the distance to the next waypoint, and not the distance to the aerodrome. In future approach designs, the distance between waypoints will be standard-ised at 5nm wherever possible as an aid in assessing distance to run to the aerodrome. The distance to the runway is also provided on the approach plate.

When within 2nm of the final approach fix, RAIM and CDI sensitivity will automati-cally transition to the approach value of ±0.3nm. This transition is gradual and com-

input from the pilot. One or more button presses may be needed to initiate a missed approach, depending on the type of equip-ment. RAIM and CDI sensitivity will automat-ically revert to terminal mode of ±1.Onm. Tracking will be provided to the missed approach point and then to the missed approach turning point and the missed approach holding point.

After conducting a missed approach, the approach must be cancelled in order to allow new waypoints or another approach to be selected.

Warnings and messages

There are many warnings and messages pro-vided by the equipment, but the most impor-

recognise the correct mode of operation for each phase of flight, particularly during instrument approach.

When first learning GPS, there is a tendency to become so engrossed in the operation of the receiver that situational awareness is lost. Remember that your first priority is always to fly the aircraft.

Differences in the operation of GPS receiver types can be confusing. Review the functions of the GPS receiver before each flight by enter-ing the complete flight plan to your destina-tion, including the instrument approach pro-cedure. Finally, do a confidence check of all tracks and distances.

As familiarity with an automated system such as GPS is gained, complacency must be

plete by the FAF. If the aircraft crosses the FAF within the

appropriate tolerance, the approach annun-ciator should indicate successful transition to the approach mode. If RAIM is not available or the approach mode is not activated, the GPS will not transition and the approach should be discontinued.

If at any time during final approach RAIM is lost or a RAIM warning appears, a missed approach should be promptly initiated.

Design obstacle clearance is based on the pilot maintaining a tracking tolerance within 1/2 scale deflection on final approach.

A missed approach can be conducted at any time by activating the missed approach mode. This is the only part of the actual approach, other than holding, which requires manual

tant are those that indicate RAIM availability and the mode in which the receiver is operating.

You should be aware of how RAIM avail-ability is indicated on the receiver you are using. Some receivers have an annunciator which illuminates when RAIM is available - others illuminate when it is not available.

The TSO design allows the approach to con-tinue for 5 minutes after RAIM is lost, however a RAIM warning during the approach will nor-mally only illuminate after this time, so a missed approach should be commenced immediately a loss of RAIM is indicated.

Human factors

A lack of mode awareness is a well-recognised hazard associated with computerised or auto-matic flight systems. You must be able to

guarded against. Always use all navigational information available to cross check GPS information and always check your own pro-ficiency in the operation of the equipment.

To a properly prepared pilot, GPS/NPA are safe, accurate and easy to fly and have the potential to ensure safe instrument approach procedures at any aerodrome in Australia.

However, if you do encounter any opera-tional problems in practising GPS/NPA5 you should report them to the Airservices Aus-tralia GNSS Program Office.

Contact the GNSS Operations Manager, Michael Smith, on 06 268 4193, or Ian Mallett of CASA on 06 268 5475.

Dick Reynoldson is a flying operations inspector for flight crew licensing, CASA.

FLIGHT SAFETY AUSTRALIA AUTUMN 1997 25