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Customer Services

A340-500 Arctic flightsTwo flights exceeding 10 hours each were required forthe A340-500 fuel system certification and for theevaluation of the ADIRS performance. In addition, thebehaviour of the navigation system and the ElectronicInstrument System (EIS) had to be checked in the polararea including flying over the North Pole itself.

Both flights were planned and operated by the AirbusFlight Test Division: Toulouse-Keflavik and Keflavik-Toulouse, each time via the North Pole.

This article recalls some aspects of the polar navigationobserved during these flights.

Capt. Michel BrandtAirbus Test Pilot

Flight Operations Support

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DIGRESSION ONMAGNETIC HEADING

Traditionally, the polar area was definedas an area of magnetic compassuncertainty and the limit was sometimesset where the earth magnetic field is lessthan 6 microtesla. Except for the standbycompass, modern aircraft no longer havemagnetic sensors “slaving” the gyro-compass to the magnetic north. Themagnetic variation is extracted fromtables in the Air Data/Inertial ReferenceSystem (ADIRS) and the FlightManagement System (FMS). These tablesgive an accurate magnetic variation up toa latitude of 82° N, or 73° N in functionof the longitude. Beyond this latitudeTRUE reference must be used.Consequently, the 6 microtesla limit hasno meaning for these navigation systems.We nevertheless observed during theseflights that the standby compassindication was coherent well above thecharted 6 microtesla limit.

DIGRESSION ON GRID TRACK

When flying east to west or west to east at very high latitude on anorthodromic (great circle) route, the true track changes quite rapidly due to theconvergence of the meridians. On older aircraft, the true heading was notdirectly available to the crew. “Old style” Horizontal Situation Indicators (HSI)were set in a so-called free gyro mode on the Grid North reference. Thecharacteristic of the grid track is that it remains constant on an orthodromicroute as indicated in the drawing below, right.With our modern navigation system, the grid track would not be mandatory, asthe true track is alwaysavailable and the navigationcharts indicate the outboundand inbound true tracks ateach waypoint for cross-check. But the grid trackindication on the navigationdisplay provides the crewwith at least a convenientstable indication fornavigation monitoring inthis moving displayenvironment.

On TRUE heading

(Valid for a polar stereographic projection)

GT - TT+Long. WGT - TT-Long. E

TT VariableGT Constant

GRID North Direction

GT

GT TT

TT

GeographicPole

0º

LONG

Note: Where words are spelt in capital letters it refers to their use as cockpit labels, messages and displays.

We left Toulouse on 12 September2002 with 12 people on board, maxi-mum fuel uplift and full flight-testequipment, including ballast, whichgave a take-off-weight (TOW) of325,000kg. Our flight plan led us toVigra VHF Omnidirectional Range(VOR) in Norway to intercept thePolar Track System (PTS) route“November”. This route follows themeridian 10°E to the North Pole.

Although Magnetic Heading (MH)is available until 82°N at this long-itude, we selected TRUE headingreference as soon as we were estab-lished on the PTS. With this selec-tion, the indications on navigationdisplay (ND) (Figure 1 below) andMultipurpose Control and DisplayUnit (MCDU) were in line with theTrue Tracks (TT) given on our nav-igation charts.

When passing 65°N latitude thegrid track (GT) appeared on thenavigation display.

Although the Flight Managementand Guidance System (FMGS)remained in GPS PRIMARY modeof navigation update all the time,we were able to cross-check theFMGS navigation with the Auto-matic Direction Finder (ADF)sometimes at a very great distance,up to 500 nautical miles (nm) fromthe Non-Directional Beacon (NDB).However, the wave propagationmay not be always as good, forexample, during periods of auroraborealis.

Flying in NAVigation autopilot(NAV) mode, nothing significanthappened until approaching theNorth Pole. We observed the rela-tive position of each InertialReference System (IRS) to antici-pate the system behaviour whileflying over the North Pole.

Figure 1

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HEADING BEHAVIOUR

At about 20nm before the Pole theheading discrepancies on NDbecame noticeable and finally trig-gered, as expected, the HDG DIS-CREPANCY Electronic Central-ised Aircraft Monitor (ECAM)caution (Figure 2) and CHECKHDG on the ND and Primary FlightDisplay (PFD) (Figures 3a and 3b).

The heading discrepancy is due tothe fact that each IRS has a differ-ent position relative to the Pole.

The ECAM procedure cannot befollowed, as all heading indicationsstart to move very quickly andswitching ATT HDG on IRS3would not help. The autopilotremained engaged in NAV modeand the aircraft continued nicelystraight ahead.

When flying exactly (with GPSaccuracy) over the North Pole theND display swung over by onehundred eighty degrees from aTrue Heading (TH) close to 000° toa TH close to 180° (the drift wassmall). But very quickly all head-ings were again in agreement asshown on the flight test tracesgiven in Figure 4 (below), and theECAM caution disappeared.

At 20nm outbound from the NorthPole, everything was nominal andit was time for us to turn left tointercept “another” 180° course tothe VOR of Thule (THT).

Despite the magnetic variation of66°W at Thule, the VOR THT ismagnetically oriented (some VORsin north Canada are oriented to thegeographic North). As we were fly-ing with TRUE reference, the VORneedle on ND in MAP mode orROSE NAV mode is automaticallycorrected with the magnetic varia-tion, so as to get a TRUE bearing.Otherwise the needle would notindicate the direction to THT. Thisis marked by the magenta color ofthe needle and the label CORR next

Sec

359.00

287.20

215.40

143.60

71.80

0.00

0

GMT START: 13:54:56

HDG of each IRS plotted versus time in seconds.

20 40 60 80 100 120

The red line indicates thetime of the flight over the

North Pole.Before reaching the

North Pole the headingcalculated by each IRS

was close to 360°/000°.After passing the North

Pole all headingsconverged rapidly toapproximately 180°."

Note: The TAXI CAMERA was selected “on” in flight so the flight testengineers could see the landscape from their station in the cabin.

Figure 2 Figure 3a

Figure 3b

Figure 4

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to the VOR2 display as shown inFigures 5 and 6. The indications ofthe Digital Distance and RadioMagnetic Indicator (DDRMI) andthe ND in ROSE VOR mode are notcorrected, as they would be on aconventional aircraft. When wewere at 77nm to THT, the DDRMIneedle of VOR2 (not shown here)was pointing towards 255° mag-netic instead of 184° True as shownin Figure 5.

After passing THT (Figure 6) theoutbound true track to the 7440Nwaypoint was 095°. On this leg thetrue track changed significantly,from 095° to 123° due to the con-vergence of the meridians, but thegrid track remained convenientlyconstant and equal to 163°.

Leaving the oceanic airspace wedeselected TRUE to revert to mag-netic reference and continued todestination, arriving at Keflavikafter 10 hours 45 minutes flighttime, having enough fuel to returnto Toulouse with more than therequired reserves.

The next day we left Keflavik forToulouse and headed north east tojoin and follow the PTS route“Romeo” up to ROGSO, a way-point located 230nm to the north ofThule. At this waypoint we turnedright to the North Pole. On PTS“Romeo” we selected TRUE refer-ence, as it is better for navigationmonitoring; but this time, beforecrossing 82°N, we deselectedTRUE to revert to magnetic refer-ence in order to see how the ECAMwill advise us to select the rightheading reference.

Figure 5

Figure 6

Ellesmere Island

Flight test engineers at station

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With the polar navigation capability of the Airbus Long Rangefamily of aircraft using the flight management system, polar flightsare no longer different from standard navigation.

Conclusion

As shown on Figure 7, we were ona magnetic track of 090° on courseto Alert NDB (identification codeLT) corresponding to a true track of025°, giving a magnetic variationof 65°W.

When we reached 82°N, we got theamber message SELECT TRUEREF on ND (Figure 7) and onMCDU (Figure 8).

Intentionally we did not follow thisinstruction, waiting for the nextstep, the ECAM caution at 82°30’ N. At this latitude, the IRSsswitched automatically to TRUEreference, which triggered theEXTREME LATITUDE ECAMamber caution (Figure 9).

The autopilot went off as well,because each IRS crosses the lati-tude limit at slightly differenttimes. We performed the ECAMprocedure, confirming the TRUEreference selection and re-engagedthe autopilot.

For the purpose of the test, wedecided to fly nearby the NorthPole this time with autopilot inHDG mode. As the North Pole wasa turning point, to join the PTS“November” route southward wepassed abeam it by about 10nm.This distance was sufficient to limitthe heading discrepancy, so that wewere able to steer the aircraft withsufficient accuracy. The recom-mended procedure is to fly with theautopilot in NAV mode, but in a sit-uation where the BACK UP NAVwould have to be used, this testconfirmed that flying in HDG iseasy, even close to the North Pole.

We continued heading south on PTS“November” until Trondheim VORin Norway. Leaving the oceanicarea, we resumed normal navigationand landed in Toulouse after 10hours flight time.

Figure 7 Figure 8

Figure 9

The time of exotic instruments like theastrocompass illustrated here is gone.

CONTACT DETAILS

Capt. Michel BrandtAirbus Test PilotFlight Operations SupportTel: +33 561 93 35 52Fax: +33 561 93 29 68michel.brandt@airbus.com

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