( http://www.caa.co.uk/docs/33/FOD200316web.pdf ) -14 May 2003 (file inserted immediately below) “The aircraft cleared terrain by 56 ft.” (SSA = Sector Safe Altitude) |
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SAFETY REGULATION GROUP FLIGHT OPERATIONS DEPARTMENT COMMUNICATION Flight Operations Department, Aviation House, Gatwick Airport South, Gatwick West Sussex, RH6 0YR Website address : http://www.caa.co.uk/publications16/2003 1 OPERATIONAL CONSIDERATIONS WHEN THE ACCURACY OR RELIABILITY OF NAVIGATION EQUIPMENT IS IN DOUBT DURING THE APPROACH PHASE 1.1 Introduction 1.1.1 The Air Accidents Investigation Branch (AAIB) is participating in an investigation into a serious incident where an Airbus A320 aircraft carried out two go-arounds, the first, after unreliable VOR navigation information and the second, after experiencing an EGPWS "Too Low Terrain " alert. Whilst the technical investigation continues, it is considered prudent to remind operators and their flight crews of the following recommendations and advice. 1.2 Background 1.2.1 The same crew and aircraft flew from London (EGLL) to Addis Ababa (HAAB) via Alexandria (HEBA) operating without incident to this en-route stop, disembarking passengers and refuelling. A long desert leg was flown via Khartoum with few navigation aids available for updating aircraft position. The aircraft was not fitted with GPS. On arrival at Addis Ababa VOR/DME (ADS), the crew were cleared to carry out a standard VOR/DME approach to runway 25L. There was a thunderstorm around 5 nm southwest of the airfield and therefore ADF information was not used. Navigation accuracy was low inbound and changed to high in the area of the VOR. The VOR/DME identified correctly and indications appeared normal up to the start of the procedure, but during the outbound leg an unexpected large correction left was required to acquire the radial. 1.2.2 After flying the ADS DME 13 nm arc, both a left turn to intercept the correct 249 degree inbound QDM, and descent to 11200 FT AMSL was commenced. The VOR radial started fluctuating during the descent and the indications disappeared. With no adequate visual reference, a standard missed approach was flown and the aircraft joined the hold. The crew confirmed with ATC that the VOR/DME was serviceable and the crew carried out a navigation accuracy check that appeared normal and elected to carry out a further approach. 1.2.3 Again the VOR fluctuated occasionally during the inbound leg and a "400 FT" radio altimeter callout was heard approximately 5 nm from the airfield at which point the Captain initiated a go-around and the crew heard the EGPWS Terrain Clearance Floor "Too Low Terrain" alert. It would appear that the aircraft was around 2.6 nm north of intended position. The aircraft cleared terrain by 56 ft. 1.2.4 Evidence so far indicates that the VOR was transmitting an erroneous signal that the aircraft may have used to update the Flight Management Guidance System (FMGS) position. The map showed the aircraft to be in approximately the expected position, but because this aircraft is not fittedFLIGHT OPERATIONS DEPARTMENT COMMUNICATION - 16/2003 with GPS, any error transmitted from the VOR could result in an error in the computed FMGS position because position refinement is taken only from ground-based navigation aids in non-GPS aircraft. Similarly, the EGPWS terrain mapping position information was in error due to the navigational inaccuracy and therefore failed to work as expected. 1.3 Advice to Flight Crews 1.3.1 Flight crews are reminded of the necessity of carefully monitoring the portrayal of navigational information, especially when flying in areas of limited navigational aids. In the event that the accuracy or reliability of navigation information is in doubt, or if conflicting navigational information is evident, then an immediate go around should be flown when below SSA. If an EGPWS alert is received then crews are reminded to carry out the correct actions immediately with the required vigour. 1.4 Recommendations 1.4.1 Operators should ensure that the details of this incident, and the advice contained above, are brought to the attention of their flight crews. 1.4.2 Operators should endeavour to use aircraft with GPS on routes that involve long sectors both over water and terrain that terminate in remote areas served with few navigation aids. This will ensure that both FMS position update computations and EGPWS are provided with a choice of information sources from ground-based and satellite navigation systems
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Some Pilot Commentary |
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I always assumed GPS was a necessary component of EGPWS. Tell me why I shouldn't be worried that it isn't. | ||||||||||||||
a necessary component of EGPWS? No Tell me why I shouldn't be worried Looks like you know! I'm pretty sure there has been a serious suggestion that plates should carry typical radalt readings for a NPA? 400' on a VOR approach WOULD raise a question mark and a few figures on the plate would be VERY useful |
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It is not permitted to rely
on GPS as a primary means of navigation. Nothing, I suspect, to do with
suspicion of GPS accuracy in this case - my guess (and it is only a guess)
is that this A320 was fitted with INS. Similarly, INS is not to be used as a primary means of navigation. The question, however, has to be asked - in this situation, which would have been more accurate - GPS or VOR/DME? |
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"the question must be asked,
why not?" - cost, pure and simple. NOT anti US, however hard you try Capt Stable - "my guess (and it is only a guess) is that this A320 was fitted with INS." - the report says GPS NOT FITTED, so the approach was, I understand, being flown 'raw data', but on a dodgy VOR |
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My suspicion is that the INS hadn't had a chance to update itself after the long transit, hence the reason for flying it raw data. | ||||||||||||||
Generally it is not permitted to fly approaches using INS (twin IRS, anyway), so it is possible this would HAVE to be flown in RD regardless of the INS state? | ||||||||||||||
Interesting comments from
some, who really should know better. INS/GPS not approved as a primary means of navigation? Hmm, have not noticed all that many VOR/DME's at 30West...or 140West for that matter. Last I noticed, pressure pattern navigation was not authorized on the OTS...but hey, maybe I missed an amendment. Some EGPWS are equipped with their own dedicated GPS (and terrain data base), thus are able to give warnings as needed. Best piece of kit to come down the pike in many a moon, IMHO. Should (IF used correctly) eliminate CFIT, and that is no small achievement, wouldn't you say? Several US air-carriers use GPS for approaches (primary means of navigation)...Alaska Airlines the most, so far as I know. |
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