British Mediterranean Airways Flight 6711

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British Mediterranean Airways Flight 6711
An Airbus A320 jet airliner in British Airways livery, with landing gear and flaps extended, in front of a cloudy sky.
The incident aircraft in June 2003 at Heathrow Airport.
Incident
Date31 March 2003 (2003-03-31)
SummaryNear-controlled flight into terrain due to VOR navaid malfunction
SiteAddis Ababa, Ethiopia
9°04′59″N 38°52′59″E / 9.083°N 38.883°E / 9.083; 38.883Coordinates: 9°04′59″N 38°52′59″E / 9.083°N 38.883°E / 9.083; 38.883
Fatal error: The format of the coordinate could not be determined. Parsing failed.


Aircraft
Aircraft typeAirbus A320-231
OperatorBritish Mediterranean Airways
IATA flight No.KJ6711
ICAO flight No.LAJ6711
Call signBMED 6711
RegistrationG-MEDA
Flight originHeathrow Airport, London, U.K.
StopoverBorg El Arab Airport, Alexandria, Egypt
DestinationAddis Ababa Bole International Airport, Addis Ababa, Ethiopia
Occupants75
Passengers67
Crew8
Fatalities0
Injuries0
Survivors75 (all)

British Mediterranean Airways Flight 6711 was a scheduled passenger flight from Heathrow Airport in London to Bole Airport in Addis Ababa, Ethiopia, with a stopover at Borg El Arab Airport in Alexandria, Egypt. On 31 March 2003, the Airbus A320 operating the flight came dangerously close to high terrain due to an undetected malfunction of a ground-based VOR navigational aid; the aircraft was not damaged, and no injuries or fatalities occurred to the 75 passengers and crew aboard.[1][2]

Incident[edit]

The flight proceeded uneventfully until reaching the vicinity of Addis Ababa. Nearing the airport, the pilots tuned the aircraft's navigational equipment to pick up signals from the two ground-based navigational aids at or near the airport; the aircraft's VOR receiver was tuned to the ADS VOR, while the automatic direction finder (ADF) was tuned to pick up signals from the AB non-directional beacon (NDB). After the flight was cleared for a VOR/DME approach to runway 25L using the ADS VOR,[lower-alpha 1] it flew over the VOR at 2315 hours UTC and turned outbound to follow the procedure turn for the approach. On the inbound leg of the approach, the aircraft's flight management system (FMS) indicated that the aircraft was on the correct approach path. However, unbeknownst to the pilots or to air traffic control (ATC), the aircraft was, in fact, over three nautical miles north of the prescribed course; the aircraft was not equipped with a GPS receiver, which would have indicated that the flight was off course.[1][lower-alpha 2] The ADF, tuned to the AB NDB, indicated that the aircraft was to the right of the approach course, but the flightcrew disregarded this indication due to the presence of cumulonimbus activity to the south (cumulonimbi are typically associated with thunderstorms, a known source of interference for ADF/NDB navigation).[lower-alpha 3] Although the flightcrew were able to make out ground lights, they could not identify the airport, and the captain performed a go-around when the VOR signal was lost shortly afterwards.[1]

After the first go-around, the flightcrew requested, and received, confirmation from ATC that the VOR was functional; they also obtained the information that the localizer for runway 25R was operating, but the aircraft was not equipped with approach plates for a localizer approach to this runway, so the pilots, after double-checkng the accuracy of the FMS by reference to the ADS VOR, carried out a second VOR/DME approach to runway 25L.[1] When inbound on the second approach, at a DME distance of 6 nm, the aircraft's enhanced ground proximity warning system (EGPWS) automatically called out that the aircraft was 400 feet above terrain. The captain initially acknowledged this callout, but then realized that it was far too early in the approach for the aircraft to be this close to the ground, and initiated a second go-around; as he did so, the EGPWS sounded a "TOO LOW TERRAIN" warning, indicating that the aircraft was in danger of impacting the ground.[1] The aircraft came within 56 feet of terrain before its descent was arrested, but at no point did the flightcrew have any visual contact with the ground. Following the second go-around, the flightcrew again discussed with ATC the possibility of using the runway 25R localizer for an approach to runway 25R followed by a landing on runway 25L, but were unable to pick up the localizer signal on the aircraft's navigational equipment, and, with the accuracy of the ADS VOR signal in serious doubt, they diverted to Ambouli Airport in Djibouti, where the flight landed safely at 0042 UTC.[1]

The next day (1 April), the flight returned to Addis Ababa; during their time on the ground in Djibouti, the flightcrew had realigned the aircraft's IRS (which provided the base data for the FMS) and set the FMS to not use the suspect ADS VOR for positional updates.[lower-alpha 4] During the course of an approach and landing in visual meteorological conditions, the flightcrew observed large errors in the indications from the VOR, which showed the aircraft as much as 30° off track when it was visually confirmed to be on the proper approach course, and reported the malfunctioning VOR to ATC.[1]

Investigation[edit]

Upon returning to the UK, the incident was reported internally; upon review of data from the aircraft's quick access recorder (QAR), which revealed the aircraft's extreme proximity to terrain during the second approach, airline management notified the AAIB, who began an investigation.[1] Due to the time delay, no relevant information could be retrieved from the aircraft's primary flight recorders, but data from the QAR and the EGPWS was available.[1] This established that the EGPWS had itself been functioning properly, but that it had been fed faulty FMS data (used to determine the aircraft's position in relation to the local terrain), causing it to think that the aircraft was on the correct approach path, over a relatively low, flat valley, when it was actually well to the north, on a path taking it low over a ridge of high terrain.[1] As a result of the inaccurate FMS position, the EGPWS's forward looking terrain avoidance (FLTA) function, designed to warn the flightcrew when approaching rapidly-rising terrain, failed to function, as it "saw" the flat terrain along the proper approach path, rather than the high ridge that the aircraft nearly impacted, while the premature descent alerting/terrain clearance floor (PDA/TCF)[lower-alpha 5] function was desensitized by the error in the aircraft's calculated position, and activated later than it otherwise would have;[lower-alpha 6] however, due to the precipitous terrain ahead of and below the aircraft, the EGPWS warning was delayed only by approximately a second and a half.[lower-alpha 7][1]

Following the incident, the ADS VOR was physically inspected, and a capacitor in the circuitry used to generate the rotating component of the VOR signal was found to be full of water, which had presumably gained entry during heavy rain on the night of 31 March. A few weeks previously, the VOR antenna's protective housing had been removed in order to install a new DME, and, when the housing was reinstalled, it was not properly sealed against water ingress. Simulations of the types of error likely to be introduced to the VOR signal by the waterlogged capacitor were consistent with the behavior observed by the crew of Flight 6711.[1]

As the aircraft was not equipped with GPS, the EGPWS relied solely on the aircraft's FMS-calculated position (obtained from the IRS and updated using signals from ground-based navigational aids) to determine the aircraft's location relative to terrain. Although FMS positional data is known to be susceptible to errors caused by the slow drift of the IRS, and flightcrews are required to crosscheck this data against that provided by ground-based navigational aids (a procedure which the incident flightcrew performed in full), this procedure does not guard against the case of data transmitted by a terrestrial navaid being itself inaccurate.[1] The FMS itself does perform internal sanity checking of positional data from ground-based installations, and will automatically reject data that has been determined to be unreasonable, but rejection of such data does not undo previous positional updates based on data later flagged as unreliable, nor does it prevent the FMS from making use of further data from the same source should the new data fall within acceptable bounds. Additionally, problems that the FMS detects with data from ground-based navaids are not annunciated to the flightcrew, despite this potentially being crucial information for the flightcrew to know - especially at lower altitudes in the terminal area, where fewer navigational aids are within line-of-sight of the aircraft's navigation receivers, and where errors in navigation are potentially far more catastrophic than those occurring at high altitude en route.[lower-alpha 8][1] Reliability standards for the FMS and EGPWS do not account for the possibility of the aircraft being provided with erroneous external data.[1]

As a result, while the FMS determined the VOR signal to be erroneous while on the outbound leg of the first approach,[lower-alpha 9] and stopped accepting new positional updates from it, it continued calculating inertial position data based on the aircraft's calculated position at that point, which included a considerable bias introduced by the previous positional updates from the unreliable VOR, causing the FMS-calculated position to be several miles away from the aircraft's actual position despite the rejection of new positional updates from the VOR. As the aircraft drew closer to the VOR during the first go-around, the VOR error detected by the FMS decreased below the threshold level, causing the FMS to once again accept positional updates from a source that it had already flagged as unreliable. During the second approach, much the same situation occurred, with the FMS ceasing to accept positional updates from the VOR partway through the outbound leg of the approach, and once again accepting VOR-provided data as the aircraft came close to the VOR. None of this was made apparent to the flightcrew.[1]

Two types of safety measures are normally in place to prevent an aid to radionavigation from presenting a hazard by transmitting erroneous signals:

  • Before being placed into use following installation or servicing, radionavigational aids are flight-checked using aircraft specially equipped to measure and analyze the signals they emit. The Ethiopian Civil Aviation Authority (CAA) relied on the Ethiopian Air Force to provide aircraft for flight checks,[lower-alpha 10] but this only covered scheduled checks, and not the rechecking of navaids after unscheduled maintenance.[1]
  • Ground-based navigational aids, such as VOR and ILS stations, are equipped with sensors to automatically monitor the signals they emit and shut them down if they begin to transmit erroneous signals; however, the monitor system for the ADS VOR had been rendered inoperative when cabling used by the system was severed by construction work at the airport.[1]

No NOTAM was ever issued by the Ethiopian CAA regarding the unreliability of the ADS VOR.[1]

The investigation also found several other ways in which erroneous navigational data from an external navigational aid could potentially result in the corruption of FMS-calculated positional data, in some cases despite the presence of numerous good sources of navigational data alongside the malfunctioning navaid; additionally, even in the absence of erroneous external data, FMS reliability standards were quite low for a system being relied on to provide critical terrain data, allowing a significant chance of navigation errors large enough to potentially result in the EGPWS completely missing high terrain ahead of the aircraft.[1]

The AAIB released its final report in January 2008; the Ethiopian CAA, formally in charge of the investigation, still had not released any report as of June 2010 (over seven years after the incident).[2] The AAIB report made a total of six safety recommendations. Two of these were to the Ethiopian CAA, regarding navigational-aid maintenance and the issuance of NOTAMs, and one was to the International Civil Aviation Organization, regarding standards for systems used to monitor the performance of navigational aids. The remaining three were made to the European Aviation Safety Agency (EASA) and the U.S. Federal Aviation Administration (FAA), and concerned the vulnerability of EGPWS to failures of ground-based navigational aids;[1][2] one recommendation called for the EASA and FAA to review EGPWS certification requirements to reduce the likelihood of a navigational-aid failure causing a loss of EGPWS protection and a possible CFIT, and for the standards for the accuracy of these navigational aids to be tightened to ensure the safety of aircraft relying on these installations for EGPWS protection, while the final two recommendations called for the EASA and FAA to consider the possibility of alerting flightcrews to FMS-detected navigational-aid faults.[1] These last three recommendations were formally rejected by the EASA, which considered the verification of the accuracy of ground-based navigational aids to be the responsibility of ground-based agencies and personnel and to be outside the scope of standard aircraft navigational equipment, as well as expressing concerns about the technical difficulty of including this sort of monitoring functionality in standard airborne navigation receivers.[3][4][5]

See also[edit]

  • [[]]

Notes[edit]

  1. The ADS VOR had a colocated DME beacon, which functioned properly throughout and did not contribute to the incident.
  2. Following the incident with Flight 6711, British Mediterranean suspended the use of non-GPS-equipped aircraft on flights to Addis Ababa.
  3. The investigation later concluded that the ADF indications were most likely accurate, but that the flightcrew's decision to disregard these indications was reasonable given the information available to them.
  4. Inertial navigation systems slowly drift over time, producing a gradually-increasing error between the aircraft's calculated position and its actual position; for this reason, the FMS periodically updates the IRS-derived position using information from other sources, such as the aircraft's GPS receiver (if equipped) or ground-based navigational aids.
  5. PDA is the term generally used in the industry; TCF is the name used for this function by Honeywell Aerospace, the manufacturer of the aircraft's EGPWS.
  6. PDA sounds an alert if the aircraft's height above terrain, as measured by its radar altimeter, is below a certain minimum safe value. This minimum value varies depending on the aircraft's distance from a runway; the closer the aircraft is to a runway, the closer it can come to terrain before PDA activates. As the error in the aircraft's FMS-calculated position caused the aircraft to appear to be closer to the airport than it actually was, this caused PDA to trigger a warning at a lower height above terrain than it would have had the FMS position been accurate.
  7. PDA is designed to guard against an aircraft descending to an excessively-low height above terrain, not to warn of terrain rising steeply ahead of an aircraft; calculations made during the investigation indicated that it would not have prevented the aircraft from coming dangerously close to terrain even had the FMS-calculated aircraft position been accurate.
  8. The FMS was originally designed to be used for en-route navigation, and was only later coopted to provide the EGPWS with terrain data.
  9. The data transmitted by the ADS VOR had been initially classified as reliable, allowing the FMS to use it for positional updates, due to the aircraft's initial approach to the VOR having been along a bearing where there was little or no error in the VOR data.
  10. The Ethiopian CAA had the necessary equipment for flight checks, with the exception of an aircraft in which to fly the equipment.

References[edit]

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 "Report on the accident to Airbus A-320-231, G-MEDA, On approach to Addis Abeba Airport, Ethiopia, 31 March 2003" (PDF). Air Accidents Investigation Branch. Archived from the original (PDF) on 12 May 2021. Retrieved 12 May 2021.
  2. 2.0 2.1 2.2 "Report: British Mediterranean A320 at Addis Abeba on Mar 31st 2003, wet VOR nearly causes two CFITs". avherald.com. 10 June 2010. Archived from the original on 13 May 2021. Retrieved 12 May 2021.
  3. "Annual Safety Report 2011" (PDF). Air Accidents Investigation Branch. Archived from the original (PDF) on 21 May 2020. Retrieved 26 July 2021. ...European Aviation Safety Agency (EASA) - Terrain Awareness and Warning System (TAWS) is not part of the aircraft navigation systems and it shall not be used as mitigation means to detect navigation system or data problems. The TAWS certification policy assumes that the signal received from the ground station, VHF Omnidirectional radio Range (VOR) is correct, and ground stations shall be adequately monitored and controlled by the responsible bodies (Airport and Air Traffic Control).
  4. "Annual Safety Recommendations Review 2013" (PDF). European Aviation Safety Agency. Archived from the original (PDF) on 17 May 2021. Retrieved 26 July 2021. The function of the Terrain Awareness Warning System (TAWS) is to provide information and alert to the flight crew in order to detect a potentially hazardous terrain situation and take effective action to avoid a Controlled Flight Into Terrain (CFIT) ... the certification policy assumes that the signal value is correct when it is validated, and ground stations are adequately monitored and controlled by the responsible bodies [airport and Air Traffic Control (ATC)] ... TAWS is not part of the aircraft navigation systems and therefore shall not be used as mitigation means to detect navigation system/data problems or to set navigational data accuracy requirements.
  5. "Annual Safety Recommendations Review 2014" (PDF). European Aviation Safety Agency. Archived from the original (PDF) on 20 June 2021. Retrieved 26 July 2021. ... [h]owever, there is no requirement in the EASA regulatory framework for airborne systems to check ground station performance. The Safety Recommendation (SR) is requesting a new service from the on-board systems for which they were not designed ... [t]his is why Flight Inspection services use special aircraft equipped with sensors that are dedicated and calibrated for this purpose. It is beyond the capability of typical aircraft and FMS equipment and why the correct functioning of the ground-based radio navigation system is the obligation of the Air Navigation Service Provider (ANSP).


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