British Airways A320 at Geneva on Dec 30th 2017, failure of yaw damper

Last Update: November 23, 2020 / 12:14:01 GMT/Zulu time

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Photo of British Airways G-EUUW, Airbus A320 — British Airways G-EUUW, Airbus A320 (Photo credit: ATom.UK / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Dec 30, 2017

Classification
Incident

Airline
British Airways

Flight number
BA-728

Departure
London Heathrow, United Kingdom

Destination
Geneva, Switzerland

Aircraft Registration
G-EUUW

Aircraft Type
Airbus A320

ICAO Type Designator
A320

A British Airways Airbus A320-200, registration G-EUUW performing flight BA-728 from London Heathrow,EN (UK) to Geneva (Switzerland) with 141 passengers and 7 crew, was on approach to Geneva maintaining FL110 in icing conditions, when the aircraft's yaw damper failed. The aircraft continued for a safe landing on runway 23 about 30 minutes later.

Switzerland's SUST rated the occurrence a serious incident and opened an investigation.

The SUST released their Summary Report concluding:

As flight BA728 was passing through a turbulent layer in icing conditions during descent to Geneva Airport, both FACs of the Airbus A320 simultaneously rejected all 3 IRs due to a significant lateral body acceleration, leading to the loss of the yaw damper function. As a consequence, the active Autopilot, Autothrust (A/THR) and both Flight Directors (FD) disengaged, and the flight controls reverted to alternate law.

The flight crew successfully reset FAC 1 which led to the recovery of one yaw damper, normal law, A/THR and FD 1. Since the FAC 2 was not reset, the landing capability of the aircraft remained limited to CAT 1(10) without AP and the subsequent ILS approach was flown manually with the A/THR engaged. There is no evidence showing that the resetting of FAC 2 would not have been successful. Such a reset would have enabled the flight crew to re-engage AP 1 and/or 2 and hence would have offered the option of an automatic landing, as required in low visibility conditions, for example.

However, the investigation also showed that it is generally not known by the pilots and not mentioned in the FCOM that in all cases the FMGC needs at least 2 IRs valid and consistent in each of the FACs to enable the re-engagement of one or both Autopilots. This means that if there is an AUTO FLT YAW DAMPER 1(2) fault due to 2 IRs rejections in one or both FACs, the APs are not available unless both FACs are reset. This is inconsistent with the status page that suggests the AP availability on the non-affected side (cf. annex 2).

In a statement, dated 11 October 2019, the manufacturer confirmed that it is planning improvements to the system messages on the status page or increased AP availability in case of IRS rejection in the FAC. In the meantime, Airbus has stated that they have updated the QRH system reset table to require a reset of the FAC when AP is inoperative following an AUTO FLT YAW DAMPER 1(2) fault.

For this reason, the STSB decided not to make a safety recommendation and concludes in regard to Article 45 of the Ordinance on the Safety Investigation of Transport Incidents (OSITI) hereby the investigation with the present summary report.

The SUST summarized the sequence of events:

The aircraft started its descent from cruising Flight Level (FL) 290 towards FL 230 at 11:14:50 UTC. At that time the aircraft was about 50 NM west of Dijon. At 11:17:58 UTC the aircraft levelled off at FL 230 with an indicated airspeed of 300 kt. Two minutes later the flight crew selected a speed of 240 kt. At 11:22:25 UTC the flight crew continued further with their descent. According to the operator’s report, the aircraft encountered icing conditions and a build-up of moderate icing was noticed by the flight crew while descending through FL 200. According to the recorded data, both engine and wing anti icing were switched on when leaving FL 230 with a selected airspeed of 230 kt.

The aircraft was descending through FL 190 when the flight crew called Geneva’s Air Traffic Control Officer (ATCO) at 11:26:37 UTC on the approach frequency 136.25 MHz, telling him they were descending to FL 160 in a right turn towards the waypoint SOVAD (cf. annex 1). The ATCO confirmed radar contact and cleared the flight to FL 130 with the information that the flight crew could expect radar vectoring to runway 23. Immediately afterwards, the flight crew – according to the operator’s report – reported severe vibrations and noise when passing FL 180, followed by multiple system failures.

The flight data show that at 11:27:45 UTC, crossing FL 174 with an airspeed of 229 kt, the active Autopilot (AP) 2, Autothrust (A/THR) and both Flight Directors (FD) disengaged, and the fly-by-wire flight control system reverted to alternate law. The AP disconnection master warning was triggered, and the AUTO FLT YAW DAMPER SYS amber caution was displayed on the ECAM.

At the same time the flight crew called the ATCO and asked for a „more quickly descent”. The ATCO did not understand since the request was drowned out by a „cavalry charge”4 aural warning. The flight crew had to repeat the request, thereupon the ATCO cleared the flight to FL 100. The aircraft then descended with a maximum rate of descent of 3300 ft/min at FL 160.

At 11:28:29 UTC the ATCO advised the flight crew to turn left on heading (HDG) 125 in order to position the aircraft for a left hand downwind. The flight crew acknowledged the instruction and turned left. One minute later the flight crew called the ATCO and informed him that they had some flight control problems and that they requested radar vectors but did not want to start the approach yet. The aircraft was descending through FL 145 with a decreasing speed of 272 kt. The flight crew had in the meantime selected an airspeed of 270 kt. The ATCO acknowledged this message and asked whether they would like to join the holding over the radio beacon SPR, but the flight crew requested further radar vectors. The ATCO continued radar vectoring and told them to report ready for approach.

After further deliberations the flight crew informed the ATCO at 11:33:59 UTC that they were still not ready for the approach and requested to stay under radar vectoring and to maintain FL 110. Thereupon the ATCO advised the flight crew to turn left on HDG 250.

It can be assumed that in the meantime the flight crew had started the checklist related to the ECAM message because the Flight Augmentation Computer 1 (FAC 1) was successfully reset at 11:35:35 UTC. Following the successful reset, normal law was recovered, and the flight crew regained A/THR and Flight Director 1 (FD1) but neither AP 1 nor AP 2 were available. In addition, the ECAM E/WD AUTO FLT YAW DAMPER SYS amber caution was replaced by the AUTO FLT YAW DAMP 2 amber caution.

After a frequency change to GVA final, the crew sent the following urgency call at 11:36:03 UTC: „Speedbird 728 Pan-Pan Pan-Pan we are just turning onto a heading of 250 degrees, maintaining flight level 110, standby for our intentions”. Three minutes later the flight crew advised that they were now able to join a holding pattern. Subsequently, the ATC cleared BA728 to hold over SOVAD (cf. annex 1). Attempts to re-engage AP1 were recorded at 11:36:28 and 11:37:09 UTC.

At 11:39:27 UTC the ATCO enquired about the reason for the urgency message. The flight crew answered with „flight control system failures”. The aircraft entered the SOVAD holding pattern at 11:41:52 UTC with an airspeed of 250 kt. The ATCO asked the flight crew to relay the number of souls on board and whether they had any dangerous goods on board.

At 11:46:01 UTC the flight crew informed the ATCO that they had lost their autopilots and requested to stay in the holding for another 10 minutes before commencing an approach. They then confirmed that no dangerous goods were on board.

At 11:51:16 UTC the flight crew reported „ready for approach”. The ATCO provided radar vectors to the instrument landing system (ILS) runway 23 and, approximately one minute later, cleared the flight to descend to 7000 ft on QNH5 1015. He then requested once more from the flight crew the number of souls on board and the latter answered with 148.

When receiving the approach clearance to runway 23 at 11:54:54 UTC, the aircraft was passing an altitude of 8800 ft with a speed of 207 kt. At 11:56:19 UTC the flight crew reported „established on runway 23” and, after having changed to the tower frequency, received the landing clearance at 12:03:07 UTC.

After an ILS approach flown manually, the aircraft touched down at 12:05:43 UTC. The aircraft taxied under own power to the given parking stand and the passengers disembarked normally.

The SUST analysed:

Technical aspects

The DFDR data and the failure messages of the Post Flight Report (PFR) show that the yaw damper function was lost in both FAC at 11:27:45 UTC, resulting in the disengagement of the autopilot active at the time of the failure (AP 2), Autothrust (A/THR) and both Flight Directors (FD) and the reversion of the flight controls system to alternate law. The master caution was triggered, and the AUTO FLT YAW DAMPER SYS message was displayed on the ECAM.

The loss of the yaw damper function can occur when the FAC rejects its own IR and IR 3. The QAR data show that none of the 3 IR registered any faults. Additionally, the manufacturer analysed the BITE8 data from the FACs and confirmed that each FAC rejected all 3 IR. Furthermore, no other rejections were recorded previously, indicating that the IR rejections occurred simultaneously and not sequentially.

As outlined above, the FAC can reject an IR due to yaw rate or lateral body accelerations. The DFDR data do not show any significant yaw rate change but it shows a significant lateral body acceleration at the time the rejections occurred. Since the parameters are sampled only once per second, the frequency and the maximum acceleration cannot be deducted. However, it can be assumed that the IRs were rejected due to these lateral body accelerations.

According to the checklist (cf. annex 4), FAC 1 and FAC 2 should be reset by pushing the respective push button „off then on”. The reset unlatches the faults and resets the monitoring function, thus recovering the yaw damper function, FD and A/THR related to the respective FAC, provided the fault does not exist anymore. This happened, when the flight crew successfully cycled the FAC 1 push button. It also means, that at least 2 IR were valid again.

After the successful reset of FAC 1, the status of the auto flight system was now similar to a rejection in FAC 2 of IR 2 and 3, or of all 3 IRs and therefore the ECAM display changed to AUTO FLIGHT YAW DAMPER 2. Even if this new ECAM display does not explicitly require a FAC reset, based on the actual situation, a reset of FAC 2 would have been the next logical step.

Based on the analysed BITE data from both FACs, a reset of FAC 2 would have been in all probability successful. Although the operator’s report states that the flight crew did attempt the reset without success, there is no evidence of FAC 2 having been reset. Cycling a FAC push button generates a push button switch message in the PFR; this was the case for FAC 1 but not for FAC 2.

Operational aspects

According to the operator’s report, the flight crew encountered moderated icing conditions when descending through FL 200. They switched on the engine and wing anti-ice systems when leaving FL 230 and selected a lower airspeed. It can be assumed that this was done as a precautionary measure in order to fly at the appropriate speed when entering the expected turbulence zone. This was adapted to the situation and a safety-conscious decision.

The DFDR data show that the flight crew made a FAC 1 reset at 11:35:35 UTC, 8 minutes after the master caution was displayed. It can be assumed that this reset was performed according to the first action point of the respective ECAM checklist (cf. annex 4). In the absence of the CVR recording it remains unclear as to why the flight crew delayed the application of the first action of the checklist.

The reset of FAC 1 was successful and, as a next step, a reset of FAC 2 should have been performed. However, it must be noted that after the successful reset of FAC 1 the ECAM display changed to AUTO FLT YAW DAMPER 2 and a reset of FAC 2 is not explicitly required anymore. Although the operator’s report states that the flight crew did reset FAC 2 without success, there is no indication in the analysed data that a reset of FAC 2 was executed.

Without a reset of FAC 2 neither AP 1 nor AP 2 could be re-engaged. The flight crew attempted to re-engage the AP 1 twice, once at 11:36:28 UTC and again at 11:37:07 UTC. Pilots are generally not aware that the FMGC needs authorization from both FACs to recover the AP and that the status page is inaccurate in suggesting that the AP of the non-affected side is available. The re-engagement of A/THR at 11:36:40 UTC was meaningful and helped the flight crew to reduce their workload.

After having sent an urgency message and according to the operator’s report, the flight crew opted to join a holding pattern in all likelihood to analyse the situation following the operator’s guidelines using the acronym DODAR, because the status page did not reflect the actual status of the aircraft. This was an appropriate decision but did not result in the recovery of the autopilots as detailed above and the approach had to be flown without any of the autopilots engaged.