On 24 August 2010, an Airbus A321-200 being operated by British Midland on a scheduled public transport service from Khartoum to Beirut experienced, during cruise at FL360 in night Instrument Meteorological Conditions (IMC), an electrical malfunction which was accompanied by intermittent loss of the display on both pilots’ Electronic Flight Instrument System and an uncommanded change to a left wing low attitude. De-selection of the No 1 generator and subsequent return of the rudder trim, which had not previously been intentionally moved, to neutral removed all abnormalities and the planned flight was completed without further event with no damage to the aircraft or injuries to the 49 occupants.
A Field Investigation was carried out by the UK AAIB. The event was not reported promptly by the Operator and as a result both the Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) recordings were overwritten. Attempts to recover data from Non Volatile Memory in various equipment also failed because of this delay. However the QAR, powered by the No 2 Generator and recording similar data to the FDR, had continued to function during the incident and the routinely downloaded data from the flight was available for the Investigation. In respect of the delay in the reporting of the event to the AAIB, it was considered that the consequent delay in interviewing the flight crew “might have reduced the quality of their accounts”.
It was established that the intermittent failure of both main electronic displays and the uncommanded application of left rudder trim which were the main symptoms had been accompanied by a ‘chattering’ sound caused by the rapid cycling of electrical relays, at times continuously. The flight crew also advised the Investigation that the aircraft “did not seem to respond as expected to control inputs” and that flight deck lighting had also failed intermittently. The ECAM had displayed a number of messages and cautions but the ESAM display itself had also become intermittent. Reversion to Alternate Law occurred and the aircraft commander passed control to the First Officer, who had been left with relatively more EFIS functionality than he had, although he was obliged to fly by reference to the SBY flight Instruments which had remained fully operational.
The flight crew stated that all functions were restored after the No 1 generator had been selected ‘off’ as part of the response to one of the many ECAM fault messages. When it was reselected on as required by the Quick Reference Handbook (QRH) drill being followed, the previous malfunctions returned and it was selected off again and left off.
When the APU had been started to regain full electrical power, the flight crew saw that the rudder trim indicated several units from neutral although they had not made any inputs. It became apparent that the aircraft had deviated approximately 20 nm off track during the incident.
The main concerns of the Investigation were why a single generator fault should have affected both left and right side instrumentation and how the flight control system might have been affected.
It was considered that the reason for the annunciation of “numerous and significant symptoms, including malfunctioning electronic displays and uncommanded rudder trim input”, not all of which had been obvious to the flight crew, had not been evident at the time. Since the ECAM had not clearly identified the root cause of the malfunction and there had been no fault captions active on the overhead panel, there had been no procedures available to assist the flight crew in effectively diagnosing the problem. Because of this, a Safety Recommendation had been made during the Investigation and the requested response had followed:
- That Airbus alert all operators of A320-series aircraft of the possibility that an electrical power generation system fault may not be clearly annunciated on the ECAM, and may lead to uncommanded rudder trim operation. (2010-092)
In respect of the track deviation which occurred, Airbus initially advised that:
“a reset of the Flight Augmentation Computer (FAC), caused by an electrical power interruption, may cause a small incremental offset in the rudder trim. Multiple electrical power interruptions can result in multiple increments which could, cumulatively, produce a significant rudder trim input”.
Later Airbus had concluded that the aircraft had responded normally to rudder trim inputs, suggesting that the rudder input alone had been the cause of the lateral deviation from the intended track.
However, it was nevertheless considered that good evidence had been found that a fault had existed in the No 1 Generator which had affected aircraft equipment and caused the incremental rudder trim inputs which had occurred. It was considered that the malfunction of the unit might have been attributable to a short in the jumper lead on the Generator. Whilst this lead was clearly damaged and a plausible failure mechanism was identified, this explanation was only valid if Display Management Computer (DMC) No 3 had been selected to power the First Officer’s EFIS Screens, which is not consistent with the report filed by the flight crew. No conclusive cause for the malfunction could therefore be established.
Safety Action by Airbus in response to a number of events involving screen blanking, which had already been in hand when the Incident investigated here occurred, had involved making changes to the QRH for EFIS screen failures. Even though this Investigation was unable to determine cause with any degree of certainty, the QRH update which was in progress was amended to include a reference to the possibility that generator malfunction could affect the rudder trim system.
The Final Report of the Investigation AAIB Bulletin: 10/2011 EW/C2010/08/08 was published on 6 October 2011. No further Safety Recommendations were made.