This serious incident was notified to the Air Accidents Investigation Branch (AAIB) by the London Terminal Control Centre (LTCC) on 11 November 2005, the day of the occurrence, and the investigation began that day. The following Inspectors participated in the investigation:
Mr R D G Carter Investigator-in-charge
Mr P Sleight Engineering
Mr J Firth Operations
Mr J R James Flight Recorders
About four and half hours into a flight from Lagos, Nigeria, the autopilot pitch trim failed and subsequently the stabiliser trim system failed. Attempts were made to re-engage the stabiliser trim channels, resulting in channel 2 appearing to engage with no response to trim commands, and channel 1 engaging intermittently. During the flight the stabiliser occasionally trimmed nose down, despite applications of nose-up trim commands. The trim eventually reached almost full nose down. To counteract this, both flight crew members had to apply prolonged aft pressure on the control column. The aircraft diverted to London Heathrow for a landing with flap retracted, although the QRH required 20° flap following a stabiliser trim failure. The commander made the decision as the crew considered that applying flap would substantially increase the control column load required to maintain level flight.
Subsequent investigation found contamination, formed by electro-migration in the presence of moisture, within the Horizontal Stabiliser Trim Control Unit (HSTCU). The moisture was probably created by humid air condensing on the cooling motherboard during prolonged flight at altitude.
The investigation identified the following causal factors:
In the absence of a mechanical backup system or sufficient physical separation of the control channels, there was insufficient protection within the design of the HSTCU against the effects of environmental contamination.
The airworthiness requirements relating to the design and installation of electronic components did not sufficiently address the specific effects of fluid and moisture contamination as a source of common cause failures.
One Safety Recommendation is made in this report and one was made earlier in the investigation.
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VP-BJM 2-2008.pdf (790.01 kb)