A just-released report reveals that a popular upper-range business jet had a potentially fatal design flaw which could disable both channels of a pitch control system simultaneously, with no reversion system.
It also raises, for the second time, the issue of an aircraft being directed onto an approach low over central London when its flight control system was compromised.
The UK Air Accident Investigation Branch (AAIB) report describes the crew's loss of control over the horizontal stabiliser in a Bombardier Challenger 604 business jet (VP-BJM) en-route from Lagos, Nigeria to Farnborough, UK on 11 November 2005.
It was diverted to London Heathrow because Farnborough's runway was not long enough for a flapless approach, and carried out a safe landing there.
The cause, says the report, was the ingress of moisture - apparently condensation from normal ground-level humidity - into the single horizontal stabiliser trim control unit (HSTCU) box which contained the control circuitry for both the "independent" channels that controlled the two separate stabiliser actuators.
The AAIB concluded: "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 Board adds: "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."
The report notes that both the aircraft and system manufacturers were aware that there had been similar occurrences previously.
The crew of the Challenger, cruising at 40,000ft (12,200m) some 4.5h into its flight to the UK, began to get indications of intermittent horizontal stabiliser malfunctions.
Varying the horizontal stabiliser pitch via the HSTCU provides autopilot pitch control and trim control for manual flight.
Eventually the captain, operating his control column pitch control switch in the "nose-up" sense, observed effective trim control reversal providing nose-down pitch almost to its limits, which he countered with elevator input.
The crew requested a diversion to London Stansted. For a high proportion of the remainder of the flight the two pilots had to fly the aircraft manually, which demanded heavy nose-up inputs on the control column to operate the elevators as the sole pitch control.
The captain, worried about the heavy control inputs needed, requested a diversion to Heathrow airport, and this request was accepted. The aircraft landed on runway 27L.
A safety recommendation in the report relates to the training of controllers to consider the balance of risk involved in directing an aircraft with impaired controls or power over major conurbations, rather than vectoring them to airports away from urban areas.
The report says the manufacturer and the Canadian and US aviation authorities have acted on recommendations to proof the HSTCU against moisture ingress, and will incorporate this consideration into future design certification.