This week there have been two incidents of modern jet aircraft severely disabled by electrical or electronic problems, and in both cases water contamination of critical electrical components was the problem.
In one case, a Bombardier Challenger 604 lost control of its horizontal stabiliser in the other, a Qantas Boeing 747-400 lost power from all four engine-driven generators simultaneously.
So what? These things happen. And, in both cases, the crew were able to recover the aircraft safely.
Actually, these things shouldn't happen, and as the industry progresses toward the much-vaunted "more-electric aeroplane" design principle, and toward progressively more integrated avionic systems, water- and moisture-proofing of electric and electronic systems and components will have to be taken much more seriously at the design stage. By implication, the alternative power backups will not be there. Hydraulics and mechanical systems such as cables and bell-cranks can continue to work - at least for a while - even when submerged in water, but busbars, simple electrical junctions, avionics, solenoids, switches and even slightly damaged wiring looms can't be certain of surviving high humidity and condensation, let alone submersion or direct fluid leakage.
When reading some incident and accident reports, it is possible to "hear" the incredulity in the investigator's words, and to visualise the raised eyebrows that go with it. So it is in the case of the UK Air Accidents Investigation Branch's examination of the horizontal stabiliser trim control unit (HSTCU) in the Challenger. The single-box HSTCU had been contaminated by moisture from humid air over a period of time, leading to contamination of the pair of separate printed-circuit boards associated with the two supposedly independent channels that operated the stabiliser actuators. The contamination resulted in electro-migration, which led to short-circuiting and intermittent, uncommanded operation of the stabiliser whenever the humidity turned to condensation as the box cooled at altitude.
"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." That AAIB quotation may have applied to the Challenger, but it could equally have applied to the 747-400, and the principles evoked will be even more critical in the even more-electric 787.
Source: Flight International
