UK safety board spotlights electronics corrosion

 - February 8, 2008, 4:40 AM

The UK Air Accidents Investigation Branch (AAIB) last month issued a sweeping recommendation that the EASA, FAA, Transport Canada and other aviation agencies amend requirements for the design and installation of electronic components in aircraft so that fluid and moisture contamination, as a source of common cause failures, is specifically taken into account. This recommendation stems from a November 2005 incident involving a 2004 Bombardier Challenger 604 (registration VP-BJM) that experienced stabilizer pitch trim failure due to corrosion of the horizontal stabilizer trim control unit (HSTCU) circuit board. The airplane had logged only 202 flying hours at the time of the incident.

According to the AAIB, moisture created by humid air condensing on the cooling motherboard during repeated prolonged flight at altitude over time corroded the solder, leading to the HSTCU’s eventual failure. The AAIB is concerned that avionics motherboards on other aircraft could be affected in a similar manner.

On Nov. 11, 2005, about four-and-a-half hours into a six-hour flight from Lagos, Nigeria, to Farnborough Airport just outside London, the Challenger’s autopilot pitch trim failed and subsequently the stabilizer trim system failed. The crew tried to re-engage the stabilizer trim channels, resulting in channel 2 appearing to engage with no response to trim commands and channel 1 engaging intermittently.

During the flight the stabilizer occasionally trimmed nose down, despite applications of nose-up trim commands. The trim eventually reached almost full nose down, and both crewmembers had to apply prolonged aft pressure on the control column.

The crew declared an emergency after fighting the controls for about 90 minutes and decided to divert to London Heathrow for a landing with flaps retracted, although the aircraft flight manual required 20 degrees of flap following a stabilizer trim failure. However, the pilot elected for a no-flap landing because the crew believed applying flaps would substantially increase the control column load required to maintain level flight.

Before beginning the approach, the pilots tapped a passenger who was an employee of the operator and “previously held a type rating appropriate to the aircraft but did not form part of the required operating crew on this flight” to help in the cockpit, the AAIB said.

“The pilots attempted to fly a stabilized approach at a target indicated airspeed of approximately 160 knots and a successful landing was achieved [at London Heathrow] by the coordinated efforts of the commander and copilot operating the primary flight controls. The off-duty employee… closed the thrust levers on touchdown. The commander then applied reverse thrust progressively to bring the aircraft to a safe taxiing speed” before taxiing to a parking spot and shutting down the engines. The pilots, flight attendant and two passengers were not injured.

A Proactive Response

Bombardier Aerospace said it quickly informed Challenger 604 operators of the incident via several “advisory wires” and distributed guidance to pilots outlining how to deal with an HSTCU failure. Meanwhile, the manufacturer “worked closely” with Transport Canada and the AAIB to identify the problem and implement a fix.

By October 2006, Bombardier developed a solution to the corrosion problem–a new motherboard with “conformal coating” to prevent moisture from forming on the board. The company also did an “extensive review of the Challenger 604’s other critical black boxes,” Challenger and Global director of customer support engineering David Fields told AIN, “and found that they already had the proper coating,” requiring no modifications.

Bombardier then began the task of retrofitting the 366 in-service Challenger 604s with the new HSTCU motherboards, which it completed this past December– one month before the AAIB published its final report on the incident. Fields said
the Montreal-based airframer picked up the parts and labor costs to retrofit the entire fleet, regardless of whether the airplanes were in warranty.

Three more HSTCU failures were reported in Challenger 604s following the London incident but before the entire fleet could be outfitted with the new motherboards; all were safely dealt with by the crews per the Bombardier-issued guidance. According to Fields, Bombardier did see varying degrees of HSTCU motherboard corrosion during the retrofit process. No other Challenger 604 HSTCU failures or problems have been reported since the fleet was fitted with the upgraded circuit boards.