The UK’s Air Accidents Investigation Branch (AAIB) is investigating the Nov. 11, 2005, incident in which a Bahamas-registered Bombardier Challenger 604 lost its autopilot. According to a UK AAIB bulletin, VP-BJM was cruising at FL400 for 4.5 hours on a flight from Lagos, Nigeria, to Farnborough, England, when the crew received an “autopilot pitch trim” caution.
The “stab trim” and “Mach trim” cautions displayed soon after. The crew corrected the problem by re-engaging the stabilizer using trim channel one, but the two cautions soon displayed again.
When the pilot began a descent to the destination, more attempts at re-engagement resulted in loss of autopilot connection. There were also indications of intermittency with stabilizer trim channel two, while attempts to apply nose-up trim, using the yoke-mounted switches, resulted in nose-down trim commands.
The crew did not attempt further trim engagements and re-engaged the autopilot, but they did not succeed before the stabilizer had almost reached full nose-down trim.
Concerned about the amount of physical effort necessary to land the aircraft and planning to make a flapless approach and landing to avoid unnecessary trim changes, the crew requested a diversion to London Heathrow Airport, where there was more than 12,000 feet of runway available.
The crew landed safely an hour after the onset of the malfunction.
The flight data recorder (FDR) showed that during the flight at FL400 the horizontal stab trim actuator showed a change of state for one minute. It indicated intermittent changes during the descent. The FDR recorded several stabilizer movements during the rest of the flight, all of them nose-down and culminating with just 0.2 degrees short of maximum.
The stabilizer trim incorporates two independent command channels within the horizontal stabilizer trim control unit (HSTCU). In normal flight channel one is used for primary control; channel two is the backup.
These channels receive the same inputs, through two rear connectors, from the autopilot, manual trim switches and the disconnect switches on the yokes.
Mechanics found no faults during checks following arrival at London Heathrow, and the HSTCU was removed and sent to the manufacturer for examination. Investigators found that the rear connector pins were heavily contaminated, with evidence of a short circuit caused by the contaminant.
After finding a contaminant in one of the HSTCU’s printed-circuit boards during servicing, the manufacturer issued a service information letter in May 2003 warning of possible contamination and recommending that operators apply tape to the top and sides of the unit. Investigators are trying to establish the type and source of the contaminant.