History of the Flight
The aircraft, a DHC-8-311 registered G-BRYW, was on a scheduled passenger transport flight from Aberdeen to Manchester on 28 September 2005. The autopilot was unserviceable, so the aircraft was being hand-flown by the commander, who was also the handling pilot. At 0829 UTC, while cruising at FL230, the commander found that the elevator trim hand wheel could not be moved forward of its current position, restricting nose-down trim. The trim could be moved rearward and then forward, but only back to the initial position.
When the Quick Reference Handbook (QRH) drill for an elevator manual trim failure was actioned, the standby electric trim system—which allows elevator trim to be controlled electrically via the autopilot elevator servo—was found to be unable to move the elevator pitch trim wheels in either direction. The out-of-trim forces were manageable, and the flight continued to its destination. As the aircraft descended through FL150, manual elevator trim operation improved, allowing some nose-down trim input.
Subsequent Examination
Later examination of the aircraft did not identify any obvious defect that might have caused the incident. As a precaution, the trim screwjack actuator drive chains were cleaned and lubricated.
Elevator Trim System Description
The elevator trim (pitch trim) is controlled via two trim tabs, one at the outboard trailing edge of each elevator. In normal operation, trim tab position is controlled manually via the captain's or co-pilot's trim hand wheels on the centre console. The hand wheels are mounted on a common shaft and connected to the trim tab screwjacks by cables and pulleys. Movement of the hand wheels is transmitted through the system to provide rotary input to each screwjack via a chain driving a sprocket. The output side is connected to the trim tab by a push rod and idler assembly. An elevator trim tab position indicator is mechanically operated by and located alongside the captain's hand wheel.
A standby elevator trim system is provided to maintain control in the event of a trim cable break forward of the elevator trim servo location. In standby mode, elevator trim is commanded electrically to drive the autopilot elevator trim servo. The standby system is armed by selecting a guarded switch, and trim is controlled via spring-loaded switches on the pilot's and co-pilot's side consoles.
Other Similar Occurrences
The operator had experienced a number of other similar events on other DHC-8 aircraft, which were reported to the AAIB. These incidents are briefly described:
- 14 July 2005: G-BRYX, SOU-MAN. At FL101, a 'NOSE DN PITCH MISTRIM' occurred with pitch trim jammed nose-down. Hand wheels were stiff. Screwjacks lubricated after incident.
- 5 July 2005: G-BRYX, MAN-GLA. At FL150, a similar annunciation occurred; hand wheel could not move nose-down, but nose-up was available. Standby nose-down trim failed. Previous flights in very wet and icy conditions. Moisture contamination found in right-hand actuator.
- 30 July 2005: G-NVSB, EDI-MAN. At FL200 in IMC, annunciation occurred. Hand wheels would not move nose-down; nose-up available with difficulty. Standby nose-down trim inoperative. Trim returned to normal after exiting icing conditions. Screwjacks lubricated; last lubrication was 395 hours prior.
- 27 September 2005: G-NVSA, EDI-MAN. During descent, autopilot failed to adjust trim. Hand wheels could not move forward but rearward possible. Trim returned to normal around FL080 at -3°C. No faults found.
- 1 October 2005: G-NVSA, GLA-MAN. Stiff elevator trim reported in flight; screwjacks cleaned and re-lubricated.
- 12 November 2005: G-NVSA, ABZ-MAN. At FL190, annunciation occurred; hand wheels could not move forward. Autopilot disconnected; standby pitch trim inoperative. Trim normal at FL090. Hardened grease found in mechanism; cleaned and re-lubricated.
- 17 November 2005: G-NVSA, MAN-ABZ. At FL170 in VMC, 'ELEVATOR MISMATCH' annunciation. Autopilot disconnected; standby electric trim inoperative. Trim normal after leveling out. Water found in trim screwjacks.
Modifications and Manufacturer's Advice
In-service operation of the DHC-8 has shown that elevator trim screwjacks can accumulate water internally, which can freeze at altitude, causing restrictions. This led to modifications 8/0415 (drain hole) and 8/0569 (grease fitting). Modification 8/0415 was mandated by the FAA under Airworthiness Directive 86-25-03.
The incidents occurred on aircraft fitted with modified screwjacks. Service experience showed that these can still be susceptible to moisture ingress, which freezes, causing restrictions. The problem can usually be eliminated by more frequent greasing.
Bombardier Aerospace recommended greasing at a 'C' Check (5,000 flying hours) but issued advice in October 2005 encouraging more frequent lubrication: 'Operators continue to report in-flight elevator trim screwjack freezing... In the interim, Operators are encouraged to perform the following: Lubricate the elevator trim screwjack while moving the elevator trim control through its full range of movement. Continue lubricating until clean grease (moisture-free) is expelled from the drain hole. After lubrication, cycle the screwjack through its full range a minimum of fifteen times to remove excess grease.'
The operator subsequently increased the lubrication frequency of its elevator trim screwjacks in accordance with this advice.