What happened
On 12 January 2009, a Boeing 737-73V, registration G-EZJK, was conducting a customer demonstration flight west of Norwich, Norfolk. The flight was part of a maintenance and delivery process for an aircraft nearing the end of its lease. During the flight, the crew performed a manual reversion check, which involved isolating the individual hydraulic systems by switching the flight control switches to the OFF position to test the aircraft's operation on a single hydraulic system.
While the aircraft was at FL150, the co-pilot moved the flight control switches to the OFF position. Immediately following this, the aircraft experienced a sudden nose-down pitch. The commander attempted to apply significant backpressure on the controls, but the aircraft maintained a nose-down attitude of 2.81 degrees and began descending at rates peaking at 21,000 f/p m.
To manage the upset, the commander executed a left bank of 91.2 degrees to unload the pressure on the elevators. The crew then re-engaged the flight control switches, which restored normal control forces. The flight was subsequently abandoned, and the aircraft returned to Southend without further incident or structural damage.
The investigation
The AAIB investigation focused on the cause of the sudden pitch change during the hydraulic isolation procedure. Investigators examined the aircraft's maintenance history, noting that the aircraft had recently undergone maintenance to prepare it for a new operator.
Technical analysis revealed that the aircraft's elevator balance tab setting had been adjusted during the recent maintenance period. The investigation also reviewed the cockpit voice recordings, which indicated confusion between the two pilots regarding whether hydraulic power had been restored during the maneuver.
Findings
- The primary cause of the pitch-down incident was the adjustment made to the elevator balance tab setting during the preceding maintenance period.
- The aircraft was performing a planned manual reversion test to assess flight control functionality.
- There was a lack of coordination between the crew members during the recovery phase, leading to a misunderstanding regarding the status of the flight control switches.
- The aircraft's descent rate reached a peak of 21,000 fpm during the upset.