What happened
On 12 January 2009, a Boeing 737-73V, registration G-EZJK, was conducting a post-maintenance flight control check west of Norwich. The flight was intended to verify the serviceability of the aircraft's controls following an adjustment to the elevator balance tab setting. The crew, consisting of a pilot and co-pilot, were accompanied by two observers.
During the procedure, the crew isolated the individual hydraulic systems to perform a manual reversion check. Upon switching the flight control switches to the OFF position to remove hydraulic assistance, the aircraft experienced a sudden and rapid nose-down pitch. The aircraft descended approximately 9,000 feet, reaching a maximum vertical descent rate of 20,000 ft/min and an airspeed of 429 knots. During the excursion, the aircraft reached a bank angle of 91 degrees.
The pilot attempted to recover the aircraft by rolling the wings level and eventually re-engaging the hydraulic systems. The aircraft was eventually recovered at approximately 5,600 feet altitude. Following the incident, the crew returned to Southend Airport, where a post-flight inspection found no structural damage or deformation.
The investigation
The AAIB examined the flight data recorder and cockpit voice recorder to reconstruct the sequence of events. The investigation revealed that while the crew intended to test the controls, there was confusion regarding the status of the hydraulic switches. The commander believed hydraulic power had been restored, but the switches had actually remained in the OFF position. Additionally, the investigation found that the aircraft' and the crew's lack of coordination during the maintenance-to-flight transition contributed to the event.
Findings
- The primary cause of the upset was the unidentified mis-rigging of the elevator tab, which caused significant asymmetrical flight control forces during the manual reversion test.
- The lack of adequate planning and communication between the aircraft operator and the maintenance provider regarding the pre-maintenance delivery flight contributed to the incident.
- There was no formal, written procedure to manage the exchange of information or the recording of test results between maintenance personnel and the flight crew.
- The commander's decision to roll the aircraft to unload pressure, while based on training, was complicated by the fact that the hydraulic switches were not correctly repositioned.