What happened
On the night of 19 March 2007, a Lockheed L188C, registration G-FIZU, departed London Stansted Airport for a scheduled freight flight to Edinburgh. Shortly after rotation, the flight crew experienced intense aircraft instability, characterized by erratic yawing, pitching, and rolling, accompanied by loud, fluctuating noises from the propellers. The crew noted that engine RPM and horsepower gauges were oscillating rapidly.
As the aircraft climbed, the crew observed the No 2 engine running down and subsequently shut it down. Following this, the remaining three engines continued to suffer from fluctuating parameters. During the final approach, both the No 1 and No 3 engines appeared to lose power. The aircraft landed using only the No 4 engine, with no injuries to the two crew members or the passenger on board.
The investigation
The AAIB investigation focused on the mechanical and electrical components of the propeller control systems. While the aircraft was equipped with Hamilton Standard propellers, the investigation centered on the synchrophaser unit. Examination of the synchrophaser unit's circuit boards revealed that the power supply board had overheated, showing localized burning. X-ray analysis confirmed that several resistors had failed internally, and a resistor connector had melted and shorted across other connections.
Flight data from the aircraft's recorder showed that the No 2 engine torque decreased shortly after takeoff power was reached. The investigation also noted that the operator's existing checklists lacked specific procedures for managing multiple propeller malfunctions, a deficiency that had likely occurred during a previous manual update.
Findings
- The primary cause of the incident was a failure within the power supply circuit board of the propeller synchrophaser unit.
- This electrical failure sent spurious commands to the speed bias motors, causing significant fluctuations in fuel flow and engine parameters across all four engines.
- The No 2 engine exceeded its maximum governor fuel topping limit of 14,500 rpm, triggering a rapid reduction in fuel flow which the crew interpreted as an engine rundown.
- The No 1 and No 3 engines eventually experienced 'fuel topping' events, leading to the observed loss of RPM and torque during the approach.
- The crew's ability to manage the emergency was hindered by the darkness, the physical movement of the aircraft, and the lack of specific procedures for multiple engine malfunctions in the operator's manuals.