What happened
On 11 January 2007, a Jetstream 4100, registration G-MAJE, was conducting a commercial passenger flight from Leeds Bradford to Southampton. During the descent through 7,500 feet, approximately 27 nautical miles north of Southampton, the aircraft experienced a significant reduction in engine torque. The crew initially observed torque dropping from 40% to 10% on both engines, leading them to believe a double engine failure had occurred. This prompted the transmission of a ‘Mayday’ call.
As the pilot attempted to level the aircraft at 7,000 feet by advancing the power levers, the aircraft yawed to the right. Upon investigation, the crew discovered that the left engine was operating normally, but the right engine speed had significantly reduced. Shortly after this realization, the right engine spontaneously restarted without any manual intervention from the crew. The ‘Mayday’ was subsequently downgraded to a ‘Pan’ call, and the flight continued to a safe landing at Southampton.
The investigation
An AAIB field investigation examined the aircraft's flight recorders and the engine anti-icing systems. The investigation focused on the engine air intakes, noting that G-MAJE was fitted with the original version of the engine air intakes. These older intakes were known to be susceptible to ice accumulation in the region between the upper lip and the propeller spinner.
Engineers examined the engines and anti-icing systems following the incident. Ground tests conducted at full power revealed no technical defects, and the aircraft was returned to service. The investigation also considered the weather conditions, noting that the aircraft had been flying through cloud layers with temperatures between -5°C and +8.5° and had experienced airframe icing.
Findings
- The right engine rundown likely resulted from the ingestion of ice that had accumulated on the intake lip while the aircraft flew through cloud tops.
- As the aircraft descended into warmer air, this accumulated ice likely slipped into the engine air intake.
- The crew's initial belief in a dual engine failure was caused by the low power settings and the initial torque drop on both engines.
- It was not determined why the right engine failed to auto-restart within the expected five-second window or why the failure was isolated to only one engine.