What happened
On March 6, 2010, a Bombardier Learjet 60, registration OH-III, operated by Jetflite Oy, experienced a simultaneous shutdown of both engines while taxiing at Helsinki-Vantaa Airport. The aircraft was preparing for a business flight to Moscow. While crossing runway 22L, the crew increased engine power to comply with a request to expedite the crossing and subsequently reduced the power. During this specific sequence of throttle movements, both engines failed immediately.
The crew managed the situation without any damage to the aircraft. The aircraft was eventually towed away from the runway with the electrical systems still active and the Auxiliary Power Unit (APU) running. No injuries or fatalities occurred, and there were no passengers on board.
The investigation
Investigators examined the fuel quality, engine components, and digital flight data. Fuel samples showed no significant contamination or water. Inspections of the engine intake ducts, turbines, and fuel pumps revealed no mechanical failures or leaks. Data retrieved from the Flight Data Recorder (FDR) and the Full Authority Digital Engine Control (FADEC) confirmed an uncommanded shutdown (error code "QF") and provided a precise timeline of the throttle movements.
Subsequent testing by the engine manufacturer, Pratt & Whitney Canada (PWC), and the investigators revealed that the shutdown could be replicated by mimicking the exact throttle movement: a rapid increase from idle to high power, followed by a rapid return to idle at a specific N2 RPM. This movement caused the engine to become prone to shutting down due to a lean fuel-air mixture.
Findings
- The primary cause of the dual engine shutdown was a disturbance in the fuel-air mixture triggered by a specific sequence of throttle movements.
- This phenomenon was specifically linked to engines equipped with Lucas-type hydromechanical units (HMU). When an alternative Woodward-type HMU was installed, the shutdown tendency disappeared.
- The investigation found that the engine control system was unable to manage the fuel-air mixture correctly during the rapid power fluctuations.
- While the manufacturer stated the risk of an in-flight shutdown was extremely remote, the investigation noted that the engine's tendency to shut down was reproducible even under simulated flight conditions.