What happened
On March 13, 2005, a Pilatus PC 6 operated by a flying club was conducting skydiving operations near Lens, France. After successfully dropping the first skydiver at 4,000 feet, the pilot began a climb toward 13,000 feet. While passing through 4,500 feet, the Pratt & Whitney PT6A-27 turboprop engine suddenly lost power, causing the propeller to automatically move into the feathered position.
Recognizing the emergency, the pilot instructed the four remaining skydivers on board to evacuate the aircraft. The pilot maintained the aircraft's best glide speed, manually moved the propeller control to the feathered position, and performed a 180-degree turn to approach the runway. Despite attempts to restart the engine, the pilot was unable to restore power and subsequently performed an emergency landing at the aerodrome. There were no injuries or damage to the aircraft resulting from the landing.
The investigation
Investigators performed an initial inspection of the engine, which revealed a failure in the high-pressure fuel pump drive. Specifically, the splines on the drive shaft and the sleeve were found to be broken. Subsequent testing of the fuel pump itself showed that the unit was functioning according to manufacturer specifications, ruling out the possibility that an abnormally high torque load had caused the failure.
A detailed metallurgical examination of the drive shaft and the splined sleeve was conducted. This analysis revealed that the chrome plating on the intact sections of both components was flaking. Furthermore, cracks were discovered beneath the chrome layer within the nitrided (hardened) zone of the metal. To confirm these findings, investigators examined a high-pressure pump from a different engine in good working order; this component also exhibited micro-cracks located beneath the chrome surface.
Findings
- The primary cause of the engine power loss was the failure of the high-pressure fuel pump drive splines.
- Metallurgical analysis indicated that cracks originated in the nitrided layer beneath the chrome plating.
- The flaking of the chrome surface was a result of these subsurface cracks propagating to the surface.
- The release of metallic abrasive particles from this degradation likely accelerated the wear, eventually leading to the complete destruction of the splines.
- This failure mechanism was noted to be consistent with similar incidents involving a Pilatus PC 6 (registration F-GEBS) and a Beech 90 (registration F-ZBBF).
Safety action
Following the investigation, the BEA notified EASA regarding the repetitive nature of these drive failures. In response, Pratt & Whitney Canada issued Service Bulletin No. 1645, which was later amended to recommend a specific inspection schedule for removing high-pressure pumps from service to check the condition of the splines. Data from the manufacturer indicated that significant wear was present in a notable portion of the pumps inspected.