What happened
On the evening of an unnamed date in 2002, a Bell 205 helicopter, registration C-FPAZ, was performing forest fire suppression duties for the Province of Saskatchewan. While the aircraft was maneuvering toward a drop point near Chitek Lake at an altitude of approximately 150 feet and a speed of less than 40 knots, the pilot observed a low-rotor-rpm warning and a yellow caution light.
In an attempt to manage the loss of rotor speed, the pilot released the water load and reduced torque. However, the rotor rpm failed to recover. The pilot attempted to regain rpm by reducing collective after clearing nearby trees, but the aircraft entered a rapid descent. The helicopter struck an earthen brush pile and rolled onto its side. The pilot sustained serious injuries and was evacuated to Saskatoon.
The investigation
Investigators examined the wreckage and found substantial damage to the main rotor, tail boom, and cabin caused by high-impact forces. The investigation focused on the transmission mounting structure, which had been bent forward, causing the free-floating main input drive shaft to spin out of its forward coupling and disengage from both the engine and the transmission.
Analysis of the drive shaft's splined end revealed extensive heat damage and helical scoring, indicating the shaft had been spinning within the coupling prior to the crash. Evidence suggested that the engine was no longer driving the rotors, leading to the decay in rotor rpm. The investigation also looked into recent maintenance, noting that a grease seal on the forward main input drive shaft coupling had been replaced three days prior to the accident. While the replacement was documented, there was no formal certification statement entered in the helicopter's journey log for that specific work.
Findings
- The main input drive shaft failure at the forward coupling was the primary cause of the accident.
- This failure likely stemmed from a progressive issue or damage that was not detectable during the recent grease seal replacement.
- The decoupling of the engine from the transmission led to the loss of rotor rpm and the subsequent rapid descent.
- The accident occurred at an altitude and airspeed that placed the aircraft in a flight regime where a successful autorotation was nearly impossible to establish.
- Pilots performing external load operations are at a heightened risk because they are not restricted by the height-velocity limitations found in the standard flight manual, leaving them with reduced margins for error during drivetrain malfunctions.