What happened
During the landing phase of a flight, the helicopter's forward fan shaft bearing seized, leading to extreme heat and torque. This mechanical stress caused the fan shaft to fail just behind the bearing. The pilot responded to the failure by reducing throttle, following established Federal Aviation Administration guidelines. To manage the situation, the pilot executed a shallow approach, utilizing ground effect and translational lift to decrease the power required for the maneuver. By maintaining a higher airspeed and a running landing, the pilot successfully mitigated the yaw effect caused by the failure, allowing the aircraft to be landed safely.
The investigation
Investigators examined the failed components and found that the bearing's inner race had disintegrated, with metal deposits present on the bearing seat. Evidence of excessive heat was visible around the fan shaft and bearing components. The investigation revealed that the shaft had been spinning independently of the bearing following the seizure. Additionally, scoring on the hanger and outer race suggested periods where the bearing and shaft were spinning together. While the pilot had reported noises two days prior to the incident, it could not be confirmed if these sounds were related to the eventual bearing failure.
Regarding lubrication, no grease was found following the failure, though investigators noted it might have been displaced by the intense heat. The aft fan shaft bearing, however, showed no signs of improper lubrication or corrosion, displaying only small areas of spalling likely caused by a foreign contaminant. Due to the extensive damage to the forward bearing, the specific cause of the initial seizure could not be determined.
Findings
- The forward fan shaft bearing seized and overheated, which directly led to the failure of the fan shaft.
- The pilot's decision to use a shallow approach and a running landing helped maintain control and mitigate yaw.
- Localised damage in the aft bearing was attributed to a foreign contaminant rather than manufacturing defects or misalignment.