What happened
On April 2, 2014, a Robinson R22 Beta II helicopter, registration OK-OBA, was performing a private flight from Podhoř กรณี (LKPN) to Benešov (LKBE). Approximately five minutes after takeoff, the pilot experienced a sudden impact and vibration throughout the aircraft. The pilot initially suspected a bird strike or an engine malfunction and immediately decided to abort the flight and return to the departure airfield.
During the descent, the pilot noted that engine temperatures had reached their maximum allowable limits. The helicopter landed safely on the grass area in front of a hangar at LKPN. Upon landing, the pilot observed the smell of burning oil and smoke emanating from the engine compartment. The pilot used an onboard fire extinguisher to cool the area, but smoke continued to rise due to the extreme heat. The pilot subsequently requested assistance from the Fire Rescue Service of the Czech Republic due to fears of an explosion.
The investigation
The ÚZPLN investigation focused on the mechanical integrity of the engine cooling system. The investigation established that the engine was functioning normally until the mechanical failure occurred. Technical inspections of the aircraft revealed that the cooling fan had detached from the lower bearing of the expansion mechanism. This failure was caused by the breakage of the fan shaft cone. The detached fan subsequently damaged the fiberglass fan shroud.
Investigators also examined the maintenance history of the aircraft. The helicopter had undergone its last 100-hour inspection in April 2013 and had flown 69 hours since that service. No previous defects or hard landings had been recorded in the aircraft logs. The investigation also noted that the pilot's decision to land near the hangar was a critical factor in managing the emergency effectively.
Findings
- The primary cause of the incident was the failure of the engine cooling fan due to the breakage of the shaft cone within the fan shaft and lower bearing assembly.
- The breakage of the shaft cone was likely caused by a fatigue fracture or a material defect.
- The failure of the fan led to a loss of engine cooling, causing temperatures to exceed the permitted upper limits.
- The pilot's rapid decision to return to the departure airfield prevented more extensive damage to the aircraft.
- The smoke observed after landing was not a fire, but rather the vaporization of lubricants (oil and grease) that had been exposed to engine temperatures exceeding 260 °C.