24th of June 2020, Bearing Failure, Blog # 709
- V2Aviation
- Jun 22
- 4 min read
After undergoing maintenance, a Bell 407 helicopter made a successful 56-minute flight from the MRO facility to the aircraft base, a private site near Peldon (Essex, England). It was an uneventful flight, after which the helicopter was fuelled in anticipation of the next day's flights.
The next day, the pilot performed a walk-around inspection of the aircraft, which included opening the engine cowlings and checking the fluid levels; he did not find any abnormalities. The engine oil level was at the full mark. The first flight to the town of Wellsbourne was uneventful. Before embarking on the second flight, another walk-around inspection was conducted, and the engine oil level was checked again; it was still at the full mark. There were 620 lbs of fuel remaining. At 10:39 lt (local time), the aircraft took off from Wellsbourne, with a passenger on board. As the aircraft was flying south of Wellsbourne, people on the ground noticed a “contrail” or smoke coming from the helicopter, above the cabin but below the rotor of the aircraft. While observing the aircraft, one person reported hearing a popping noise, and the smoke turning grey as the helicopter started descending.
The Pilot and passenger were unaware of the observations made on the ground and continued the flight at ~1800 ft amsl (~1700 ft agl). All indications were normal as the aircraft flew at a speed of 118 kts with the engine torque varying between 70 and 75%.
This changed when an amber FADEC fault light appeared on the Caution and Warning Panel (CWP). When the pilot checked the instruments, he saw the NR was at 100% and the NP was at 90%. The pilot also noticed a reduction in engine power. While analysing the problem, ~27 seconds after the fault light appeared, a “mechanical snapping noise”, a violent yaw to the left, the red engine out warning illuminated, the torque dropped to 3%, and a horn was heard.
It was obvious the engine had failed, the collective was fully lowered, while the cyclic was moved rearward to reduce the speed to 60 knots. Once in a stable autorotation, the pilot declared a Mayday after selecting a field for landing. During the autorotation, the pilot recalled that he just focused on NR (rotor RPM), airspeed and his selected field; he did not notice any further engine indications and did not switch anything off. As the helicopter approached the ground, he flared and raised the collective, bringing the helicopter to a nose-high attitude. He levelled the helicopter and it touched down, then skipped forward a few feet. As soon as the helicopter had stopped, the pilot described “dumping the collective” as quickly as possible. The passenger immediately said she could smell smoke and that they needed to get out. The pilot recalled seeing a coloured liquid on the windscreen which he described as “off white coloured”. They both exited the helicopter through their respective doors; as they looked back, they could see flames below the rotor. The pilot had no recollection of switching anything off before exiting the helicopter, as his focus was on getting himself and his passenger out safely. Both the pilot and the passenger escaped without injuries. During the landing, the tailboom had separated from the fuselage and remained free from fire damage; it was found to be covered in oil. The aircraft burned down completely.
The aircraft had flown a total of 3 hours and 30 minutes since leaving the maintenance facility. An investigation was launched by the Air Accidents Investigation Branch.
After an extensive investigation, it was concluded that the uncontained failure of the gas-producing turbine rotors occurred due to insufficient oil being supplied to the No. 8 main engine bearings. The lack of oil was the result of a leak that occurred during the accident flight. Due to the extent of the fire damage to the helicopter, it was not possible to establish the source of the leak.
The investigation established that the gas-producing rotors were not contained because the engine was not required to be designed or certified to contain such a catastrophic failure.
The uncontained failure resulted in damage to the fuel and oil pipes in the engine compartment, and the leaking fluids ignited in the presence of the hot components. As the helicopter fuel valve remained open, and the booster pumps continued to run after the occupants had left the helicopter, fuel continued to be pumped into the engine compartment through the damaged component, further feeding the fire.
As there was no fire detection system fitted to the helicopter, and because of the extensive fire damage, it was not possible to establish if the helicopter met the fire protection certification requirements. However, the protection offered by the engine bay fire walls enabled the pilot to make a controlled landing and the occupants to safely vacate the helicopter. Although the NR was below the speed specified in the RFM during the autorotation, the pilot was able to land safely. The rapid lowering of the collective after landing resulted in a main rotor blade striking and severing the tail boom.
The full AAIB report with all the investigation details can be accessed by clicking on the .pdf file below;
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