Bearing Seizure Caused Fatal AW169 Crash, UK Probe Finds
LONDON—The UK’s Air Accident Investigation Branch (AAIB) is recommending changes to certification standards over the use of bearings in critical applications on helicopters.
The recommendations follow a five-year investigation into the fatal crash of a Leonardo AW169 helicopter—registered G-VSKP—shortly after takeoff from King Power Stadium, Leicester, England, on Oct. 27, 2018.
The accident—the first-ever loss of an AW169—killed all five people onboard, including Vichai Srivaddhanaprabha, the Thai chairman and owner of Leicester City Football Club.
The AAIB’s comprehensive investigation, published Sept. 5, finds that moments after the helicopter lifted out of the stadium, the tail rotor’s duplex bearing—which connected the aircraft’s rotating tail rotor assembly with the nonrotating tail rotor actuator control shaft—seized, disconnecting yaw control from the pilot’s inputs.
“An increasing right yaw rapidly developed, despite the immediate application of corrective control inputs from the pilot,” the report says, noting that the helicopter’s rotation rate peaked at 209 deg. per second. The aircraft began rapidly falling and struck the ground on land near the stadium at a rate of descent that “likely exceeded the helicopter’s design requirements.”
Fuel onboard the helicopter ignited, and the aircraft burst into flames with such ferocity that emergency services and stadium staff were “not able to gain access to the helicopter.”
The AAIB’s investigation had immediately focused on the anti-torque system. It was discovered that the inner and outer races of both sides of the bearing had become damaged by Rolling Contact Fatigue—a type of surface damage resulting from repeated rolling or rolling and sliding contact between curved surfaces such as the race and ball in a bearing. Manufacturer Leonardo had used a hybrid bearing, which used ceramic ball bearings made from silicon nitride.
Although lighter, the ceramic balls had a higher contact pressure in the bearing. Combined with deterioration in the grease used to lubricate the bearing, this caused the ceramic balls to slide rather than roll. This ultimately led to wear and fatigue fractures that resulted in the bearing seizing.
The report says, however, that the manufacturer is not at fault, because there are no explicit requirements within the European Union Aviation Safety Agency’s (EASA) CS29 requirements governing certification of large rotorcraft that define how rolling contact fatigue should be addressed within the design process or any compliance test activity for any type of bearing, even if they are used in safety-critical applications. Leonardo Helicopters has since changed the bearing design and begun using steel rather than ceramic balls.
The company has also added an accelerometer sensor on the tail to the servo-actuator control lever. This allows monitoring of the vibration signature of the duplex bearing through the aircraft’s health and usage monitoring system.
Other AAIB recommendations call for manufacturers and regulators to assess the performance and reliability of components identified as critical parts before certification, and for EASA to introduce additional requirements to CS29 to address premature rolling contact fatigue failure across bearings used in safety-critical applications.