The helicopter crash which killed the chairman of Leicester City and four other people was caused by a pin that had come loose in the tail rotor control mechanism, a report has revealed.
The Air Accidents Investigation Branch found that the pin had become disconnected, resulting in the helicopter becoming unstable and preventing the pilot from controlling it.
Leicester City chairman Vichai Srivaddhanaprabha, two of his staff Nursara Suknamai and Kaveporn Punpare, pilot Eric Swaffer and Mr Swaffer’s girlfriend Izabela Roza Lechowicz were killed when the aircraft crashed into a car park close to the King Power stadium on 27 October.
Investigators found the pilot’s pedals became disconnected from the tail rotor and caused the aircraft to make an uncontrollable right turn before it spun and fell, hitting the ground in a ball of flames.
The pedals and tail rotor are linked by a mechanism which failed after parts of it had become disconnected and there was a “build-up of black grease” on one component.
The failure led to the pitch of the tail rotor blades being changed “until they reached the physical limit of their travel”, investigators said.
The report said: “The initiating cause and exact sequence of the failure that resulted in the loss of tail rotor control is being investigated as a priority.”
“The tail rotor actuator control shaft became disconnected from the actuator lever mechanism,” it added.
“The disconnection stopped the feedback mechanism for the tail rotor actuator from operating and the tail rotor actuator from responding to yaw control inputs.”
This loss of the feedback mechanism meant the yaw stops were ineffective and the tail rotor actuator was able to continue changing the pitch of the tail rotor blades “until they reached the physical limit of their travel”.
“This resulted in an uncontrollable right yaw.”
It went on: “Sufficient force and torque had been applied to the castellated nut on the actuator end of the control shaft to friction weld it to the pin carrier and to shear the installed split pin.
“Whilst the shaft was rotating and a yaw control input was applied, the shaft ‘unscrewed’ from the nut, disconnecting the shaft from the actuator level mechanism, and causing the nut to become welded to the pin carrier.”
It added: “The threaded portion of the control shaft, at the actuator end, was inside the outer shaft and contained the remains of the split pin. The top and bottom of the split pin had been sheared off in rotation.”