The seizure of the tail rotor bearing of the helicopter carrying Leicester City's chairman and four others led to a "sequence of failures" causing it to crash and kill everyone on board, a new report reveals.
On Saturday 27 October 2018, a helicopter belonging to the chairman of the club took off from the pitch in the King Power Stadium.
Shortly after taking off at 7.37pm, the helicopter began to rapidly descend, crashing onto a clearing next to the stadium.
All five people on board died in the crash - the club chairman, Vichai Srivaddhanaprabha, pilot Eric Swaffer and his partner Izabela Lechowicz, and two of Khun Vichai's staff, Nusara Suknamai and Kaveporn Punpare.
A report has now been published by the Air Accident Investigation Branch looking into the incident, which was described as a "tragic accident".
It reveals the seizure of the tail rotor duplex bearing initiated a sequence of failures. It culminated in the unrecoverable loss of control.
The impact of the crash damaged the lower fuselage and the helicopter's fuel tanks which result a "significant leak" and in turn caused a fire that engulfed the fuselage.
The condition of the tail rotor duplex bearing could not have been predicted or identified by existing maintenance requirements prior to the accident, the report said.
Investigators have now made eight safety recommendations to address "weaknesses or omissions identified in the regulations for the certification of large helicopters".
Mark Jarvis, principle inspector of the the Air Accident Investigation Branch, spoke to ITV News Central.
He said: "It is something that is classified as a catastrophic event in the certification programme for the helicopters and as a result there is no drill the pilot can carry out to try and mitigate this happening.
"Once the bearing had failed there was nothing the pilot could do to prevent the accident.
"He (the pilot) did everything possible but where it landed is essentially just where it landed and there is no choice of where it landed.
"While the investigation has been ongoing. We have identified a number of improvements to the immediate airworthiness of this helicopter type can be and those changes have already been put in train.
"We've made eight safety recommendations as part of this report which will help to ensure and improve certification standards and the monitoring of helicopters in the future."
Here at ITV Central, we have compiled a list of the key findings from the report:
What happened on Saturday 27 October 2018?
At 7.37pm the helicopter, carrying the pilot and four passengers, lifted off from the centre spot of the pitch at the King Power Stadium.
The helicopter moved forward and then began to climb out of the stadium on a "rearward flightpath while maintaining a northerly heading and with an average rate of climb of between 600 and 700 ft/min".
Passing through a height of approximately 250ft, the pilot began the transition to forward flight by pitching the helicopter nose-down and the landing gear was retracted.
The report reads: "The helicopter was briefly established in a right turn before an increasing right yaw rapidly developed, despite the immediate application of corrective control inputs from the pilot.
"The helicopter reached a radio altimeter height of approximately 430ft, before descending with a high rotation rate.
"At approximately 75 ft from the ground, the collective was fully raised to cushion the touchdown."
The report says the helicopter struck the ground on a stepped concrete surface, coming to rest on its left side.
Fuselage and fuel tanks damaged
The impact, which the report says likely exceeded the helicopter’s design requirements, damaged the lower fuselage and the helicopter’s fuel tanks, resulting in a significant fuel leak.
It reads: "The fuel ignited shortly after the helicopter came to rest and an intense post-impact fire rapidly engulfed the fuselage.
"The pilot and four passengers were fatally injured in the accident."
According to the report, there were several key findings made by the investigators.
Seizure of the tail rotor duplex bearing: The report says it initiated a "sequence of failures" in the tail rotor pitch control mechanism which culminated in the "unrecoverable loss of control" of the tail rotor blade pitch angle and the blades moving to their physical limit of travel.
Increasing rate of rotation: The force of the main rotor blade, coupled with the "negative" tail rotor blade pitch angle resulted in an increasing rate of rotation of the helicopter "in yaw". This made effective control of the helicopter’s flightpath impossible.
Tail rotor damage: the tail rotor duplex bearing likely experienced a combination of "dynamic axial and bending moment loads which generated internal contact pressures sufficient to result in lubrication breakdown and the balls sliding across the race surface". This caused "premature, surface initiated rolling contact fatigue damage" to accumulate until the bearing seized.
The pilots yaw control pedals became ineffective after the TRA shaft detached, resulting in the pilot being unable to control the direction or rate of yaw of the helicopter.
The loss of yaw control was irrecoverable.
Without effective yaw control the pilot was unable to control the trajectory of helicopter.
First responders arrived at the accident site within one minute of the helicopter striking the ground and attempted to gain access to the cockpit and cabin.
They were unable to do so due to the strength of the cockpit windscreen and the rapid increase in the intensity of the fire.
The helicopter was compliant with all applicable airworthiness requirements and had been correctly maintained and was certified for release to service prior to the accidental flight.
During the course of the investigation and as a result of the findings made, the helicopter manufacturer has issued sixteen Service Bulletins and EASA has published nine Airworthiness Directives for the continued airworthiness of the AW169 and AW189 helicopter types.
The investigation found the following contributory factors for the accident:
The load survey flight test results were not shared by the helicopter manufacturer with the bearing manufacturer in order to validate the original analysis of the theoretical load spectrum and assess the continued suitability of the bearing for this application - nor were they required to be by the regulatory requirements and guidance.
There were no design or test requirements in Certification Specification 29 which explicitly addressed rolling contact fatigue in bearings identified as critical parts; while the certification testing of the duplex bearing met the airworthiness authority’s acceptable means of compliance, it was not sufficiently representative of operational demands to identify the failure mode.
The manufacturer of the helicopter did not implement a routine inspection requirement for critical part bearings removed from service to review their condition against original design and certification assumptions - nor were they required to by the regulatory requirements and guidance.
Although the failure of the duplex bearing was classified as catastrophic in the certification failure analysis, the various failure sequences and possible risk reduction and mitigation measures within the wider tail rotor control system were not fully considered in the certification process - the regulatory guidance stated that this was not required.
Eight Safety Recommendations have been made in this report. These have been made to EASA to address weaknesses or omissions identified in the regulations for the certification of large helicopters - this falls under Certification Specification 29.
The recommendations address the main findings of the investigation and include: validation of design data by suppliers post-test; premature rolling contact fatigue in bearings; life limits, load spectrum safety margin and inspection programmes for critical parts and assessment and mitigation of catastrophic failure modes in systems.
Crispin Orr, Chief Inspector of Air Accidents, said: “This was a tragic accident in which five people sadly lost their lives. Our thoughts are with their loved ones, and everyone affected.
“The AAIB has carried out an extensive investigation to establish why the accident happened and how safety can be improved.
"This involved a multi-disciplinary team of skilled investigators from the AAIB, supported by a wide range of experts from industry, academia, and safety investigation authorities from around the world.
“As a result, safety action has already been taken for the continued airworthiness of the AW169 and AW189 helicopter fleets.
“Today we are publishing our final report which sets out eight recommendations to enhance safety in the longer term, regarding the design, validation and in-service monitoring of safety critical components on large helicopters.”
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