Air Accidents Investigation Branch
Introduction:
The Air Accidents Investigation Branch (AAIB) became aware of this accident during the evening of 27 October 2018. In exercise of his powers, the Chief Inspector of Air Accidents ordered an investigation to be carried out in accordance with the provisions of Regulation (EU) 996/2010 and the UK Civil Aviation (Investigation of Air Accidents and Incidents) Regulations 2018.
The sole objective of the investigation of an accident or incident under these regulations is the prevention of future accidents and incidents. It shall not be the purpose of such an investigation to apportion blame or liability.
In accordance with established international arrangements, the Agenzia Nazionale per la Sicurezza del Volo (ANSV) of Italy, representing the State of Design and Manufacture of the helicopter, appointed an Accredited Representative (Accrep) to participate in the investigation. The Transportation Safety Board (TSB) of Canada, representing the State of Design and Manufacture for the helicopters engines, the National Transportation Safety Board (NTSB) of the USA, representing the State of Design and Manufacture of the tail rotor actuator and the Bureau dEnqutes et dAnalyses pour la scurit de laviation civile (BEA) of France representing the State of Design and Manufacture of the tail rotor duplex bearing, also appointed Accreps.
Experts were appointed by the Aircraft Accident Investigation Committee of Thailand and the State Commission on Aircraft Accidents Investigation of Poland.
The helicopter, bearing, tail rotor actuator and grease manufacturers, the operator, the European Union Aviation Safety Agency (EASA), and the UK Civil Aviation Authority (CAA) also assisted the AAIB investigation.
Summary:
At 1937 hrs 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 250 ft, the pilot began the transition to forward flight by pitching the helicopter nosedown and the landing gear was retracted. 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 430 ft before descending with a high rotation rate. At approximately 75 ft from the ground the collective was fully raised to cushion the touchdown.
The helicopter struck the ground on a stepped concrete surface, coming to rest on its left side. The impact, which likely exceeded the helicopters design requirements, damaged the lower fuselage and the helicopters fuel tanks which resulted in a significant fuel leak. The fuel ignited shortly after the helicopter came to rest and an intense post-impact fire rapidly engulfed the fuselage.
The investigation found the following causal factors for this accident:
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Seizure of the tail rotor duplex bearing 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.
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The unopposed main rotor torque couple and negative tail rotor blade pitch angle resulted in an increasing rate of rotation of the helicopter in yaw, which induced pitch and roll deviations and made effective control of the helicopters flightpath impossible.
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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 investigation found the following contributory factors for this accident:
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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.
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There were no design or test requirements in Certification Specification29 which explicitly addressed rolling contact fatigue in bearings identified as critical parts; while the certification testing of the duplex bearing met the airworthiness authoritys acceptable means of compliance, it was not sufficiently representative of operational demands to identify the failure mode.
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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.
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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.
AAIB Special Bulletin S1/2018, published on 14 November 2018 and AAIB Special Bulletin S2/2018, published on 6 December 2018, provided initial information on the circumstances of this accident.
During the course of this 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.
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 - 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.
Animation
This animation is intended to help visualise and explain the effect of the bearing failure. The graphics are not a precise depiction of the aircrafts behaviour or mechanical components. Comprehensive information about the circumstances of the accident and the reasons for the severity of the outcome are contained in the investigation final report.