This is an informal precis of an AAIB investigation interim report into the Leicester helicopter crash.
Julian Bray the aviation expert and broadcaster suggested on BBC Wales earlier today that the initial findings appear to clear the pilot, as the report strongly suggests, a left yaw [turn] foot pedal control actually pitched the helicopter uncontrollably into a maximum right yaw, spiralling the helicopter nose first onto stepped concrete.
Multiple linked mechanical failures and a control rod effectively unscrewing itself in the process added to the complex process. The investigation is being aided by the recovery of the combined voice and data recorder by the AAIB team fitted to this Augusta AW169, and decoded at AAIB Farnborough.
Julian Bray the aviation expert and broadcaster suggested on BBC Wales earlier today that the initial findings appear to clear the pilot, as the report strongly suggests, a left yaw [turn] foot pedal control actually pitched the helicopter uncontrollably into a maximum right yaw, spiralling the helicopter nose first onto stepped concrete.
Multiple linked mechanical failures and a control rod effectively unscrewing itself in the process added to the complex process. The investigation is being aided by the recovery of the combined voice and data recorder by the AAIB team fitted to this Augusta AW169, and decoded at AAIB Farnborough.
Aircraft Type and Registration: Agusta AW169, G-VSKP
No & Type of Engines: 2 Pratt & Whitney Canada PW210A turboshaft enginesYear of Manufacture: 2016 (Serial no: 69018)
Location King Power Stadium, Leicester
Date & Time (UTC): 27 October 2018 at 1937 hrs
Type of Flight: Private
Persons on Board: Crew - 1 Passengers - 4
Injuries: Crew - 1 (Fatal) Passengers - 4 (Fatal)
Nature of Damage: Aircraft destroyed
Commander’s Licence: Airline Transport Pilot’s Licence (A and H)
Commander’s Age: 53 years
Commander’s Flying Experience: TBA
Last 90 days - 40 hours Last 28 days - 7 hours
Information Source: AAIB Field Investigation
The investigation
The accident occurred at 19371 hours on 27 October 2018. The AAIB published Special Bulletin S1/2018 on 14 November 2018 to provide preliminary information gathered from the site investigation, subsequent technical investigation, recorded data, and other sources.
This second Special Bulletin provides information on the findings to date of a detailed examination of the helicopter’s yaw control system.
The tail rotor control system was first inspected at the crash site. This identified that the input lever mechanism was not attached to the control shaft. The pin, spacers and one
of the locating bearings were missing from the lever. The locking nut and pin carrier were found loose in the tail rotor fairing and were bonded together (they should be separate components). The threads of the nut appeared to be undamaged. There was no evidence of the split pin, and the control shaft threaded section had moved inside the outer shaft and was no longer visible.
The control shaft, the locking nut and pin carrier, and the duplex bearing/sliding unit assembly were removed from the wreckage and inspected in detail. The locking nut on the bearing end of the control shaft was found to have a torque load significantly higher than the required assembly value. The inner races of the bearing could only be rotated a few degrees in either direction by hand. There was a build-up of black grease inside the slider unit around the inboard face of the duplex bearing. The section of the control shaft adjacent to this bearing face showed evidence of burnt-on grease and was discoloured along its length.
The control shaft, the locking nut and pin carrier, and the duplex bearing/sliding unit assembly were removed from the wreckage and inspected in detail. The locking nut on the bearing end of the control shaft was found to have a torque load significantly higher than the required assembly value. The inner races of the bearing could only be rotated a few degrees in either direction by hand. There was a build-up of black grease inside the slider unit around the inboard face of the duplex bearing. The section of the control shaft adjacent to this bearing face showed evidence of burnt-on grease and was discoloured along its length.
The components were then inspected using a Computed Tomography (CT) Scanner. This uses x-rays to image the inside of the components, then recreates them as a 3-D model.
The results showed that the nut and pin carrier were friction welded together. 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. The scan of the bearing showed fractures to the bearing cages and significant damage to the surface of the inner bearing races, the damage being worse on the inboard bearing race
where there was also evidence of sub-surface damage. The scan also showed evidence of debris accumulating in the bearing raceways The bearing was then removed from the sliding unit and disassembled, revealing evidence of relative rotation between the sliding unit and the bearing outer ring. The debris present on the CT scan was identified as a combination of black dust and metallic particles.
Failure sequence
The evidence to date shows that the loss of control of the helicopter resulted from the tail rotor actuator control shaft, becoming disconnected from the actuator lever mechanism. Disconnection of the control shaft prevented the feedback
mechanism for the tail rotor actuator from operating and also the tail rotor actuator from responding to yaw control inputs.
Loss of the feedback rendered the yaw stops ineffective, allowing the tail rotor actuator 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 [turn] yaw.
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.
The observed condition of the duplex bearing and the increased torque load on the castellated nut that remained on the spider end of the shaft is consistent with rotation of the tail rotor actuator control shaft.
Whilst the shaft was rotating and a yaw control input was applied, the shaft “unscrewed” from the nut, disconnecting the shaft from the actuator lever mechanism, and causing the nut to become welded to the pin carrier.
Safety actions
precautionary inspection of the tail rotor control assembly on all helicopters in
the global fleet. On 6 November the manufacturer also issued ASB 189-213
for AW189 helicopters, which have a similar tail rotor control system.
AAIB investigations are conducted in accordance with Annex 13 to the ICAO Convention on International Civil Aviation,
EU Regulation No 996/2010 and The 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 is not the purpose of such an investigation to apportion blame or liability.
Accordingly, it is inappropriate that AAIB reports should be used to assign fault or blame or determine liability, since neither the investigation nor the reporting process has been undertaken for that purpose.
The European Aviation Safety Agency (EASA), in its capacity as the regulator
responsible for the type design approval of the AW169 and AW189, issued
Airworthiness Directive 2018-0241-E dated 7 November 2018 to mandate
these inspections.
Airworthiness Directive 2018-0241-E dated 7 November 2018 to mandate
these inspections.
https://www.gov.uk/government/news/update-on-leicester-helicopter-accident-g-vskp?utm_source=9dabe4fe-6523-4a70-b57e-83d1017a08c0&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate
On 19 November 2018 the EASA issued AD 2018-0250-E, superseding
AD 2018-0241-E, to require a precautionary one-time inspection of the tail
rotor duplex bearing and, depending on findings, applicable corrective actions. On 21 November 2018 the helicopter manufacturer published Emergency Alert Service Bulletin ASB169-125 for AW169 helicopters, and ASB189-214
for AW189 helicopters, giving further instructions for a one-time inspection
of the tail rotor duplex bearing. The EASA issued AD 2018-0252-E on
21 November 2018, superseding AD 2018-0250-E and mandating this
inspection.
On 30 November 2018 the helicopter manufacturer published Emergency Alert Service Bulletin ASB 169-126 for AW169 helicopters, and ASB 189-217
for AW189 helicopters, introducing repetitive inspections of the castellated nut
that secures the tail rotor actuator control shaft to the actuator lever mechanism,
and the tail rotor duplex bearing. The EASA issued AD 2018‑0261‑E on
30 November 2018 mandating the repetitive inspections.
Ongoing investigation
The initiating cause and exact sequence of the failure that resulted in the loss of tail rotor control is being investigated as a priority. Work continues to identify the cause of the image observed to the duplex bearing and to establish its contribution to the failure
sequence. The AAIB is working with relevant organisations to identify any other factor that may have contributed to the loss of tail rotor control.
The other areas of investigation specified in Special Bulletin S1/2018 will continue, and the AAIB will report any significant developments as the investigation progresses.
@JULIANBRAY +44(0)1733 345581, Journalist & Broadcaster, Aviation Security & Airline Operations Analyst/expert, ... http://feeds.feedburner.com/BraysDuckhouseBlogwww.aviationcomment.com, ... http://www.freelancedirectory.org/user.php?user=8121 ...
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