Investigation Summary
The accident was traced to the failure of a modified spiral bevel ring gear in the forward transmission. This gear was responsible for transmitting power and maintaining the precise timing between the forward and aft rotor systems. In 1985, the transmission had been modified according to a Boeing service bulletin intended to address torque loss issues. However, the modification introduced unforeseen stresses. During the approach to Sumburgh, the ring gear teeth failed, causing the forward rotor to desynchronize from the aft rotor. Because the rotor paths overlap by approximately 80%, synchronization is critical to prevent the blades from striking each other. Upon losing timing, the blades collided, resulting in the immediate structural failure of the helicopter. The aircraft plunged from an altitude of approximately 150 feet into the sea. Investigators found that the certification and testing program for the modified gear had been inadequate, failing to identify the fatigue risks that led to the gear's eventual disintegration.
Final Conclusions
The Air Accidents Investigation Branch (AAIB) concluded that the primary cause was the fatigue failure of the forward transmission spiral bevel ring gear. This failure was facilitated by an inadequate test program for the modified component and a lack of effective condition monitoring or inspection protocols that could have detected the impending failure. The investigation resulted in the publication of AAIB Report 2/1988.
Video Analysis
Untested Modification Kills 45: The 1986 Chinook Helicopter Crash — A detailed documentary examining the mechanical failure and the testing oversights that led to the 1986 Sumburgh Chinook disaster.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Captain Pushp Vaid (45) survived. He was a former Indian Air Force pilot who joined British International Helicopters in 1975 and began flying the Chinook in 1982.
Systems & Failure Modes
Forward Transmission Spiral Bevel Ring Gear
A critical component that transmits rotational force to the forward rotor and maintains synchronization with the aft rotor.
Rotor Synchronization Shaft
A series of shafts connecting the transmissions to ensure the two rotor systems do not collide during operation.
ADELT
An Automatically Deployable Location Transmitter designed to eject and signal the aircraft's position upon impact, which failed in this instance.
Interesting Facts
- 01The accident is the deadliest civilian helicopter crash in European history.
- 02The forward and aft rotors of the Chinook overlap by 80%, making mechanical synchronization vital.
- 03The modified bevel ring gear failed after approximately 547 hours of service following its installation.
- 04The Automatically Deployable Location Transmitter (ADELT) failed to operate due to impact damage.
- 05Following the accident, the oil industry withdrew the Chinook from offshore passenger operations; surviving airframes were sold to Columbia Helicopters for heavy-lift, non-passenger use.
Safety Actions & Advisories
Modification Certification Review
Recommended that certification procedures be reviewed to ensure all modifications to vital components are adequately scrutinized and tested before approval.
Condition Monitoring Requirements
Called for the CAA to incorporate specifications for suitable condition monitoring systems into helicopter airworthiness requirements.
ADELT Equipment Review
Recommended a review of ADELT requirements regarding location, crashworthiness, and power supply after the beacon failed to deploy.