Investigation Summary
The accident occurred while the pilot was performing a planned simulation of an autopilot failure. The Westland Wessex features a folding tail section designed to reduce its parking footprint. This design utilizes a disconnect coupling consisting of two toothed flanges that must mesh to transmit power to the tail rotor. Investigators determined that when the tail was last unfolded, these flanges failed to engage completely. Contributing factors to this incomplete engagement included the hardening of grease within the mechanism and possible spline wear, which prevented the components from seating correctly. The stresses induced by the autopilot failure simulation caused the partially meshed flanges to slip entirely. This led to a momentary loss of drive, followed by a violent re-engagement as the rotor speed dropped. The resulting shock loading caused an overload failure of the No. 3 tail rotor drive shaft. Deprived of the tail rotor's anti-torque force, the helicopter entered an unrecoverable spin. The aircraft impacted the water and submerged almost immediately, hindering rescue efforts for the passengers seated in the rear cabin.
Final Conclusions
The Ministry of Defence Board of Inquiry (BOI) concluded the primary cause was the overload failure of the No. 3 tail rotor drive shaft due to the incomplete engagement of the tail fold disconnect coupling. The investigation found the pilot's actions during the emergency were logical and correct, but the mechanical failure rendered the aircraft uncontrollable. The final report was released in November 1995.
Video Analysis
Llyn Padarn Helicopter Crash Of 1993 — A detailed look at the 1993 Llyn Padarn helicopter crash involving an RAF Westland Wessex, featuring amateur footage of the accident and the subsequent investigation into the tail rotor failure.
If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Tail Rotor Drive Disconnect Coupling
A mechanism consisting of two toothed flanges that allows the tail section to be folded for storage. It must be perfectly meshed to safely transmit torque to the tail rotor.
No. 3 Tail Rotor Drive Shaft
A component of the transmission train that failed due to sudden shock loading when the disconnect coupling re-engaged after slipping.
Interesting Facts
- 01The crash was captured on amateur video by a bystander.
- 02The Westland Wessex fleet was grounded for non-emergency use following the accident.
- 03The three fatalities were all Air Training Corps (ATC) cadets.
- 04A civilian inquest into the deaths was opened in Llandudno in January 1996.
- 05The aircraft, MSN WA/146, was manufactured in 1964.
Safety Actions & Advisories
Servicing Schedule Amendment
The Wessex servicing schedule was updated to include a mandatory visual check for full engagement of the disconnect coupling before flight.
Simulator Training
Enhanced training for Wessex pilots was implemented to include simulator-based tail rotor emergency procedures.
Passenger Safety Equipment
A recommendation was made to provide protective headgear for all helicopter passengers.