On 6 November 1986, a Boeing-Vertol Model 234LR Chinook helicopter returning workers from the Brent oilfield crashed on approach to land at Sumburgh Airport in the Shetland Islands.

At 2.5 mi (4.0 km) from the runway the helicopter had a catastrophic forward transmission failure which caused the tandem rotor blades to collide.

[4] On 6 November the first flight was delayed due to an oil leak from an engine gearbox that was soon rectified and the aircraft left Sumburgh at 08:58 with 40 passengers for the Brent Field.

The helicopter was largely broken up by the clashing rotor blades in mid air, hit the sea and quickly sank.

A Coastguard search and rescue Sikorsky S-61 helicopter which had just departed Sumburgh Airport on training flight reported liferafts in the sea.

The accident was caused by the failure of a modified bevel ring gear in the forward transmission which allowed the twin rotors to collide when synchronisation was lost.