S-76s will stay in operation during the investigation

The Civil Aviation Authority has ruled out grounding the 29 Sikorsky S-76 helicopters operating in the UK while Tuesday's crash is investigated.

Experts said that the only explanation for the crash, which killed 11 people, was mechanical failure in the main or tail rotor system. But they were mystified by why it was not predicted by standard maintenance checks.

The CAA said there was "no justification" at present for stopping the aircraft flying because the cause of the accident was not known. Most S-76s are still being used for North Sea flights, though a number have been grounded voluntarily by their operators. The Air Accident Investigation Board is investigating, but has not set a completion date.

A spokesman for Sikorsky said that since the S-76 was introduced in 1978, more than 500 had been sold. Reports suggest S-76s have been involved in a handful of crashes, though Sikorsky could not confirm the number. Independently, the model is rated as having an excellent safety record. "The model has three million flight hours," Sikorsky's spokesman said.

Captain Peter Morgan, a former pilot with Bristow Helicopters, which operated the crashed model, said: "To get out of the pilot's control, either the helicopter's blades or tail rotor must have fallen off, or there was a transmission failure which stopped the drive to the main rotor or the tail rotor turning."

Pieces breaking off inside the gear system of the main rotor can destroy the aircraft. But there are usually warnings because small particles break off before the main failure, and those would normally be picked up by magnetic filters in the oil system.

Michael Neale, of Neale Consulting Engineers, a crash investigation company, said: "Those filters are inspected routinely every 50 flying hours, or 25 hours if something has been found before."

Bristow declined to comment on any aspect of the helicopter's maintenance.

Bristow uses the electronic Health and Usage Monitoring System, provided by the aerospace company Meggitt, which can predict gear failure by analysing vibration in the engine and motors. But pilots also make other standard checks. Capt Morgan said it was common for pilots to check the bolts that attach the blades to the rotors.