An inquest opens at Wells today into the deaths of four West-based Navy helicopter crew who died when their aircraft crashed into the sea off Cornwall on a night search-and-rescue mission.

The crash of the Lynx helicopter, based at Royal Naval Air Station Yeovilton, on December 8, 2004 led to changes in Navy training and the installation of cockpit voice recorders in all Lynx helicopters.

Lt Dave Cole, 34 and Lt Rob Dunn, 29, both from Dorset, Lt Jamie Mitchell, 29, from Dundee, and Leading Air Engineering Mechanic Richard Darnell, 31, from Torquay died.

The helicopter, embarked on HMS Portland, was responding to an alert for a possible man overboard from HMS Montrose when it crashed in the dark, approximately two minutes after reporting that it was 100 feet above the sea. No mayday call was received.

Neither the initial board of inquiry, nor the investigation by the Royal Navy Flight Safety and Accident Investigation Centre, was able to establish the precise cause. Both concluded that the crew were reacting to a real or perceived major aircraft malfunction.

The bulk of the aircraft was discovered upright, though badly damaged, on the sea bed. Both engines appeared to have been shut down. No evidence of a malfunction or system failure that would have caused the aircraft to depart from controlled flight, or give an indication of any malfunction, was discovered.

The board of inquiry reported: "Carrying out crash checks at low level over the sea at night is extremely hazardous and the board considered the crew would have undertaken the action only if they were firmly convinced they were dealing with a major emergency."

It said the lack of cockpit voice recorder and flight data recorder deprived the investigation of "potentially vital" information and called for a review of the policy of operating Naval aircraft without them. From autumn 2007 all Lynx have been fitted with cockpit voice recorders.

The pilot's eyesight required him to fly with service spectacles. He was thought to wear contact lenses at times but it was not possible to determine whether he wore them when flying.

The board concluded that he was not wearing approved visual correction but could not determine whether this contributed to the accident. It called for rules on corrected vision to be re-emphasised to medical personnel and air crew.

An MoD spokesman said contact lenses are not currently approved for Navy aircrew.

The board also called for consideration to be given to including live or simulated complex multi-unit search and rescue operations in command team training schedules. A search and rescue exercise is now included as routine.

The board concluded: "the flying environment experienced by the crew, ie slow speed, low level flight, on a very dark night over sea, made the response to and handling of any real or perceived major malfunction extremely difficult."

The inquest, conducted by East Somerset coroner Tony Williams, is expected to last two weeks.