INVESTIGATORS are focusing on metal fatigue as a potential cause of the crash of an RAF Hercules transport plane in Iraq that killed all 10 servicemen on board.

They believe structural failure caused by the fatigue may explain why the right wing of the C-130 Hercules appears to have snapped off in mid-flight, causing the plane to spin uncontrollably to the ground.

The crash north of Baghdad on January 30 was the worst fatal incident for British forces in Iraq since the invasion of the country in 2003.

No conclusions have yet been reached, but a source close to the investigation said no evidence had been found in the wreckage to suggest the wing had been blown off either by a missile or explosives on board. This leaves wear and tear on the frame of the plane as the emerging focus of attention.

“It’s hard to believe there might be a structural failure after all the close monitoring we do, but there is a history of surprises about metal fatigue,” said the source. “Planes do snap sometimes. You can’t always tell when.”

Fatigue problems have been found in other Hercules. About 30 planes have been grounded in the United States after cracks were found in the wing structure. Restrictions have been placed on a further 60.

It is understood cracks have been found in one aircraft where the wing joins the fuselage. British investigators are also understood to be examining evidence from the crash of a Hercules in America in 2002 in which metal fatigue caused a wing to break off in mid-flight.

A similar problem occurred two years ago in Canada, when the country’s air force grounded its Hercules after cracks were found in spars joining the wing to the fuselage.

In the past few days the Ministry of Defence (MoD) has taken the unusual step of calling in the civilian Air Accidents Investigation Branch to help find the cause of the crash of the RAF plane, built in 1967.

Officials said that if the collapse of a wing was verified, about 25 older C-130s would have to be grounded. “A wing spar problem must be at the top of the probability list in the absence of anything else,” said David Learmount, a former C-130 pilot and now operations and safety editor for Flight International magazine.

“I am surprised the RAF has given no indication about what caused this. It’s most unusual.They get a lot of tough work in tough places, including stressful desert landings,” he added. “If you keep bending a piece of metal, even if it’s designed to be bent a bit, like a coat hanger, it will eventually break.”

Other aviation sources said there may be legal implications for the MoD, the manufacturer of the aircraft and its maintainers if it is shown that cracks in the wings were avoidable or went unnoticed.

The most dramatic evidence that C-130s could be vulnerable to such problems occurred on June 17, 2002, after an American civilian firefighting C-130 had dropped its load of flame retardant on a forest fire in California.

A video taken from the ground shows the right wing suddenly folding upwards from the fuselage, with burning fuel spilling from ruptured tanks and the plane crashing in a fireball.

Other sudden losses of Hercules aircraft include a US Marine Corps KC-130 air refuelling tanker that crashed in Pakistan in January 2002 with the loss of seven lives. Some reports said a wing was some distance from the bulk of the debris. The Pentagon said this weekend that two other Hercules crashes had been attributed to metal fatigue.

The RAF emphasised that it had an excellent safety record and said the Hercules fleet is routinely and continually monitored for metal fatigue.

A defence ministry spokesman said the investigation had reached no conclusions yet and that if the president of its board of inquiry had any safety concerns he would already have grounded the fleet of Hercules. He added that the wings of the British aircraft were built differently from the grounded American planes.

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