A REPORT issued six months before an RAF Nimrod spy plane exploded over Afghanistan, killing 14 people, identified a "critical" problem with the aircraft's structural integrity.

The report by QinetiQ, a private defence company, blamed the age and outdated design of the aircraft and the "intense" way in which it was being flown. The problem involved extensive fuel leaks dating back 10 years and caused by ageing sealants breaking down, the report said.

Despite the problem, the Ministry of Defence pushed back the introduction of the new Nimrod until 2010 and cut numbers from 24 to 12 to save money.

The accident in Afghanistan took place in September 2006. The Nimrod pilot reported a fire in the bomb bay and tried to land but the aircraft exploded in mid-air, killing all 14 on board.

The board of inquiry into the explosion is believed to blame fuel leaks created by pressure from air-to-air refuelling.

Nimrods operating over Afghanistan fly from Oman and refuel in mid-air to stay operational longer. But the pressure caused by pumping in the fuel on repeated occasions was creating numerous leaks.

The QinetiQ report, published in March 2006 and released under the Freedom of Information Act (FOI), said the leaks could not be found when the Nimrods returned to their UK base and as a result were not being fixed.

The report said a civilian contractor carrying out servicing was not required to pass full details of the extent of the fuel leaks to senior officers.

Jimmy Jones, a former RAF engineer who worked on the Nimrod trials in the late 1960s, said this was a major concern. Contracting work out to civilian firms was a mistake, he said. Jones also said the report showed there was no requirement for the civilian team to do the sort of preventive maintenance routine for the RAF. The QinetiQ report said that if senior officers had known the extent of the leaks they would have been able to identify a structural problem that should have been addressed "before it became critical".

Although all Nimrods suffered leaks, the problem was worse in six aircraft fitted with special video equipment, which were the only ones flying the "intense" schedule over Afghanistan. If a leak was found and fixed, "time constraints on releasing the aircraft preclude any follow-on air testing to confirm the leak-free status", the report said.

Documents released under the FOI show that the Nimrod that exploded was one of the two aircraft worst hit by the leaks.

The RAF has now moved one of its three Reaper unmanned aircraft, which has a real-time video capability similar to the Nimrod's, to Afghanistan to ease pressure on the remaining five aircraft.

The Nimrod, introduced in 1969, was based on the Comet airliner built in 1949 "when aircraft were neither intended nor expected to remain in service for this length of time", the report said. The main problem was with the sealant on the aircraft which was deteriorating and was "peeling like masking tape from some areas".

As a result only external leaks could be sealed, making it "almost inevitable" that sealant put on top of the leaks hid corrosion underneath. The RAF primary service team was "highly committed" and working to good standards but had out-of-date instructions and inferior equipment. It was under "huge pressure" to fix the leaks and had suffered "a critical loss of expertise and experience" as a result of technicians leaving the RAF, with a "major impact" on efficiency. The MoD said the safety of all personnel "has been and will remain of paramount importance" but further comment would be unhelpful until the board of inquiry reported.

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