The BBC has seen restricted safety documents relating to military planes
Safety alerts were ignored which could have saved the lives of 31 military personnel in three separate fatal air crashes, the BBC has been told.
File on 4 has heard that before each of the three crashes military chiefs and Ministry of Defence (MoD) officials were made aware of technical problems.
Two Sea King helicopters collided in 2003, a Hercules plane crashed in 2005 and a Nimrod jet exploded in 2006.
The MoD said if its planes were not safe it did not allow them to fly.
File on 4 was shown restricted flight documents, spoke to engineers and air crew and learned of safety breaches where warnings were either ignored or over-ruled.
A fuel leak caused a Nimrod jet from RAF Kinloss in Moray to explode mid-air during operations in Afghanistan, killing all 14 personnel on board on 2 September 2006.
An inquest into the crash in 2008 found that the jet was not airworthy due to a fundamental design flaw - fuel couplings were in the same compartment as hot air piping.
The BBC has seen a restricted flight safety investigation report into an incident two years before that Nimrod crash, in which another Nimrod had a hot air leak - up to 400C - which was effectively boiling a fuel tank.
The station commander stated: "This incident... highlights that it is particularly important for all involved in operating ageing aircraft to be aware of the potential failure in areas not previously subject to inspection regimes."
This report, with seven recommendations, was copied to the Directorate of Air Safety, the MoD and Group Command.
The MoD said six out of the seven recommendations were acted on.
Another report from QinetiQ, one of the world's major defence technology companies, found 26 faults in Nimrod planes which had implications for the jets' "potential airworthiness".
But Air Vice-Marshal Stephen Hillier responded: "As an operational commander I would simply not allow my crew to fly if I felt it was otherwise.
"What the loss of the Nimrod has tragically demonstrated is that there were shortcomings in our understanding of the airworthiness of that platform."
Earlier this year the entire Nimrod fleet was grounded for safety work and the bulk of the fleet is still out of action.
FIND OUT MORE
Listen to File on 4, BBC Radio 4 2000 BST, Tuesday 13 July 2009, repeated 1700, Sunday 19 July 2009.
On 30 January 2005 a Hercules transporter plane crashed in Iraq after an inert object - possibly a single bullet - pierced a fuel tank, causing the wing to explode with the loss of all 10 people on board.
Yet three years earlier pilots and air crew had called for a relatively cheap foam safety device to be fitted to the planes, to stop fuel tanks exploding even if pierced by a bullet.
US versions of the aircraft have been fitted with the device since the Vietnam War in the late 1960s.
Former Squadron Leader Chris Seal told the BBC he had compiled a five-page document outlining problems the Hercules encountered during operations in Afghanistan.
"They had been talking about foam at high levels for some time... sadly I think they should have taken the planes out of service to fit it," he said.
Documents seen by the BBC show that the MoD Procurement Executive drew up specifications to fit in the foam in 1982.
Fourteen military personnel were killed in the Nimrod crash
However Air Vice-Marshal Hillier said: "It wasn't a question of not knowing there was the ability to fit explosive suppression foam, it was a question of prioritisation.
"The way we were using the C130 [Hercules] was not in the way the US was using the aircraft in Vietnam - our assessment of the threats was that this was not the highest priority."
The two Sea King helicopters collided in the Persian Gulf in March 2003, killing six UK military personnel and a member of the US military.
Anti-collision strobe lights - which might have helped the helicopters spot each other - were switched off by the crews in both helicopters, as they interfered with pilots' vision and were not fit for purpose, according to engineers.
A senior MoD engineer, who did not wish to be named, said he had warned in 2000 that there were concerns about fitting these lights and those they were not compliant with safety regulations.
"After my final warning that the design still wasn't acceptable or stable I left the project shortly after and what happened in the next two years I don't know.
"But you get the Board of Inquiry [into the Sea King crash] report from early 2004 which says it was non-complaint so the implication is that regulations were still not implemented properly," he said.
A Royal Navy statement said that the High Intensity Strobe Light (HISL) was fitted to all marks of Royal Navy Sea King and other service aircraft and "as with all aircraft modifications, it has been subject to stringent safety assessment".
The MoD said all of its aircraft "are operated under strict Military Aircraft Regulation with standards and management arrangements that are mandated to be as low as reasonably practicable (ALARP), at least as good as those required by legislation".