New concerns are being raised about the safety of the RAF's Nimrod spy planes, following an investigation by Panorama.
Air-to-air refuelling was suspended
One of the aircraft crashed in Afghanistan last year, shortly after air-to-air refuelling, killing all 14 people on board.
There have been two serious mid-air incidents since the crash, both after refuelling, the BBC programme found.
The Chief of Air Staff, Air Chief Marshal Sir Glenn Torpy, has said he is confident mid-air refuelling is safe.
There is still no official cause for last September's crash, but shortly before the plane came down a fire broke out on board after it had refuelled in mid-air.
Panorama has uncovered two more events, in November and December of last year, both involving fuel leaks that happened after air-to-air refuelling.
A former RAF engineer, Jimmy Jones, said he believed at least one of those leaks was so serious it could have brought the plane down.
Sir Glenn confirmed there had been 25 fuel leaks on Nimrods in the five months up to March, but said he was satisfied air-to-air refuelling is safe.
"We did suspend air-to-air refuelling for a period in November," he told Panorama.
Ex-engineer Jimmy Jones said a fuel leak last year was serious
"We looked at what we were doing, both in terms of integrity of the system, the way we were actually conducting the air-to-air refuelling, and I'm satisfied that the way we're now doing is as safe as it needs to be."
But a member of a Nimrod crew, who has not been named, said there were more problems than ever before with the aircraft.
"All sorts of equipment has failed, we've had problems with engines that we've had to shut down, fuel leaks are on the increase."
The families and friends of the 14 British servicemen who died in last year's crash - 12 of whom were based at RAF Kinloss on the Moray coast - have also expressed concerns about the safety of the rest of the Nimrod fleet.
Rayna Quilliam, whose husband Gary was killed, said it is important to know whether the deaths could have been avoided and if another catastrophe could be prevented.
"Gary joined the Air Force to serve his country, he was probably one of the most patriotic people I know," she said.
Rayna Quilliam's husband Gary died in last year's Nimrod crash
"He would have willingly put his life on the line for his country - to do his job - and that's what he did do. But, if this was a technical fault - he was not supposed to die as a result of a technical fault."
Laura Robson, girlfriend of 26-year-old co-pilot Steve Swarbrick, said he had been concerned about some of the planes he was flying.
She said: "He was always complaining that the flights were always delayed because of maintenance issues. He was always saying that there was going to be an accident, a serious one."
The Armed Forces minister, Adam Ingram, said safety had never been compromised.
The Nimrod was designed as a sea patrol and anti-submarine aircraft and entered service with the RAF 30 years ago.
Panorama: On a Wing and a Prayer is due to be broadcast on 4 June at 8.30pm