By Paul Adams
Defence correspondent, BBC News
The Nimrod aircraft has a history of fuel-related leaks
Fresh fears have arisen about the safety of an ageing model of RAF reconnaissance plane, following an incident in southern Afghanistan.
On Monday a Nimrod surveillance aircraft sent out a mayday call after crew members spotted fuel spraying into an empty bomb bay.
The crew's log, leaked to the BBC, reported the bomb bay doors "to be wet with fuel". The aircraft landed safely.
The MoD says a full investigation into the incident is being carried out.
The log entry says: "The bomb bay heating mixing chamber cladding was soaked with fuel. Fuel was also observed on the pipework on the roof of the bomb bay area."
The incident appears to resemble the circumstances which led to another Nimrod, tail number XV230, exploding north west of Kandahar in September last year, killing all 14 crew members aboard.
The explosion was also preceded by a fuel leak in the bomb bay.
Both Nimrods were in the process of air-to-air refuelling, with fuel passing at high pressure throughout the aircraft and into several tanks.
In a statement, the MoD confirmed there had been a further incident affecting a Nimrod - tail number XV235 - and that after an initial investigation, the air-to-air refuelling system was isolated.
The aircraft has since been flown back to its operating base for further engineering investigation.
An MoD spokesman added: "As a precautionary measure air-to-air refuelling has been suspended for all Nimrod aircraft, until the results of a full investigation have been considered.
"As always the safety of our crews remains paramount."
An official Board of Inquiry into the loss of XV230 is expected to be published shortly and is likely to focus on fuel leaks and air-to-air refuelling.
Defence sources suggest that modified operating procedures introduced after the crash may have helped to avert disaster this time.
But the BBC has seen official documents that reveal the issue was examined and brought to the RAF's attention six months before the loss of XV230.
The defence company Qinetiq was asked to look at the repair of fuel leaks in the lower wing surfaces of Nimrod aircraft.
Qinetiq's experts visited RAF Kinloss in February 2006, where they had access to a number of Nimrods, including XV230.
'Peeling like tape'
Qinetiq's findings, obtained under the Freedom of Information Act, make it clear that the team had grave misgivings about the equipment and methods used to identify and repair fuel leaks on an aircraft under huge operational pressure.
The highly detailed Qinetiq report speaks of the sealant used to maintain joints on wing panels "peeling like masking tape".
It describes repair teams having to use out-of-date manuals and equipment, and the "considerable loss of expertise and experience as trade specialists have left the team".
The report says that in some cases, leaks reported while planes have been flying on operations cannot be detected by the ground maintenance crews.
This crucial point is echoed in this week's incident report, in which the author notes that it was impossible "to replicate reported airborne occurrence" using "current ground testing procedures".
But the Qinetiq report makes observations which point to the one overriding reason why critics argue the Nimrod should not still be flying: its age.
"The recurrence of significant fuel leaks," it says, "may be taken as an indication of the age-related deterioration of the basic sealing systems."
The aircraft, which entered service in the 1960s, was designed to hunt Soviet submarines during the Cold War.
Now, a decade after it should have been replaced, it is being asked to conduct surveillance missions over Iraq and Afghanistan.
It is flying longer hours in harsher conditions than ever before. And John Blakeley, a retired senior RAF engineer, says the resources need to keep it flying safely simply are not there.
"Airworthiness requires funding," he says, "and we come back to that very basic feature that there isn't enough money to do the job properly."
Jimmy Jones, an RAF engineer who worked on Nimrods in the 1960s, says he knows of four fuel leak incidents aboard Nimrods since the crash of XV230, and says that every time he hears of a new one "my blood runs cold".
The Ministry of Defence statement added that "the Board of Inquiry to determine the cause of the tragic loss of Nimrod XV230 and those on board is nearing conclusion".
"The Board's findings will be given to the families of those killed before we make any public comment and until that time it would be inappropriate for us to comment further."
Andrew Brookes, of the International Institute for Strategic Studies, said the Nimrod plays such a vital role in Afghanistan that the RAF cannot afford to ground the fleet.
He told BBC News 24: "I don't think they've got any choice unless it really was falling out of the air."
But Westminster SNP leader Angus Robertson MP, who represents the Nimrods' home base at RAF Kinloss in Moray, said the MoD needed "to get a grip of the situation".
"It is completely unfair to the service community at RAF Kinloss to have this degree of uncertainty about the safety of the Nimrod fleet," he said.
Graham Knight, the father of Ben Knight who died in the XV230 explosion, said his son's death is even harder to bear because he feels it could have been averted.
He said: "It would have been different I think if they had been shot down...that would still be a tragedy, but it would be a tragedy caused by the insurgents.
"This appears to be a tragedy caused by incompetence."