Aircraft in the RAF's fleet urgently need replacing, MPs and armed forces sources have told the BBC.
The government is upgrading the RAF's fleet of Hercules aircraft
The aircraft, which are currently being used for operations in Iraq and Afghanistan, are increasingly breaking down, they have warned.
Some VC10s and Tristars are more than 40 years old but are being used at full stretch, they said.
Air Vice Marshall Iain McNicoll said he was content that the RAF's air transport fleet was safe.
This month a VC10 tanker had to make an emergency landing in Qatar after it began losing cabin pressure because of a hole in the nose of the aircraft.
In September, a Tristar full of troops left the Afghan capital Kabul with a broken window and had to put down in Cyprus for repairs.
Conservative MP Adam Holloway said: "We're left with a fleet of frankly knackered old 1960s, 70s and early 80s aircraft and, like an old car, an old aeroplane needs much more maintenance."
The government is undertaking some upgrades and has ordered explosive-suppressant foam for all RAF Hercules aircraft.
But so far just two out of 49 Hercules aircraft have been fitted with the potentially life-saving modification.
Two Hercules aircraft have already been lost, one in Afghanistan and one in Iraq.
Ground crews are working flat-out trying to keep the aircraft operational but often they are grounded because of serious faults, BBC correspondent Paul Wood reported.
There have been long delays for ground troops going home and complaints have been posted by troops on military websites, our correspondent said.
One wrote: "The air transport force needs sorting out pronto."
Another said: "As soon as you come into contact with the RAF, everything grinds to a halt."
One source told the BBC some aircraft had clocked up the equivalent of three years' worth of flying time in just six months.
AVM Iain McNicoll said: "There is no such thing as risk-free flying and certainly there's no such thing as risk-free military aviation.
"These are demanding operations that people are engaged in. They are being flown to the limits but up to the limits, not beyond the limits.
"I'm content that what's being done with air transport force is safe."