MPs have criticised the Ministry of Defence (MoD) for the £24m cost of mothballing half the Army's fleet of new WAH-64 Apache attack helicopters.
The Army's Apache fleet will eventually total 67
The Commons Public Accounts Committee said the four-year delay in introducing the £4bn fleet was caused by the award of a separate contract to train pilots.
MPs said the "serious mistakes" meant the Army lacked £1.2bn of helicopters.
Last year, it was revealed 40 of the 67 aircraft would have to be stored because UK pilots take longer to train.
Top speed: 162 mph
Range: 285 miles
Engine: Rolls Royce
Weapons: 16 Hellfire missiles, 76 Rockets (2.75"), 1200 cannon rounds (30mm), 4 air to air missiles
The committee said the decision by the MoD to award a separate contract - under a private finance initiative (PFI) - to train the pilots was supposed to save £22m but turned out to cost an extra £24m.
The training programme was delayed partly because the government did not realise pilots in the UK need more time than pilots in the US, because there are fewer clear flying days.
Committee chairman Edward Leigh said: "As a result of serious mistakes by the Ministry of Defence in introducing the Apache, the Armed Forces have not yet been able to benefit from the enhanced operational capability that the helicopter will provide.
"Over £24m of taxpayers' money has been wasted on additional costs and 40 helicopters, worth over £1.2bn, will sit idle.
"In introducing future capabilities, the MoD must put in place much clearer programme management arrangements and realistic deadlines."
A National Audit Office report last October revealed that dozens of the new fleet would have to be kept in storage for four years due to the pilot shortage.
The NAO report warned that although the Apaches were being delivered on time, a private finance initiative (PFI) deal to train aircrew was three years late.