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The major difficulty with the purchase arose when the MoD discovered that it had neglected to include in the contract a clause that would provide access to the source codes for the highly complex software. Without them, RAF specialists were unable to check whether the adapted helicopters passed Britain’s strict airworthiness criteria.
Boeing was reluctant to hand over the codes since no request had been made for them in drawing up the contract. So the RAF said that the Chinooks could not be flown except in the most clement weather. The sky had to be cloudless and the pilots would have to operate from at least 500ft so that they could navigate by landmarks