The importance of maintenance of aircraft in airline operations is emphasised by the fact that, in British European Airways at least, 25 to 30 per cent, of the total direct cost of operating an aircraft is absorbed by maintenance costs. This applies fairly generally to all aircraft in B.E.A.'s fleet, and is also applicable to most of the world's airlines: in the U.S. the proportion is slightly lower, which may be due not to relatively lower maintenance costs, but to relatively higher crew costs there.