Since the early nineteen-seventies, when the price of fuel went up so dramatically, airlines have been seeking methods of saving fuel. Climb, cruise and descent speeds were refined to give the most economical operation. Advisory systems came on the market, but these did little more than give crews information which could be obtained from operations manuals. Then equipment such as performance management systems (PMS) and flight management systems (FMS) was produced. The potential of these systems can be measured by the number of airlines who have purchased the equipment, and also by the fact that most of the new generation of aircraft are being built with PMS as part of the standard aeroplane.