There is an old saying in the aircraft industry that you can make a brick fly, if you give it enough power. How true this is I would not care to say, but it is certainly true that the worries of the designer are lessened if he knows that he can always give the throttle a little extra push, if all else fails.

This is not to say that the design of a conventional, high powered, aircraft is a simple, hit or miss affair, that would be ridiculous, but it does serve to show the special difficulties faced by the builders of a man-propelled aircraft who have less power available to them than is used by a small electric fire.

The problem as to which engine configuration—single or twin—is most suited to the close support/tactical strike class of aircraft has faced both designers and Air Force personnel for many years. The inherent human choice on safety grounds is two engines. But, many aircraft built today are powered by one engine and good reasons for this choice have been produced. The basic arguments for one engine are the lower initial cost and better maintainability.

A comprehensive view of the overall costs incurred by selecting one layout as opposed to the other is a rarity, and yet it is possible to produce a definite indication, albeit by a broad brush, as this paper will show.

Head-up display (HUD) is an electronic presentation of flight information projected into the pilot's line of sight, enabling him to see both the outside world and relevant instrument information at the same time (see Fig. 1).

Using HUD the pilot no longer needs to alternate his attention continually between his cockpit instruments 2-3 ft (0·6-0·9 m) away and the outside world effectively at infinity. This continual change which present day pilots must make is first, very tiring and secondly it takes time; time to scan instruments, assimilate the relevant information, move the head and eyes, refocus the eyes on the outside world, adjust perhaps to a different light level and then take in the changed surroundings before returning to check the instruments. This time may add up to several seconds. In normal flight this is acceptable.