If navigation were confined to the function of keeping a craft on a desired track, and estimating its progress periodically, then a long-range turboprop would present no features of navigational interest. Navigation, however, is supposed to encompass a wider field than this. In a famous sixteenth-century definition, ‘Navigation demonstrateth how, by the shortest good way, by the aptest direction, and in the shortest time, a sufficient ship between any two places may be conducted’. The economic realities of modern airliner operation give a new emphasis to those phrases ‘By the shortest good way, by the aptest direction, and in the shortest time’. A Britannia 310, for example, which will be the first, probably the cheapest, and possibly the smallest, long-range turbineengined airliner, costs about £1 million and is capable of producing a gross revenue of £1000 per hour. The sum of payload and fuel load is limited in most long-range cases by maximum take-off weight, and the fuel for one hour of flight is equivalent in weight to about 2 5 passengers and their baggage. It is not surprising in the circumstances that quite minor refinements of navigational technique are worth tens of thousands of pounds per aircraft per year, while major improvements can alter the status of an aircraft type as an instrument of transport. Such aircraft should be considered as acutely sensitive instruments to be operated precisely according to scientifically designed techniques.