Although the aeroplane itself is the king pin of aerial spraying, without the staff to sell the proposition and the ground personnel to organise and service it, the aeroplane could not operate efficiently and economically.

Aerial spraying is undoubtedly the most dramatic operation which contractors undertake on farms in the United Kingdom. It is the one about which the general public are most aware because in the first place they are affected more by this operation in areas near farms than by other contract operations; and second if there is any untoward incident with an aeroplane they hear about it on television and read about it in the newspapers. If a contractor has an accident with a sugar beet harvester or a combine it is hardly newsworthy.

Three basic problems encountered in the use of propellers on hovercraft are erosion, corrosion and noise. The tip velocity of a 9 ft diameter hovercraft propeller can exceed Mach 0·8, giving rise to fairly high noise levels. When operating over shallow water the propeller may run in a fine suspension of sand and sea water, which at these high speeds is akin to operating in a highly corrosive vapour blast cabinet.

The subject of my paper is the programme for a British advanced ATC system which is known by the code-name Mediator. The name has no particular significance. But before I tell you the history of its development—and this development is still far from complete—and before I go through the various problems which bedevil all such large scale and imaginative plans, I should tell you one or two things about the air traffic situation in the UK. This differs slightly from that in other countries, and needs to be understood from the outset.