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.