At present the rate of failure of a weather radar set is of the order of 1 failure in 50 sector flights. Probably the most reliable unit of the equipment is the scanner, which is fortunate because it is the most difficult unit to locate in an aircraft by virtue of its size and the need for uninterrupted forward scanning. With its present size the installation of two scanners is not feasible. In a piston-engined aircraft a radar failure when confronted by prolonged turbulent cloud conditions means no loss of aircraft range due to the adoption of optimum turbulence airspeed. The serviceability rate afforded by a single equipment is, therefore, acceptable since speed is not critical to performance. In a jet the inability to pick a way around turbulent cells if the radar fails, necessitates, by the adoption of optimum turbulence airspeed and altitude, a considerable shortening of range. It may be that in a supersonic aircraft, an unserviceable weather radar will make the flight abortive, since the ability to become supersonic will depend on smooth flying conditions.