There can be no question that manoeuvrability is one of the principal requirements for a combat aircraft, and achieving the necessary amount of usable lift is never an easy design task. The lift generated by the wings ceases to be usable when it is accompanied by unacceptable levels of buffet or degraded handling or loss of control. Practically all combat aircraft have to penetrate some way into this region where things begin to go wrong, and the problem we are concerned with in the Royal Aircraft Establishment is to see how certain we can be, before the first flight of a new design, that the so-called buffet boundary will allow the specified manoeuvrability to be achieved. One line of attack is to take an existing aircraft, record and analyse its behaviour in these limiting conditions, and see whether design and test methods are good enough to predict what we find in flight.