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Aircraft crash fires differ in many ways from other flammable liquid fires, such as those in petrol storage tanks. The time element is usually critical in that people must be rescued at the earliest possible moment, and the accent therefore lies in the rapid reduction of the intensity of the fire rather than on its extinction. It is unlikely that there will be any appreciable area of free petrol surface, and in order to obtain rapid coverage of the aircraft crash and the surrounding terrain, the foam must be applied directly to the burning areas. Foam may be applied either as a jet or a spray, and opinions of operational officers differ on the relative merits of the two methods. While many tests have been made with sprayed foam on large spill fires, there is little information on the application of foam as a jet to this type of fire. An initial programme of tests was therefore planned to see whether there was any marked advantage in using one method of application in preference to the other, and at the same time the effects of varying the expansion and critical shearing stress of the foam were investigated.