During the climb at about 120 feet above ground level, the engine stopped suddenly. The student promptly lowered the nose of the aircraft to maintain flying speed. Almost immediately, the instructor took over and increased the nosedown attitude in an attempt to increase the airspeed. However, he had to pull back on the stick almost straight away to attempt a flare for landing. The aircraft had not gained sufficient airspeed to fully respond and mushed into the ground with a heavy impact. The first witness on the scene to render assistance noticed that the ignition " kill " switch was in the OFF position although both pilots were adamant they had not switched it off as a deliberate action after the crash. Subsequent inspection of the engine by a Civil Aviation Authority engineer and a test run of the engine by the company engineer failed to find any reason for the engine failure. Further experimentation, however, showed that the " kill" switch could be placed in a mid-position which would allow the engine to start and then the switch could move to either the ON or OFF position by vibration. Although the actual cause of the engine failure could not be established, it is probable that the ignition " kill " switch was placed in the OFF position inadvertently or vibrated into the OFF position from a mid-selection position.