History of Flight
The aircraft, a Piper PA-34-200T (registration G-BFLH), was conducting a training flight from Bristol Airport to Oxford (Kidlington) Airport on 17 January 2002. The commander, a 60-year-old commercial pilot with instructor ratings, occupied the right-hand seat and was instructing a student in the left-hand seat. A second student, who had flown the previous leg, sat in a rear seat.
After takeoff from Bristol, the student pilot flew an ILS approach followed by a go-around. During the go-around, the instructor simulated an engine failure by closing one throttle. With the landing gear retracted, the gear warning horn sounded. The horn is designed to emit a beeping sound when the landing gear is retracted and one or both throttles are retarded aft of a position corresponding to 14 inches manifold pressure. The student completed relevant touch drills, after which the instructor increased the engine's manifold pressure to 14 inches to simulate a feathered propeller. The horn continued, and the instructor found it loud and obtrusive. After the drill was finished, normal power was restored and the flight continued to Oxford.
On joining the hold at Oxford, the instructor simulated another failure on the same engine. Again, when increasing manifold pressure to 14 inches, the horn remained active. To silence the horn and make his comments clearly audible, the instructor pulled the landing gear warning circuit breaker. This circuit breaker also controls the landing gear control circuit, disabling the hydraulic extension and retraction motor as well as the gear position indicator lights. In this condition, the gear could not be raised or lowered using the normal selector.
During the subsequent NDB approach, go-around, and visual circuit, the instructor was busy coaching the student and looking out for other traffic. He reported that he failed to reset the gear circuit breaker, and the landing gear was not lowered—a fact missed by both pilots during landing checks. The aircraft made a wheels-up landing, touching down about one-third of the way along the runway and coming to rest at the edge, midway along its length. All three occupants vacated the aircraft uninjured.
Technical Details
The gear warning horn is activated by a micro switch in the throttle quadrant, which operates on throttle movement rather than manifold pressure. The switch position can be adjusted fore and aft to correspond to a throttle position equivalent to 14 inches manifold pressure, with a tolerance specified in the maintenance manual of 14 + 2 inches. On occasion, the micro switch becomes misaligned, causing the horn to sound even when manifold pressure is at or above 14 inches.
Post-Event Actions
The training organisation involved issued a notice to all instructors stating that the landing gear circuit breaker should not be pulled if the gear warning horn sounds with manifold pressure at 14 inches or more. Instead, such problems should be reported so that the micro switch position can be checked and adjusted if necessary.