1965 Chipmunk Accident at Birrfeld (HB-TUA)

Accident Overview and Investigation Status On 22 October 1965, a D.H. Chipmunk HB-TUA operated by Fliegerschule Birrfeld made an emergency landing at Birrfeld Airport. The aircraft was a D.H.C-1 Chipmunk Mk 22 equipped with a Gipsy Major engine rated at 147 PS and in a weight class up to 2000 kg. Its airworthiness certificate, issued on 30 March 1963, was valid until 2 April 1966. The preliminary investigation was opened on 3 November 1965, and no separate investigation was conducted by cantonal authorities. The final report was approved with four redacted changes and circulated on 20/29 December 1965. ## Occupants and Aircraft The aircraft carried two occupants: a flight instructor born in 1930, holding a commercial pilot licence issued on 15 January 1964 and valid until 13 December 1965, with 733 flight hours and 4,791 landings; and a student pilot born in 1933, holding a student pilot licence issued on 27 August 1965 and valid until 23 August 1967, as well as a lapsed Australian Commercial Pilot Licence. The student had 236 hours in Australia, including 10 hours at night and 13 hours of IFR training, a five-year interruption, and 2 hours and 13 landings in Switzerland. ## Flight Sequence and Accident The flight was a training flight that departed Birrfeld at 1515 MEZ. The purpose was emergency landing practice near the airfield. At 1613 MEZ, the aircraft made an emergency landing at Birrfeld. During the exercise, the instructor and student flew dual controls. On the opposite side to runway 08 at 700 ft above ground, the instructor engaged the carburettor preheat and then reduced power, instructing the student to select a suitable field other than the airfield and to make the approach. The instructor indicated a stubble field across from the airfield and specifically mentioned a temporary electrical line at its end toward the runway. At 3–4 m above ground, the instructor ordered a go-around and also advanced the throttle to avoid delay due to the obstacle. About one second after full throttle, the engine began to stutter and did not reach normal power. The instructor pulled the aircraft slightly up to clear the line, and because the engine still did not produce sufficient power, he decided to turn immediately and land at the airfield. This required a left turn of 70° with 10° bank at 5–6 m height. The low-wing aircraft touched the ground with its left wingtip during the manoeuvre. ## Damage and Weather There were no personal injuries. Damage to the left wing included four broken ribs, a dented wingtip skin, a bent aileron, and a bent pitot tube. Estimated repair costs were about Fr. 1,500, representing about 5% of the aircraft’s value. There were no ground damages. Weather at the time included a closed high fog layer with a base of 850 m, temperature about 4°C, visibility 4–6 km, and wind 060/3 kts. The report stated that weather probably had no influence on the accident. ## Discussion and Probable Cause The report noted that the engine ran normally during the taxi-back after the emergency landing, making carburettor icing unlikely. However, a later test run, in which the mixture control lever was pushed forward together with the throttle, produced the same stuttering as during the aborted emergency landing exercise. The mixture control lever was located on the same console as the throttle and ran parallel to it. It appeared possible to the occupants that the student had inadvertently pushed the mixture lever forward with his sleeve when applying full throttle, but this could not be confirmed after landing because the mixture lever is mechanically pulled back with the throttle. The report stated that, given the unfavourable conditions of low altitude and an obstacle, the instructor’s decision and conduct were correct and the safest option for the occupants and aircraft. The chosen turn could not have been made with a smaller bank angle due to the very limited space, and an earlier turn or a larger turn path was not possible because of the hangar and electrical line. The instructor had to clear the line on the shortest path with limited engine power, resulting in an approach to the airfield at nearly 90° to the runway. The accident was attributed to a strong loss of engine power during the go-around from an emergency landing exercise, probably due to inadvertent actuation of the mixture control lever, which forced the instructor to make an effective emergency landing under unfavourable conditions.