1964 Bücker Bü 131 Jungmann ground accident at Beromünster (HB-USL)

Accident overview and investigation status On 6 September 1964, a Bücker Bü 131 Jungmann with registration HB-USL was damaged while taxiing on the ground at Beromünster airfield in the canton of Lucerne. The Swiss Federal Accident Investigation Commission opened a preliminary investigation on 22 September 1964. The report was approved on 30 November 1964, with four amendments, and circulated in December 1964. ## Aircraft, occupants, and site The aircraft was owned and operated by Flugsportgruppe Zürcher Oberland, based in Pfäffikon, Zurich. Its maximum weight was 670 kg, and its airworthiness certificate, issued on 8 May 1963, was valid until 3 June 1965. The pilot, born in 1924, held a private pilot licence issued on 2 August 1962 and valid until 24 April 1966. His experience was 88 3/4 flight hours and 459 landings, including about 7 hours on the Bücker Jungmann. A second person was listed as a “co-pilot.” The accident occurred at the airfield’s parking area, with coordinates 657.950/227.000 and an elevation of 650 m above sea level. ## Sequence of events During an air day at Beromünster on 6 September 1964, the aircraft was parked outside its usual parking area because the airfield was heavily used. Around 1600 MEZ, the pilot prepared to taxi. A ground official gave him signals that the pilot interpreted as instructions to taxi straight toward the taxiway. The official then left. The pilot taxied slowly toward the taxi zone, which was marked with yellow flags. The aircraft suddenly pitched forward, the propeller struck the ground and broke. After exiting, the pilot found that the left wheel had struck a drainage pipe protruding about 12 cm from the ground, and that the left landing-gear strut had been bent backward. ## Damage and findings There were no injuries. Aircraft damage consisted of a bent left landing-gear strut and a broken propeller, with repair costs estimated at 2,000–3,000 Swiss francs, equivalent to 10–15 percent of the aircraft’s value. There were no ground damage. The investigation found that the incident was mainly due to a misunderstanding between the pilot and the ground official, whose signals intended to warn of a ground obstacle were interpreted as taxi clearance. The use of the same colour for taxiway and obstacle markings was considered potentially confusing. The flags were also judged to have been poorly conspicuous as an obstacle marking. The pilot’s failure to check the area immediately in front of the aircraft during an unmonitored taxi was also identified as a contributing factor.