Introduction
On 20 July 2021, at approximately 20:24 local time (UTC+2), a Bücker Bü 131B Jungmann, registration HB-UVC, crashed in a field at Altwegacker, Subingen, Switzerland. The aircraft was destroyed, the passenger sustained fatal injuries, and the pilot serious injuries. The flight operated under visual flight rules (VFR) as a private flight from and to Langenthal Airport (LSPL).
Aircraft Information
The Bücker Bü 131B Jungmann, serial number 53, was manufactured in 1939 by Dornier Werke in Altenrhein. Originally fitted with a Hirth HM-504 A2 engine, it was later converted to a Walter Minor 4-III engine in 1979. The aircraft had a history of a previous accident in 1981 due to an engine failure, resulting in substantial damage. At the time of the accident, the owner and the registered holder were private individuals. The pilot had purchased the aircraft in July 2017.
Pilot Background
The pilot, a Swiss national born in 1978, held a Private Pilot Licence (PPL(A)) issued by the Federal Office of Civil Aviation (BAZL). He had a total of approximately 1016 flight hours, including 21 hours on the accident type. He had no aerobatic training nor upset recovery training. He had never performed a spin recovery in practice, only theoretically. The pilot commenced type familiarization on HB-UVC on 26 June 2020 and completed it on 2 July 2020 after 5 hours and 7 minutes of flight time.
Flight History
On the afternoon of 20 July 2021, the pilot and the passenger prepared the aircraft for a sightseeing flight to the Biel/Gruyère region. After a full refuel, the pilot conducted a local circuit flight without the passenger around 16:30. During landing roll, the engine stopped and could not be restarted; the aircraft was pushed to the hangar. Around 17:30, the engine was restarted, and the aircraft taxied to the holding point for runway 05. At approximately 18:00, HB-UVC departed from Langenthal, heading west.
Accident Sequence
Due to retrograde amnesia, the pilot had limited recollection of most of the flight. He recalled that during the approach to Langenthal, he handed control to the passenger to prepare approach documents on his kneeboard. According to the pilot, the passenger then increased power, lowered the nose, and then pulled up steeply, apparently attempting a loop. The aircraft entered a rotation around its vertical axis and lost altitude rapidly. The pilot took over controls, which felt soft and ineffective, and attempted to recover from the spin using the theoretical technique of neutral forward stick and full rudder opposite the rotation. However, a video recording showed that the engine was not at idle but delivering increased power during the spin. At approximately 20:20, multiple witnesses observed the aircraft in a left spin over Subingen. The aircraft impacted the ground at an elevation of 411 m AMSL on a field, resulting in fatal injuries to the passenger and serious injuries to the pilot.
Investigation
The Swiss Transportation Safety Investigation Board (STSB) conducted the investigation. The emergency locator transmitter (ELT) did not activate upon impact. No probable cause was explicitly stated in the report.