What happened
On July 5, 2006, a Bücker 131 Jungmann, registration HB-UUT, was performing a ferry flight from Langenthal/Bleienbach to Granges/SO. The aircraft had recently undergone extensive structural repairs and a complete repaint following a 20-month overhaul. After completing several successful test flights earlier that day, the pilot proceeded to Granges for landing.
While established on the final approach for runway 07L, the aircraft's descent profile became significantly steeper. Approximately 235 meters before the runway threshold, the aircraft's lower right wing struck an 8-meter tall light pole located at a nearby roundabout. The impact caused the aircraft to crash onto its back in a rapeseed field. The pilot sustained serious injuries during the impact.
The investigation
Investigators examined the aircraft's engine, fuel system, and the pilot's medical status. The investigation revealed that the engine had been in storage for 21 years prior to being installed in the aircraft. Analysis of the wreckage showed that the engine was operating at low power at the time of impact and exhibited signs of an incipient seizure. Furthermore, the fuel sampled from the tanks contained lead levels significantly higher than the maximum allowed for AVGAS 100LL.
Regarding the approach, video footage of the day's previous flights indicated a pattern of a flat approach angle. While the light pole was positioned 2 meters below the ICAO-defined approach plane, the investigation focused on why the aircraft descended below the safe obstacle clearance altitude.
Findings
- The primary cause of the accident was a collision with an obstacle resulting from a final approach conducted at too low an altitude.
- The pilot was not wearing his prescribed multifocal corrective lenses, using only non-corrective wind goggles instead.
- The engine was experiencing the onset of a seizure, and high temperatures were noted near the number 2 cylinder.
- The fuel used contained excessive lead content.
- The aircraft's emergency locator transmitter (ELT) failed to activate because its deployment mechanism had remained locked following the recent overhaul.