What happened
On April 7, 2015, an Aero AT-3 R100, registration HB-SRB, was conducting a training flight near Bern-Belp when the engine failed during cruise flight. The aircraft, operated by Alp-Air Bern AG, was carrying a flight instructor and a student pilot. While flying near Rüschegg, the instructor noticed significant fluctuations in engine RPM. Shortly thereafter, the engine ceased functioning entirely, and the propeller stopped rotating.
Realizing that returning to the airport was not possible, the instructor performed an emergency landing in a grassy field near Riggisburg. Despite the engine failure, the landing was executed successfully without injuries to the occupants, though the aircraft sustained heavy damage.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) revealed that the engine had seized due to a lack of lubrication. Upon inspection of the engine, investigators found that the connecting rod of cylinder number one had effectively welded itself to the crankshaft, causing the engine to block. The investigation also noted significant metallic debris on the magnetic drain plug and within the oil filter.
Technicians had recently performed a 100-hour inspection on the aircraft. During this maintenance, the oil tank had been removed. Investigators found that the internal baffle (the swirl insert) had accidentally fallen out of the tank during the oil draining process and was not noticed because the funnel used for draining the oil made it difficult to see the tank's interior.
Findings
The primary cause of the engine failure was an interruption of the oil supply caused by the following factors:
- The missing swirl insert created the necessary conditions for the lubrication failure.
- The oil suction pipe in the tank lacked the modern slotted design, which meant the internal baffle could move and block the oil flow.
- The design of the oil draining process prevented the mechanic from detecting that the baffle was missing.
Contributing factors included a deviation from the engine manufacturer's recommended maintenance procedures and high workload and time pressure experienced by the mechanic during the recent maintenance interval.
Safety action
Following the investigation, SUST issued a safety notice regarding the modification of oil tanks. While newer Rotax 912 series engines feature modified tanks with slotted suction pipes and recessed baffles to prevent this specific failure, the notice recommends that operators of older engines retrofit these components as a cost-effective safety optimization.