Accident
On 7 April 2015, at 08:37 local time, an Aero AT-3 R100 (registration HB-SRB) made an emergency landing on a short-grass meadow near Egg, Riggisberg, in the canton of Bern, Switzerland. The aircraft sustained substantial damage, but the occupants—a flight instructor and a student pilot—were uninjured.
Flight Sequence
The flight instructor arrived at Bern-Belp Airport (LSZB) shortly after 07:00 and briefed colleagues and students. The aircraft, prepared for the first flight of the day, had 2 deciliters of engine oil added. The instructor and student, who had no prior flight experience, departed at 08:20 from runway 14 for a training flight under visual flight rules. After takeoff, the instructor handed controls to the student. The aircraft climbed south to a cruising altitude of approximately 5,500 feet QNH near Wattenwil, then turned west.
About two kilometers east-southeast of Rüschegg, the instructor noted engine RPM fluctuations of 200–300 rpm. Despite engaging carburetor heat, the fluctuations persisted. Moments later, the engine stopped completely and the propeller ceased rotating. The instructor took control, initiated a turn toward the northeast, and radioed a mayday call to Bern-Belp tower at 08:33:41. The controller cleared the aircraft for a direct approach, but the instructor replied that returning to the airport was unlikely and that an off-field landing would be necessary. Attempts to restart the engine failed. Realizing insufficient altitude to glide to the Gürbetal valley, the instructor chose to land in the Riggisberg area. The landing on a flat meadow with short grass was described as nearly normal.
Aircraft and Engine
The Aero AT-3 R100 is a single-engine, two-seat low-wing monoplane with fixed tricycle landing gear, manufactured in 2009. The engine was a Rotax 912 S2 four-cylinder horizontally opposed liquid/air-cooled engine, built in 2012, with 788:33 hours since new. The airframe had 1,903:56 hours. The last scheduled 100-hour inspection was certified on 26 March 2015 at 1,900:53 hours. Five flights with 11 landings occurred between that inspection and the accident, with no technical restrictions noted in the journey log. The fuel used was unleaded gasoline 98.
Investigation Findings
The Swiss Transportation Safety Investigation Board (STSB) examined the engine lubrication system. The Rotax 912 S2 uses a dry-sump forced lubrication system with a trochoid oil pump. Oil is drawn from the oil tank through a suction tube, passes through the oil cooler and filter, then circulates to engine bearings. The investigation found that the oil suction screen was missing from its intended location. Additionally, the suction tube inside the oil tank lacked a slot that would allow a removable plate to obstruct the lubrication circuit. The design of the oil tank drain prevented the absence of the screen from being detected during routine maintenance.
These conditions led to an interruption of oil supply, resulting in oil starvation and eventual engine seizure. The investigation also determined that the engine manufacturer's maintenance procedures were not followed, and the mechanic performing the work was under time pressure and a heavy workload.