What happened
On 12 August 2011, an Aero AT-3 R100, registration HB-SRA, was conducting a VFR training flight near Worb, Switzerland, when the engine lost power. The flight, which involved a flight instructor and a trainee pilot, was performing simulated aerodrome circuits over the A1 motorway. During the flight, the engine began to stutter and power decreased until it eventually quit completely.
Upon the engine failure, the flight instructor took control of the aircraft and initiated an emergency landing. The crew initially attempted to reach a preferred field, but a high-voltage power line prevented them from doing so. They instead targeted an alternative field in the Worbboden area. During the landing roll-out, the aircraft's right wing struck a sunflower field, causing the plane to rotate and sustain heavy damage. Both occupants escaped the aircraft without injury.
The investigation
The Swiss Accident Investigation Board (SAIB) examined the aircraft's fuel system and the circumstances leading to the fuel exhaustion. The investigation found that while the crew had used a dipstick to estimate the remaining fuel prior to takeoff, the measurement was inaccurate. The investigation also reviewed the manufacturer's technical documentation and the physical state of the fuel sensor and tank capacity.
Findings
- The primary cause of the accident was an emergency landing resulting from engine failure due to fuel exhaustion.
- The aircraft's fuel tank capacity was actually 66 litres, which was less than the 73.5 litres stated in the manufacturer's documentation.
- The fuel measurement sensor had significant design flaws; specifically, the float could become stuck on the connecting pipe, causing the cockpit gauge and the fuel reserve warning lamp to provide false readings.
- The dipstick used for manual checks was misleading, as it could indicate up to 10 litres more than the actual fuel present.
- The amount of fuel carried for the planned 40-minute training flight was insufficient for the mission.
- The aircraft's flight manual had not been updated to the most recent revision.
Safety action
Following the investigation, a safety recommendation was issued to the European Aviation Safety Agency (EASA) to review and correct the fuel quantity indication system design and manufacturer information for the entire Aero AT-3 R100 fleet. The Federal Office of Civil Aviation (FOCA) subsequently conducted a fleet inspection to ensure owners were aware of these technical deficiencies.