Accident Overview
On 7 February 2020, at approximately 14:36 local time, a Piper PA-18 (registration HB-POG) was departing from Raron airfield (LSTA) for a training flight to Trient glacier. The aircraft was operated by Capra-Air Raron. The crew consisted of an instructor and a student pilot. The purpose of the flight was to practice glacier landings.
Pre-Flight and Takeoff
The crew arrived at the airfield shortly after 14:00. The aircraft was removed from the hangar, and the pre-flight checklist was completed on the tarmac. The instructor conducted fuel drain checks on both wing tanks and the engine filter. Both fuel tanks were approximately three-quarters full, so no refueling was performed. The student occupied the front seat, the instructor the rear seat. Engine start and run-up were normal. At 14:35, the student taxied and aligned for runway 28. The takeoff roll was normal, with rotation after about 300 meters. The aircraft deviated right as per the departure procedure.
Engine Failure and Emergency Landing
At a height of approximately 40 meters, the engine lost complete power progressively, without prior fluctuations. The instructor announced 'my controls' and took over, initiating an emergency landing. Because the runway extension was obstructed by buildings, the instructor made a gentle left turn. After about 90 degrees of turn, the aircraft touched down near the threshold of runway 10 on the grassy area adjacent to the runway. The aircraft continued straight toward a small forest about 30 meters from the first ground contact. Facing an inevitable collision, the instructor guided the aircraft between two large tree trunks, which arrested its motion as both wings were torn rearward. The wreckage came to a stop after a few meters. Despite a significant fuel leak, no fire occurred. Both occupants unbuckled and exited unaided after securing the aircraft. The emergency locator transmitter activated; the instructor switched it off to avoid unnecessary search.
Technical Examination
Post-accident examination revealed:
- The tail section had minor damage; controls were functional.
- Both wings were partially torn from the fuselage and folded rearward.
- Fuel lines from tanks and vents were torn; fuel odor was present.
- The left fuel tank leaked due to impact.
- Main landing gear was partially torn and folded beside and under the fuselage.
- Cockpit controls: throttle forward, mixture rich, carburetor heat cold, primer locked.
- Magneto selector found on BOTH.
- Flap selector in first notch.
- Propeller blades showed no leading-edge impact marks; one blade was slightly damaged, the other bent rearward.
- A 'Quick Drain Valve' installed on the carburetor bowl was found in the open position; no fuel sample could be taken. This valve showed no impact deformation.
Engine and System Tests
A full engine test bench run with the Quick Drain Valve closed showed normal operation. Six tests with the valve open and a maximum of 2500 RPM demonstrated that the engine could run at high power, but these tests did not account for propeller slipstream, airspeed, or aircraft attitude during takeoff. Subsequently, three ground tests on a similar aircraft were performed simulating the accident conditions. General results showed the engine could run more or less normally up to 2300 RPM. The first test was interrupted after about 2 minutes due to a restriction in the hose attached to the open valve. The second test revealed RPM variations of about 250 RPM around a maximum static 2300 RPM, appearing after 30 seconds with the valve open. The third test eliminated the possibility of a takeoff with the fuel selector off, as the engine stopped after 7 seconds. Static tests achieved 2300 RPM, compared to the 2500 RPM reported by the crew. It remains open whether the additional 200 RPM during takeoff would have been sufficient to cause engine stoppage. Flight tests were not conducted for safety reasons.
Injuries and Damage
One occupant sustained minor injuries; the other was unharmed. The aircraft was substantially damaged. Minor ground damage occurred.