What happened
On May 19, 1985, at approximately 16:40 local time, a Piper PA-38-112 Tomahawk (HB-PHI) operated by the Flugsportgruppe Zürcher Oberland attempted to take off from Runway 05 at Langenthal airfield. The pilot, accompanied by his nine-year-old son, had landed earlier that day but decided to advance their departure due to an approaching thunderstorm.
During the takeoff roll on the grass runway, the aircraft accelerated normally for a short distance. However, as the pilot approached the midpoint of the runway, he realized the remaining distance was critically short. In the final third of the runway, with indicated airspeed near 60 knots and engine RPM at 2200, the pilot attempted to rotate. The aircraft barely lifted off before the stall warning activated. Recognizing the inability to climb, the pilot reduced power and applied wheel brakes.
The aircraft failed to stop on the grass field beyond the runway threshold. It crossed an open agricultural field and a farm track before the pilot initiated a sharp left turn to avoid a stream. The right main landing gear struck the two-meter drop-off of the bank bordering the stream, causing the aircraft to overturn and be destroyed. Both occupants exited the wreckage unaided without injury. No fire occurred, though the local fire brigade extinguished leaking fuel.
The investigation
The preliminary investigation was led by Hans-Ulrich Jordi and concluded in August 1985. Investigators examined the pilot's qualifications, aircraft maintenance records, and physical evidence at the scene.
The pilot held a valid private pilot license with 38 total flight hours, including 34 hours in the accident model within the preceding 90 days. The aircraft was airworthy, and its weight and balance were within limits. Weather conditions included variable winds around 5 knots and an approaching thunderstorm from the southwest.
Crucially, post-accident examination revealed that the engine's spark plugs were heavily fouled with lead deposits. A comparative test using a similar aircraft type under comparable conditions demonstrated that lead-fouled spark plugs resulted in significantly reduced engine acceleration, losing approximately 40-50 RPM of power output.
Findings
The accident was caused by a combination of factors that prevented the aircraft from stopping within the available runway length:
- Insufficient Acceleration: The lead-fouled spark plugs reduced engine performance, preventing the aircraft from accelerating adequately for takeoff.
- Runway Surface Resistance: The pilot failed to account for the increased rolling resistance offered by the grass runway, which can extend takeoff rolls by 30-50% compared to hard surfaces.
- Late Decision to Abort: The decision to abort the takeoff was made too late in the roll. By the time the pilot reduced power and applied brakes, there was insufficient distance remaining on the grass field to stop before the ditch.
Contributing factors included a probable tailwind component during the takeoff phase and the pilot's relatively low experience level with this specific aircraft type.
Safety action
The investigation highlighted that standard performance tables for the PA-38 only account for dry, horizontal hard-surface runways. Operators must apply significant safety margins when calculating takeoff distances on grass strips. Additionally, pilots are advised to verify wind conditions immediately prior to takeoff if radio reports are unavailable.