Incident Overview
On June 27, 1999, at approximately 18:10 local time, a Piper PA 28-236 "Dakota" with registration HB-PNF crashed into a cornfield in Seebli, within the municipality of Lupfig in the canton of Aargau. The aircraft was operated by a private pilot who had departed from Birrfeld Airport on a Visual Flight Rules (VFR) circuit flight with three passengers aboard. Due to deteriorating weather conditions, the pilot decided to abort the planned sortie and perform a local traffic pattern before landing.
Operational Sequence
The initial approach proceeded normally until the pilot initiated a go-around maneuver because another aircraft was occupying the runway. During the subsequent turn onto crosswind departure, the stall warning horn sounded. Despite maximum engine power and a climb attitude, the aircraft began to descend. The collision with the terrain could not be avoided. Witnesses on the airfield reported that the initial go-around appeared normal and was executed with full power.
Environmental Conditions
A significant weather warning had been issued for the region earlier that day, predicting thunderstorm gusts of 25–29 knots ahead of a cold front. At the time of the accident, strong shower and thunderstorm rain began around 18:00. The investigation indicated that a thunderstorm cell approaching from the south-southwest created a downdraft zone. As the aircraft turned into crosswind departure, the wind component shifted from a cross/tailwind to a significant tailwind of up to 40 knots. This sudden change reduced the aircraft's airspeed relative to the environment by more than 20 knots.
Investigation Findings
The Swiss Federal Office for Civil Aviation (BAZL) determined that there were no technical defects in the aircraft that contributed to the accident. The engine was found to be producing power at the time of impact, and all control surfaces were free and functional. The reduced engine power setting observed at the time of the crash corresponded to the throttle position, indicating cockpit input rather than mechanical failure.
The probable cause was identified as the pilot's underestimation of the thunderstorm-induced downdrafts and the sudden change in wind direction. The combination of the strong tailwind and the downdraft caused the aircraft's airspeed to drop below the lower limit of the green arc on the airspeed indicator. Consequently, the climb performance was insufficient to overcome these adverse aerodynamic effects, leading to a stall and subsequent ground impact.