What happened
On September 12, 1987, a female pilot embarked on her initial instructional flight in a Glaser-Dirks DG-400 motor glider, registration HB-2093, at the Birrfeld aerodrome in Switzerland. The aircraft had been prepared for the flight with necessary ballast to maintain center-of-gravity limits, and the pilot held a valid sailplane license with prior experience on different motor glider models.
Following a successful launch by self-start, the pilot flew away from the airfield. Communication was initially lost on the pre-selected frequency but was re-established on the common traffic advisory frequency when the instructor noted her low altitude. He instructed her to return immediately and advised caution regarding strong southwesterly winds. The pilot returned without sufficient height reserve and began her approach toward Runway 26 (grass). Witnesses observed her flying very slowly in the base leg. Due to the strong crosswind, she drifted past the runway centerline into the approach corridor of the parallel paved runway. To correct her trajectory, she executed a steep turn at approximately 50 meters above ground level.
During this maneuver, the aircraft entered a spin and crashed onto an adjacent meadow. The pilot was killed instantly upon impact. The airframe was destroyed, though there was no fire or significant third-party property damage.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the flight data, wreckage, and operational context. Key findings included:
- The aircraft's airspeed indicator had been incorrectly marked for a DG-300 model, showing a yellow arc (recommended approach speed) at 90 km/h instead of the correct 100 km/h for the DG-400.
- The pilot’s personal checklist listed an approach speed of 95 km/h, contradicting the flight manual's requirement of 100 km/h.
- Weather conditions included moderate to strong southwesterly winds with gusts up to 40 knots and turbulence.
- The aircraft was airworthy, with weight and balance within limits due to added ballast.
- The pilot had no prior experience on the DG-400 type, despite having extensive hours on other motor gliders.
Findings
The accident resulted from a combination of operational errors and environmental factors:
- Primary Cause: The aircraft exceeded its stall speed while in a steep bank during the base leg correction. The increased load factor in the turn raised the stall speed significantly, which the pilot likely underestimated due to the mislabeled airspeed indicator.
- Contributing Factor: Poor traffic pattern management. The pilot entered the circuit too low, forcing an immediate turn into the final approach without adequate altitude or time to assess wind conditions and traffic.
- Contributing Factor: Strong wind and turbulence, which caused drift and required a steep correction maneuver.
- Contributing Factor: The installation of an airspeed indicator with incorrect color markings for the DG-300, which provided misleading visual cues regarding safe approach speeds.
Safety action
The investigation highlighted critical safety issues regarding aircraft maintenance and pilot training. The misuse of interchangeable instrument panels led to the installation of a mislabeled airspeed indicator, a defect that persisted without detection during routine checks. Additionally, the case underscored the risks associated with transitioning pilots to high-performance motor gliders without adequate type-specific instruction, particularly when environmental conditions are challenging.