What happened
On the afternoon of May 23, 1993, a Piper PA-28-151 Cherokee Warrior, registered as PH-ANU, was engaged in a private flight over the Swiss Alps. The aircraft had departed from Sion earlier that day with three pilots on board, intending to conduct an hour-long scenic flight in the Zermatt region. The crew planned to cross the Theodul Pass and return via Italy.
To maximize performance during the climb, the fuel tanks were filled only halfway, providing approximately two hours and fifteen minutes of endurance. As the aircraft climbed through the Rhône Valley toward Visp and then turned into the Mattertal, the pilot noted that the climb rate was lower than expected for the aircraft's specifications. The variometer indicated a climb rate of 200 feet per minute.
Approaching the Upper Theodul Glacier at an altitude of approximately 3,280 meters (10,754 feet), the pilot realized he was positioned about 400 meters east and slightly below the summit of the Theodulhorn. Suddenly, a high-voltage power line running perpendicular to his flight path came into view. Recognizing that pulling up would not provide sufficient clearance to cross over the obstacle, the pilot decided to fly underneath it.
At this critical moment, the more experienced mountain pilot in the right seat took control and attempted a left turn to avoid the glacier surface directly ahead. The aircraft passed under the power line, but its landing gear struck the hard-packed snow on the ground. The impact caused the main wheels to break off and the nose wheel to fold back under the engine cowling. The plane skidded to a halt on a ski piste near the Testa Grigia cable car station at an altitude of 3,250 meters.
The front passenger sustained minor facial bruises, while the two pilots and the rear passenger were uninjured. The aircraft suffered heavy damage.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the wreckage and flight records. Key findings included:
- The pilot flying had no prior experience in mountain flying.
- The mixture control was confirmed to be set to "lean" during the climb.
- The altimeter was set to 1017 hPa, indicating an altitude of 10,400 feet QNH.
- The aircraft's mass and center of gravity were within permissible limits at the time of the accident.
- Meteorological conditions were excellent with clear skies and light southwest winds.
- The high-voltage line was clearly marked with orange spheres and depicted on the ICAO map onboard.
The investigation highlighted that at 3,200 meters, a conventional engine loses approximately 35% of its nominal power. For this specific aircraft, this reduced output to roughly 84 kW (105 HP). Given the weight and temperature conditions, the climb rate was likely near zero feet per minute.
Findings
The accident was caused by a combination of factors:
- Insufficient altitude: The pilot approached the high mountain pass at an inadequate height to safely cross it or maneuver around obstacles.
- Poor flight preparation: The crew failed to adequately account for the performance degradation of the aircraft at high density altitudes.
- Lack of mountain flying experience: The pilot lacked the specific skills required for navigating complex alpine terrain.
- Overestimation of aircraft capabilities: The crew overestimated the Piper Cherokee Warrior's ability to climb and maneuver in thin air at high elevations.
The STSB noted that hypoxia may have subtly impaired judgment, as symptoms can begin at 3,000 meters. However, the primary causes were operational errors related to altitude management and performance calculation.
Safety action
The report serves as a cautionary example for general aviation pilots operating in high-altitude environments. It underscores the necessity of rigorous pre-flight planning that accounts for density altitude effects on engine performance and climb rates. Pilots must ensure they have sufficient margin to clear obstacles, such as power lines and mountain ridges, especially when flying aircraft not specifically designed for high-performance mountain operations.