Accident Overview
On 29 July 2004, at approximately 10:20 local time, a Piper PA-18-180M (registration HB-POD) operated by the Sportfluggruppe Swissair was conducting a glider towing flight from Zweisimmen Airfield (LSTZ) in the Bernese Oberland, Switzerland. Shortly after takeoff, the engine lost power, forcing the pilot to execute an emergency landing in a meadow approximately 1 km north of the airfield. The aircraft sustained damage to the left main landing gear attachment area. The pilot and the glider pilot were uninjured, and the glider remained undamaged.
Aircraft and Pilot Information
The aircraft, a Piper PA-18-180M (serial number 18-7409045, built 1974), was equipped with a Lycoming O-360-A3A engine. The pilot, a 28-year-old Swiss national, held a Private Pilot License (A) and a Glider Pilot License, with a total flight experience of 182:04 hours, including 75:26 hours on the accident type. His last 90-day flight time was 10:30 hours total and 6:40 hours on type.
Flight and Engine Failure
On the morning of the accident, the pilot performed an engine run-up and observed an abnormal magneto drop: 170 RPM on the right magnet with rough running, and approximately 100 RPM on the left. After consulting a licensed mechanic, who found the spark plugs damp but satisfactory and noted continued roughness on the right magnet during a ground run, the aircraft was released for flight with a recommendation to use a leaner mixture and conduct another run-up before the second tow.
The pilot then re-entered the cockpit and did not use the checklist for the second preflight, nor did he perform another run-up. After takeoff on runway 35, engine power appeared normal, with RPM initially at 2450 and increasing to 2550. At approximately 50 meters above ground, after passing the runway end and flying over the Kleine Simme river, engine RPM dropped to 1400. The pilot briefly checked the throttle, mixture, and carburetor heat controls, then decided to land in the terrain. He radioed the glider pilot to release the tow rope, and both aircraft released. The forced landing was conducted on a flat meadow with small terrain irregularities, and the left landing gear attachment was damaged during touchdown with a relatively high sink rate and slight left bank.
Investigation Findings
After the accident, the fuel selector valve was found in the "RH" (right tank) position. However, the pilot later recalled that immediately after landing, the valve was not in the RH position but approximately midway between the right and left tanks. It could not be determined who had moved it before the investigation.
Ground runs conducted at the accident site on the following day showed normal engine starts and operation on both magnets in the "BOTH" position. When switching to the right magnet only, RPM dropped from 2000 to 1400-1500 (500-600 RPM drop), indicating a significant magneto issue. Subsequent runs on August 5 and 11, 2004, at Hausen am Albis airfield revealed a smaller right magneto drop of 200 RPM. Examination of the right magneto (Bendix P/N 10-51360-29, installed August 2002 with 225 hours) showed heavily discolored breaker points and excessive burning marks; the capacitor (P/N 10-51676 H) was defective. After replacing the capacitor, the magneto met specifications. The left magneto showed no anomalies.
Flow tests on the fuel selector valve determined that when positioned 30° to 45° from RH towards LH, the flow cross-section was significantly reduced, decreasing fuel flow by up to 30%. Ground runs with the valve in that intermediate position (30°-40° from RH) at full throttle resulted in a steady RPM drop from maximum to 1400-1500 RPM after 1-2 minutes. Returning the valve to full RH or LH restored full power.
The aircraft's weight and balance were within limits, and no other anomalies were identified.
Probable Cause
The accident was due to an emergency landing following a loss of engine power immediately after takeoff. This loss of power was most likely the result of an intermediate position of the fuel selector valve.