Incident Overview
On 30 July 2019, a privately operated Piper PA-16 Clipper, registration N5723H, suffered a sudden engine power loss during cruise flight near Welschenrohr, Switzerland. The pilot, the sole occupant, was not injured and the aircraft was not damaged. The flight had departed from Luzern-Beromünster (LSZO) at 18:12 local time for a local VFR flight.
Findings
The aircraft's engine, a Lycoming O-235-C1, had been overhauled in the United States in spring 2019 and installed in July 2019 following an annual inspection. After successful ground runs, the aircraft was released for flight on 12 July 2019. The first flight after these maintenance actions occurred on the day of the incident. The left wing tank contained approximately 50 liters of fuel, and the right tank contained about 30 liters.
During the flight, the pilot initially selected the left tank (as required for takeoff and landing) but later switched to the right tank. While climbing toward the Chasseral region, the aircraft encountered significant turbulence due to a north wind. At approximately 18:45, at an altitude of 4300 ft AMSL, the engine lost power and the propeller stopped.
Fuel System Design
The Piper PA-16's original fuel system had been modified in the 1950s by removing the fuselage tank and adding an auxiliary tank in the right wing, similar to the later PA-20 Pacer. This modification was covered by a Supplemental Type Certificate (STC) dating from the early 1970s. The left tank is designated the main tank (capacity 68 liters) and the right tank the auxiliary tank (also 68 liters). The STC requires that the aircraft be flown only straight and level when operating on the right tank, due to known fuel feed issues during climbs or descents. A placard near the tank selector stated this limitation, but it was poorly legible. Experience showed that fuel feed from the right tank could be unreliable when the fuel quantity was less than approximately 30 liters.
Pilot Actions
Upon losing engine power, the pilot immediately switched the tank selector to the left tank, set the mixture to full rich, applied carburetor heat, and lowered the nose to increase airspeed. The propeller did not windmill as expected. The pilot then searched for a suitable landing area and initiated a turn toward a meadow southwest of Welschenrohr. At 18:46:05, he broadcast a mayday call on the Grenchen airfield frequency and manually activated the emergency locator transmitter (ELT). The loud ELT alarm in the cockpit was distracting.
On final approach to the meadow, at approximately 2900 ft AMSL (600 ft above ground), the pilot pressed the starter button. The engine started after a few revolutions and resumed normal power. The pilot aborted the landing, climbed, and deactivated the ELT. He then decided to return to Luzern-Beromünster.
Aftermath
The pilot attempted to cancel the emergency on Grenchen frequency, but his transmission was not received by anyone. Grenchen tower personnel had activated emergency services and attempted to contact the aircraft without success. The ELT signal was received by the Rescue Coordination Centre (RCC) at 18:46:47, putting search and rescue on alert. The aircraft's whereabouts remained unknown for over 45 minutes until it landed at Luzern-Beromünster at 19:36 without further incident. After landing, the pilot checked the fuel system by switching to the right tank with the engine running and found no anomalies.