Factual Information
On 19 July 2020, at approximately 18:25 local time, a Piper PA-28-181 registered as LX-AVA departed from St. Gallen-Altenrhein Airport (LSZR) for a return flight to Luxembourg. The aircraft was operated by Avia Sport a.s.b.l. and piloted by a Luxembourg national holding a Private Pilot Licence (PPL(A)). The flight was conducted under Visual Flight Rules (VFR).
Approximately one minute after takeoff, while climbing at 85 KIAS at an altitude of 1550 ft QNH (about 250 ft above Lake Constance), the engine exhibited a noticeable power loss accompanied by stuttering. The pilot leveled off, but airspeed dropped to 68 KIAS. The pilot checked the fuel selector, ignition, thrust lever, mixture lever, electric fuel pump (on), carburetor heat (off), and engine instruments (all in green). No circuit breakers had tripped.
Unable to resolve the engine issue, the pilot initiated a right reversal turn. At 18:25:55, the passenger radioed that the engine was stuttering and they were returning. The aerodrome controller cleared LX-AVA for a priority landing on runway 10. The aircraft landed safely at 18:27:47. No damage occurred to the aircraft, and neither occupant was injured.
Technical Findings
The aircraft was a single-engine, low-wing, four-seat all-metal Piper PA-28-181, equipped with a Lycoming O-360-A4M engine. At the time of the incident, the airframe had accumulated 6801:36 hours. The last maintenance (100-hour/annual inspection) was performed two days prior, on 17 July 2020, at 6798:36 hours. During that inspection, compression checks at 80 PSI input pressure yielded readings of 78 PSI (cylinder 1), 76 PSI (cylinder 2), 70 PSI (cylinder 3), and 78 PSI (cylinder 4). Lycoming Service Instruction No. 1191A considers differences up to 5 PSI satisfactory and differences of 10–15 PSI warranting investigation but not necessarily immediate cylinder removal.
After the incident, an inspection on 30 July 2020 revealed very low compression on cylinder no. 3. The piston and rings were undamaged, but intake and exhaust valves showed combustion residues. A new cylinder, valves, piston, and rings were installed, after which the engine produced required power during a test run.
Analysis
The Swiss Transportation Safety Investigation Board (STSB) determined that the loss of power resulted from a leaking intake or exhaust valve on cylinder no. 3, caused by combustion residues (e.g., carbon or lead from aviation gasoline) interfering with valve seating. Such leaks can occur suddenly, particularly in engines with high operating hours, as was the case here. The compression check performed two days earlier had shown a 10 PSI difference for cylinder no. 3, which was within the manufacturer's tolerance, but the incident occurred only a few flight hours later.
The pilot's decision to initiate a reversal turn at 250 ft AGL and declare an emergency contributed to a safe outcome. The aircraft landed without further incident.