Introduction
On 26 September 2018, at 15:46 local time, a Piper J3C-65/L-4 aircraft registered as HB-OUS sustained minor damage during taxi at Grenchen Airport (LSZG). The pilot, a Swiss citizen born in 1955 holding an Airline Transport Pilot Licence (ATPL(A)), was uninjured. The aircraft was owned and operated by Regionalflugplatz, Jura-Grenchen AG.
Sequence of Events
After completing a sightseeing flight, the pilot refueled the single-engine tailwheel airplane at pump number 2. With the engine running, he taxied along the maintenance hangar towards Hangar 83, monitoring the Grenchen air traffic control frequency. During taxi, he observed a twin-engine Beechcraft BE18, registration N223CM, stationary on taxiway N with engines idling. The propellers were turning slowly. The pilot decided to continue taxiing behind the Beechcraft with the elevator held up. As he crossed behind the BE18, the Piper's right wing suddenly lifted, causing the left wingtip to strike the ground. The aircraft then pitched forward and came to rest on its nose after the propeller contacted the pavement. The pilot shut down all systems and exited the aircraft without injury. Assistants signaled the BE18 crew, who then shut down both engines.
Findings
The Beechcraft BE18 was occupied by two mechanics who were conducting a magneto test on the engines. During this test, they increased engine speed to 1,950 revolutions per minute; the maximum RPM for the BE18 at full power is 2,300. According to the Grenchen Airport operations manual, engine ground runs are only permitted at designated locations and require prior coordination with air traffic control. In this case, ATC had not been informed of the engine run, and taxiway N was not a designated site for such tests.
Analysis
The investigation highlighted that prop wash from a propeller-driven aircraft, even at low power settings, can have significant effects on a lightweight aircraft taxiing behind it. The report referenced a similar incident involving a Piper PA28 that was lifted while passing behind an Airbus A320. The recommended practice is to coordinate passing behind another aircraft via radio with the other crew or ATC, or to delay taxiing until the other aircraft has shut down its engines or moved out of the area.