Introduction
On 7 April 2018, at approximately 17:45 local time, a Robin DR 400/180 R registered as HB-KAW was involved in a serious incident at Sion aerodrome (LSGS). The aircraft was operated by Vol à voile Club Valais and was being used as a tow plane. The pilot, a Swiss citizen born in 1941, held a Private Pilot Licence (PPL(A)) and had accumulated 2,755 total flight hours, including 800 hours on type.
Sequence of Events
Earlier that day, the pilot flew an instruction flight with a motorglider (HB-2360) to Lausanne-La Blécherette and Gruyères, returning to Sion at 17:25. Before that flight, another tow pilot asked if he could perform tows after his return, and he agreed. After landing, the pilot approached the HB-KAW, which was parked on the concrete area in front of the hangar, from the rear and boarded without performing a pre-flight inspection, trusting the previous user. He did not notice that the nose wheel tow bar was still attached and aligned with the wheel.
The pilot turned on the ignition, checked the fuel level, and started the engine. He then taxied on the grass area, making a 180° left turn toward the takeoff point of runway 25 grass, where a glider (HB-1564) was waiting. During taxi, he heard a sudden noise and felt an impact, prompting him to stop the engine.
A person from the gliding group approached, observed damage to the two-blade metal propeller and the tow bar, and informed the pilot, who remained seated, that there was a small mark on the propeller. Unable to see the damage from the cockpit, the pilot did not exit the aircraft. He restarted the engine, performed engine and control checks, and noted nothing abnormal.
At 17:53, with only the pilot on board, the HB-KAW took off and conducted the planned tow flight, which lasted 6 minutes and proceeded normally. After landing, the pilot saw the propeller damage and informed the club committee and maintenance workshop. No further tows were performed.
Damage and Investigation
The propeller was severely damaged and found to be out of tolerance, making it unrepairable. The nose wheel tow bar was also damaged. After the incident, the propeller and engine were removed from the aircraft. An approved company conducted a specific inspection of the engine, and no other damage was found on the aircraft.
Analysis
The pilot's failure to conduct a pre-flight inspection was risky and resulted in him being unaware that the tow bar was still installed. After hearing the impact and stopping the engine, the damage assessment was performed by an unqualified person. The pilot assumed the damage was minimal and proceeded with the tow flight, taking significant risks. The investigation noted that seemingly minor events can lead to major safety-related damage that may only become apparent after a thorough inspection.