What happened
On June 30, 2019, a Cessna 172N, registration HB-CQM, was preparing for a private flight from Hausen am Albis (LSZN). Due to high ambient temperatures of 33°C, the pilot had anticipated a high density altitude of approximately 5,000 ft and had prepared a short-field takeoff procedure, which included leaning the fuel mixture to optimize engine performance.
During the takeoff roll on runway 27, the pilot applied full power but noted that acceleration felt lower than expected for the chosen procedure. Having previously briefed the passengers that the takeoff would be aborted if the airspeed did not reach 40 KIAS by the time the aircraft reached the fuel station, the pilot decided to terminate the takeoff. In an attempt to find more stopping distance, the pilot steered the aircraft toward the left side of the runway. Consequently, the aircraft left the paved surface, crossed the grass between the runway and the taxiway, and traveled across the taxiway before coming to a stop approximately 10 to 15 meters beyond it. There were no injuries to the pilot or the three passengers, and the aircraft sustained no damage.
The investigation
The investigation focused on the pilot's decision-making and the performance of the aircraft under high-temperature conditions. Investigators examined the pilot's preparations, noting that the use of a short-field takeoff procedure and the leaning of the mixture were appropriate for the high density altitude. The investigation also reviewed the aircraft's performance, noting that at the point of the abort, the aircraft had reached 40 KIAS, which was more than 80% of the required rotation speed and left sufficient runway remaining.
Findings
- The pilot's decision to abort the takeoff was considered safety-conscious, as the pilot had established a clear decision point during the takeoff briefing.
- A primary contributing factor was the pilot's misconception regarding acceleration; the pilot expected higher acceleration due to the short-field procedure, whereas the high density altitude actually resulted in lower engine performance compared to standard atmospheric conditions.
- The pilot lacked sufficient experience performing takeoffs at this specific airfield under high-temperature and high-weight conditions.
- The decision to steer off the runway to find more stopping distance increased the risk, as braking on grass requires significantly more distance than on pavement and introduces risks related to surface irregularities.