Background
On 28 June 2019, the pilot of a Cessna 172N, registered as HB-CQM, began preparations for a local sightseeing flight planned for 30 June at Hausen am Albis airfield (LSZN). Anticipating high temperatures, the pilot studied the effects of high density altitude and planned to use a lean mixture and short field takeoff procedure as outlined in the aircraft flight manual (AFM). The pilot calculated a density altitude of up to 5000 ft for the planned departure at 15:30 local time.
Incident
On 30 June 2019, the pilot arrived at the airfield around 13:30 LT with his son and conducted preflight preparations. Two additional passengers joined later, and the pilot and three passengers boarded HB-CQM. Before taxiing onto runway 27, the pilot briefed that he would abort the takeoff if the indicated airspeed was less than 40 KIAS at the gas station along the runway.
After completing all takeoff preparations, including optimizing engine performance, the pilot applied takeoff power and released the brakes, noting an engine speed of 2500 RPM. He perceived the acceleration as slower than usual, expecting stronger acceleration from the short field procedure. At about the gas station, the airspeed indicator read 40 KIAS. Believing the speed increase was too slow, the pilot aborted the takeoff a few seconds later. The aircraft departed the runway to the left, rolled over grass between the runway and taxiway, crossed the taxiway at the runway end, and came to a stop approximately 10 to 15 meters later, undamaged. The pilot reported he steered left intuitively, assuming more distance to stop was available in that direction.
Weather
At the time of the incident, a high-pressure system brought sunny and hot weather. Temperature was 33 °C with a dew point of 17 °C. Wind was calm, visibility 25 km, and QNH 1019 hPa. The density altitude under these conditions was approximately 4300 ft. The MeteoSwiss aviation weather forecast noted temperatures over 30 °C in lowlands and slightly elevated areas, advising attention to density altitude.
Aircraft and Procedure
The Cessna 172N had no recording devices and no technical limitations were reported. The AFM specifies that for a short field takeoff, flaps are set to 10°, and the aircraft is held until takeoff power is set, then rotated at 48 KIAS instead of the normal 55 KIAS. According to the AFM, leaning the mixture is required for takeoffs above 3000 ft airport elevation; although the field elevation is lower, the density altitude of 4300 ft warranted leaning.
The calculated takeoff roll distance for the short field procedure under the prevailing conditions was about 1100 ft (335 m). The available runway for takeoff on runway 27 is 600 m, with a total length of 700 m. The distance from the runway start to the gas station is approximately 360 m. Braking marks indicated that the wheels were locked over a long distance.
Analysis
The pilot's preflight preparations were thorough and forward-looking regarding weather conditions. The short field procedure was applied correctly, reducing rotation speed and takeoff roll. At 40 KIAS, the aircraft had reached over 80% of the required rotation speed, and with about 340 m of runway remaining, a safety margin existed. However, the pilot's lack of experience on type, with only 12.19 hours on the Cessna 172N, likely meant he had not previously taken off from Hausen am Albis under high temperature and heavy weight conditions. This lack of experiential reference for acceleration relative to runway length, combined with the high density altitude, led to his decision to abort—a decision that was understandable and safety-conscious. The incident also demonstrated that stopping on grass requires significantly more distance than on paved surfaces and that leaving the runway introduces additional risks from terrain or obstacles.
Conclusions
The pilot abandoned the takeoff due to perceived insufficient acceleration, which was influenced by high density altitude and his limited experience on type. The takeoff was aborted safely, and the aircraft sustained no damage. No injuries occurred.