What happened
On July 23, 1998, a Zlin 143L, registration HB-TCF, was performing a takeoff from the grass runway 05 at Reutte-Höfen airfield in Austria. During the takeoff roll, the pilot noticed that the propeller RPM was only 2300, which was lower than the standard 2400 RPM required for takeoff. Although the aircraft managed to lift off and reach an initial altitude of approximately 35 to 40 meters, the engine power continued to decline shortly after departure.
Unable to maintain level flight, the pilot attempted to flatten the flight path to preserve altitude, but the descent became uncontrollable. The aircraft entered a forced landing into a nearby wooded area. The impact with tree branches significantly decelerated the aircraft before it slid into the forest floor. While the aircraft sustained total loss (destruction of the airframe), all four occupants—the pilot and three passengers—remained uninjured.
The investigation
The investigation focused on why the engine failed to produce sufficient power despite no obvious mechanical failure or fuel exhaustion. Technical examinations of the Lycoming O-540-J3A5 engine revealed that the engine was running on an excessively rich fuel mixture. During a test run on a dynamometer, it was found that the engine could not reach takeoff power without the pilot manually adjusting the mixture control beyond two-thirds of its travel.
Investigators also analyzed the environmental conditions. At the time of the accident, the ambient temperature was approximately 28-30°C, and the airfield elevation was 2803 ft MSL. These conditions resulted in a high density altitude of roughly 4800 ft, which significantly impacted engine performance and required a longer takeoff roll.
Findings
- The primary cause of the power loss was a very rich default fuel mixture setting combined with the high density altitude.
- The pilot did not utilize a takeoff procedure that would have identified the power deficiency, such as performing a full-power check with brakes applied prior to the takeoff roll.
- The high temperature and high density altitude further degraded the engine's ability to produce necessary thrust.
- The takeoff roll on the grass runway was constrained by the high density altitude, leaving a narrow margin for error regarding obstacle clearance.