Cessna 172M N1443U accident near Pahokee, Florida

Probable cause

The flight instructor's misjudgment of the airplane's position and airspeed relative to the runway while returning to the airport after a partial loss of engine power. Contributing to the accident was the separation of the engine's No. 3 cylinder exhaust valve head from the stem, and the operator’s exceedence of the engine manufacturer's recommended time between overhauls.

NTSB analysis

The flight instructor stated that, as the airplane climbed through about 500 feet above the ground after takeoff on the intended dual instructional cross-country flight, the engine started shaking, sputtering, and making “alarming” noises. The flight instructor took control of the airplane, noted nothing abnormal with the mixture, fuel selectors, engine instruments, or fuel quantity, and decided to return to the departure airport to land. The flight instructor noted that there was not a total loss of engine power, but it was not possible to maintain the airplane’s altitude. When the flight instructor realized that the airplane was too high and too fast to land on the intended runway, she moved the throttle to idle; however, she did not use flaps or slip the airplane to reach the runway. The airplane overflew the entire length of the runway, and the flight instructor elected to land the airplane in a lake on the left to avoid obstacles straight ahead and on the right. Postaccident examination of the engine revealed that the No. 3 cylinder exhaust valve head had sheared off at the stem. The stem, spring, and rocker arm were still intact, and the No. 3 piston exhibited signs of mechanical damage from the sheared head inside the combustion chamber. At the time of the accident, the engine had 2,229.5 hours of operation since major overhaul. The engine manufacturer's recommended time between overhauls was 2,000 hours.

Text from the NTSB. Open the NTSB docket for ERA11LA429 for the full record.