Cessna TU206 N878AH accident near Twentynine Palms, California

Probable cause

The pilot’s decision to takeoff with a known mechanical engine anomaly, and the failure of the No. 2-cylinder exhaust valve, which resulted in a total loss of engine power during the initial climb and subsequent impact with terrain.

NTSB analysis

The pilot reported that during the first cross-country flight after an annual inspection, he experienced a rough running engine. He noted that when the engine roughness began, the airplane’s airspeed decreased by 23 knots, and the No. 2 cylinder head temperature decreased from 1,350° F to 650° F. He decided to continue flight at the slower airspeed. The engine roughness once again returned, and the pilot elected to land as soon as practical and have a mechanic look at the engine. However, there were no mechanics available, so the pilot decided to refuel and takeoff. The first takeoff attempt was unsuccessful because the pilot felt a slight hesitation in the engine power during the takeoff roll, and he aborted the takeoff. During the second takeoff attempt, at 250 ft agl, the engine lost power. The airplane descended to the ground, impacted a sand berm, and nosed over. It sustained substantial damage to the right wing and wing left strut

A postaccident examination of the engine revealed that the No. 2-cylinder exhaust valve failed. The No. 2 cylinder exhaust valve was missing about 45 percent of the valve head and foreign object compression signatures were sustained to the No. 2 piston and cylinder head. The missing exhaust valve head was not present in the No. 2 cylinder. Additionally, the No. 5 cylinder piston and cylinder head sustained foreign object compression signatures. The No. 5 cylinder intake valve exhibited material deformation around the valve seat.

The engine total time was 2,180.1 hours when the annual inspection was completed. The engine had acquired 3 hours since the last annual. The engine was overhauled after acquiring 909 hours. At the time of the accident, the engine had amassed 1,274.1 hours since major overhaul. It is likely that the initial engine roughness and partial loss of engine power, followed by the total loss of engine power during takeoff was due to the failure of the exhaust valve.

Text from the NTSB. Open the NTSB docket for WPR21LA059 for the full record.