Piper PA 28R-201T N5966V accident near Nipton, California

Probable cause

A loss of engine power due to the detonation of the No. 5 cylinder due to excessive leaning at high power settings, which resulted in a forced landing and the subsequent hard touchdown.

NTSB analysis

About 40 minutes after takeoff and while the airplane was at a cruise altitude of 10,000 ft mean sea level, the pilot began leaning the engine mixture. After about 5 minutes, the pilot observed that the engine was running roughly, the No. 5 cylinder head temperature had spiked, and the airplane was unable to maintain altitude. The pilot could not restore engine power and subsequently made a forced landing on a highway. The airplane touched down hard, which resulted in substantial damage to both wings.

A postaccident engine examination confirmed continuity to the accessory section. Spark was produced on all six top spark plug leads, and thumb compression was obtained on all cylinders except the No. 5 cylinder. The top spark plugs were removed, and all except the No. 5 spark plug exhibited dark-colored combustion deposits; the No. 5 spark plug exhibited a molten, metal-like substance around the electrode. Cylinder Nos. 1 and 3 exhibited minor signs of detonation. The No. 5 cylinder exhibited signs of detonation/preignition with material missing from around the circumference of the piston face, and the No. 5 cylinder exhaust and intake valves showed signs of mechanical damage. The detonation was consistent with excessive leaning at high power settings.

Text from the NTSB. Open the NTSB docket for WPR18LA033 for the full record.