CEN19LA199 · Zenair CH750 STOL

During a personal flight the airplane had a total loss of engine power while in a descent. The pilot was unable to restart the engine and a forced landing was completed to rough terrain, during which the airplane landed hard and nosed over. The airplane sustained substantial damage to the forward fuselage, left wing, and rudder.

Flight and engine data recovered from a primary flight display (PFD) established that about 38 seconds after the pilot reduced engine power to descend the exhaust gas temperature (EGT) for cylinder Nos. 1 and 3 began to decrease rapidly. During the next 45 seconds, the EGT decreased 335°C and 461°C for the No. 1 and No. 3 cylinders, respectively. However, in contrast, during the same 45 second period, the EGT for cylinder Nos. 2 and 4 increased about 60ºC. The EGT for cylinder Nos. 2 and 4 also began to decrease rapidly when the fuel pressure dropped as low as 7.5 pounds per square inch. The engine stopped rotating about 10 seconds before the PFD ceased recording data, at which time the EGT was about 90°C and 210° for cylinder Nos. 1 and 3 and cylinder Nos. 2 and 4, respectively. The final data recorded showed the airplane in a right turn about 60 feet above the ground, and in a 542 feet per minute descent at 48 knots indicated airspeed.

Examination of the fuel system did not reveal any evidence of a mechanical failure. Neither wing tank contained any recoverable fuel; however, the airplane sat inverted several days before it was recovered to an upright position. The pilot stated the left and right fuel tanks contained about 11 gallons and 9 gallons, respectively, before the first flight of the day. Based on recovered PFD data and the expected fuel consumption rate of 7.9 gallons per hour at 2,700 rpm, the engine likely used about 11 gallons of fuel before the accident. The pilot stated that most of the flight was flown using fuel from the left tank and that the loss of engine power had occurred a few minutes after he switched to the right fuel tank. At the accident site, the fuel selector switch was found positioned to use fuel from the left tank; however, it is possible that the pilot switched fuel tanks during his troubleshooting after the loss of engine power.

Additionally, as discussed previously, the EGT for cylinder Nos. 2 and 4 initially continued to increase despite the rapid decrease of EGT for cylinders Nos. 1 and 3, which suggests fuel was still present for engine combustion at that point. Therefore, it is unlikely that fuel exhaustion occurred during the flight.

An engine examination revealed the No. 3 exhaust valve was stuck open. A borescope inspection of the No. 3 cylinder did not reveal any mechanical issues or interference between the piston and the stuck exhaust valve. A borescope inspection of the crankshaft and camshaft did not reveal any anomalies or mechanical damage.

The airplane was equipped with an experimental reciprocating engine equipped with an electronic fuel injection and ignition system. The engine control unit (ECU) continuously controls the air-fuel mixture and ignition timing based on readings from multiple sensors, including a crankshaft position sensor that provides the engine speed and position of each cylinder’s piston and valves. It is likely that the stuck No. 3 exhaust valve adversely affected cylinder compression, engine timing, and fuel delivery to the cylinders.

Verbatim NTSB analysis and probable cause from the NTSB dataset