History of Flight
On June 29, 2019, about 1246 central daylight time, a Zenair CH750 STOL airplane, registration N170MV, was substantially damaged during an accident near Paducah, Kentucky. The pilot, who sustained serious injuries, was operating the airplane under Title 14 Code of Federal Regulations Part 91 as a personal flight. According to the pilot, the flight's purpose was to overfly his business and property, where he was constructing a private grass airstrip. A normal preflight inspection was completed at Mayfield Graves County Airport (M25), Mayfield, Kentucky.
Data from the primary flight display (PFD) indicated that the engine was started at 1115:52, and the airplane departed M25 at 1121:03 after a short taxi. The pilot reported landing at Kentucky Dam State Park Airport (M34), Calvert City, Kentucky, to adjust seat restraints. At 1223:30, he landed at M34 and shut down the engine at 1224:39. The engine was restarted at 1230:07 while on the ramp, and the airplane departed on runway 28 at 1230:59.
During cruise, the knob fell off the throttle lever; the pilot reinstalled it after a couple of attempts, and the engine continued operating normally. The PFD data showed that the pilot maintained about 2,750 rpm in a cruise climb, with fuel pressure around 31 psi. At 1243:05, fuel pressure decreased to 20 psi over 49 seconds before returning to 31 psi, while exhaust gas temperature (EGT) for all four cylinders increased by about 30°C and then returned to previous levels.
At 1244:19, the airplane reached 4,741 ft msl, its highest altitude, and engine speed decreased from 2,700 to 2,200 rpm with corresponding drops in fuel pressure and EGT. At 1244:57, during a sustained descent, EGT for cylinders No. 1 and 3 began decreasing rapidly, and fuel pressure fell below 23 psi with engine speed about 2,100 rpm. Between 1244:57 and 1245:42, EGT decreased 335°C for No. 1 and 461°C for No. 3, while EGT for cylinders No. 2 and 4 increased about 60°C. At 1245:34, indicated airspeed was about 114 knots with a descent rate of 3,200 fpm. At 1245:42, EGT for cylinders No. 2 and 4 also began decreasing rapidly as fuel pressure dropped to 7.5 psi.
At 1246:01, fuel pressure dropped to 7.5 psi. The engine momentarily stopped rotating at 1246:23. Between 1246:23 and 1246:31, fuel pressure fluctuated between 10 and 20 psi while engine speed varied between 87 and 304 rpm. At 1246:31, the engine stopped a second time, oil pressure dropped to 0 psi, and EGT was about 90°C for cylinders No. 1 and 3 and 210°C for No. 2 and 4. Fuel pressure increased to 31 psi at the end of the recorded data. At 1246:41, about 10 seconds after the second stoppage, the final recorded data showed the airplane in a right turn about 60 ft above ground, descending at 542 fpm and 48 knots, about 263 ft south of the accident site (elevation 372 ft msl).
The pilot stated that a total loss of engine power occurred about 0.5 mile west of his property. Unable to restart the engine, he performed a forced landing on an unfinished portion of the airstrip, during which the airplane landed hard and nosed over.
Aircraft Information
The airplane was an experimental Zenair CH750 STOL equipped with an UL Power 350i reciprocating engine featuring electronic fuel injection and ignition. The engine control unit (ECU) continuously adjusts air-fuel mixture and ignition timing based on sensor inputs, including a crankshaft position sensor. According to the manufacturer, relative fuel pressure (the difference between fuel line pressure and intake manifold pressure) must be at least 35 psi for proper mixture; below this, the mixture becomes too lean, reducing power output.
The pilot reported that the fuel sensors in the wing tanks were not calibrated, making the PFD fuel quantity display unreliable. He used a calibrated dipstick during preflight, finding about 11 gallons in the left tank and 9 gallons in the right tank. The pilot stated typical fuel consumption was 3-5 gallons per hour; however, the engine manufacturer specifies about 7.9 gallons per hour at 2,700 rpm. Based on PFD data, the engine operated for 1 hour 25 minutes, consuming about 11 gallons of fuel. The pilot said most of the flight used fuel from the left tank, and the power loss occurred a few minutes after switching to the right tank.
Wreckage and Impact
The airplane came to rest inverted on a rough dirt area of the pilot's property that was being developed into an airstrip. The main wreckage consisted of the entire airplane, with substantial damage to the forward fuselage, left wing, and rudder. Flight control continuity was confirmed from the cockpit to the control surfaces. Neither wing tank contained recoverable fuel; the airplane had been inverted for several days before recovery. Recovery personnel noted no evidence of a fuel spill under either wing. Fuel was present in the fuel lines within the engine compartment.
The fuel line between the fine fuel filter and the engine contained fuel with no obstructions. Fuel was also present in the return line. Both fuel pumps functioned when connected to a 12-volt battery, and continuity tests confirmed proper wiring. No worn or loose wires were found. The fuel shutoff valve was on, and the fuel tank selector switch was positioned to the left tank. The selector switch was wired correctly to an electric selector valve, which operated correctly during functional testing.
Engine and Fuel System Examination
Engine crankshaft rotation through the propeller confirmed internal continuity. The No. 3 cylinder exhaust rocker moved freely, but the exhaust valve was stuck open. Borescope inspection of the No. 3 cylinder revealed no mechanical issues or interference between the piston and the stuck valve. The No. 3 cylinder and head were removed to extract the stuck exhaust valve, which had to be hammered out of the valve guide; after extraction, the valve stem did not appear bent when rolled on a flat surface. Borescope inspection of the crankshaft and camshaft showed no anomalies. Fuel injectors from each cylinder ejected a clean spray pattern when tested.
Fuel was present in the fuel lines, and the fuel pumps and selector valve functioned properly. The reason for the fuel pressure drop and subsequent engine stoppages was not determined from the examination.