HISTORY OF FLIGHTOn June 29, 2019, about 1246 central daylight time, a Zenair CH750 STOL airplane, N170MV, was substantially damaged when it was involved in an accident near Paducah, Kentucky. The pilot sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.
The pilot stated that the purpose of the flight was to fly over his business and then over his property where he was building a private grass airstrip. He completed a normal preflight inspection of the airplane at his hanger located at Mayfield Graves County Airport (M25), Mayfield, Kentucky.
According to data downloaded from a primary flight display (PFD), at 1115:52, the pilot started the engine and after a short taxi departed M25 at 1121:03. The pilot stated that he landed at Kentucky Dam State Park Airport (M34), Calvert City, Kentucky, to adjust his seat restraints before continuing toward his property. At 1223:30, he landed at M34 and shut down the engine at 1224:39.
At 1230:07, the pilot started the engine while on the ramp at M34 and departed on runway 28 at 1230:59. The pilot stated that during cruise flight the knob fell off the throttle lever; however, he was able to reinstall the knob after a couple of attempts. He stated that the engine continued to operate normally while he reinstalled the throttle knob.
According to the PFD data, the pilot maintained an engine speed of about 2,750 rpm while in a cruise climb. The fuel pressure remained about 31 pounds per square inch (psi) until 1243:05, when over the next 49 seconds, the fuel pressure decreased to 20 psi before it returned to 31 psi, and the exhaust gas temperature (EGT) for all four engine cylinders increased about 30°C before returning to their previous temperatures, as shown in figure 1 and figure 2.
At 1244:19, the airplane had climbed to 4,741 ft msl, the highest recorded altitude for the flight, and the engine speed decreased from 2,700 rpm to 2,200 rpm with corresponding decreases in fuel pressure and EGT. However, at 1244:57, while the airplane was in a sustained descent, the EGT for cylinder Nos. 1 and 3 began to decrease rapidly, and fuel pressure decreased below 23 psi with the engine speed about 2,100 rpm.
Between 12:44:57 and 1245:42, the EGT decreased 335°C (626°C to 291°C) and 461°C (659°C to 198°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 (640°C to 700°C), as shown in figure 2.
About 1245:34, the airplane’s indicated airspeed and descent rate were about 114 knots and 3,200 feet per minute (fpm), respectively. At 1245:42, The EGT for cylinder Nos. 2 and 4 also began to decrease rapidly when the fuel pressure dropped as low as 7.5 psi.
At 1246:01, the fuel pressure had decreased to 7.5 psi and, at 1246:23, the engine momentarily stopped rotating. Between 1246:23 and 1246:31, the fuel pressure increased from 10 psi to 20 psi while the engine speed varied between 87 rpm and 304 rpm. At 1246:31, the engine stopped a second time and the oil pressure decreased to 0 psi. At this point, the EGT was about 90°C and 210° for cylinder Nos. 1 and 3 and cylinder Nos. 2 and 4, respectively. The fuel pressure continued to increase and reached 31 psi at the end of the recorded data.
At 1246:41, about 10 seconds after the engine stopped the second time, the final recorded data showed the airplane in a right turn about 60 feet above the ground, and in a 542 fpm descent at 48 knots indicated airspeed. The final recorded position was about 263 ft south of the accident site. The elevation at the accident site was 372 ft msl.
The pilot stated that the airplane had a total loss of engine power when it was about 0.5 mile west of his property. He was unable to restart the engine and a forced landing was completed on an unfinished potion of the airstrip, during which the airplane landed hard and nosed over.
AIRCRAFT INFORMATIONThe airplane was equipped with an experimental UL Power 350i 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.
According to the manufacturer, the relative fuel pressure used by the ECU to determine the air-fuel mixture is the difference between the fuel line pressure and the pressure in the intake manifold. If the relative fuel pressure falls below 35 psi, the mixture becomes too lean and the engine will not have the correct air-fuel ratio and, as a result, will have reduced power output.
The pilot stated that the fuel sensors in the wing tanks were not calibrated properly and, as such, the fuel quantities displayed on the PFD were unreliable/inaccurate. During his preflight, he used a calibrated dip stick to determine that the left and right fuel tanks contained about 11 gallons and 9 gallons, respectively. The pilot stated that the engine typically used 3-5 gallons of fuel per hour; however, according to the engine manufacturer specifications the expected fuel consumption for the UL Power 350i engine is about 7.9 gallons per hour at 2,700 rpm.
Based on the recovered PFD data, the engine was operated a total of 1 hour 25 minutes on the day of the accident and, as such, the airplane would have used about 11 gallons of fuel. The pilot stated that most of the flight was flown using fuel from the left tank, and that the loss of engine power occurred a few minutes after he had switched to the right fuel tank. AIRPORT INFORMATIONThe airplane was equipped with an experimental UL Power 350i 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.
According to the manufacturer, the relative fuel pressure used by the ECU to determine the air-fuel mixture is the difference between the fuel line pressure and the pressure in the intake manifold. If the relative fuel pressure falls below 35 psi, the mixture becomes too lean and the engine will not have the correct air-fuel ratio and, as a result, will have reduced power output.
The pilot stated that the fuel sensors in the wing tanks were not calibrated properly and, as such, the fuel quantities displayed on the PFD were unreliable/inaccurate. During his preflight, he used a calibrated dip stick to determine that the left and right fuel tanks contained about 11 gallons and 9 gallons, respectively. The pilot stated that the engine typically used 3-5 gallons of fuel per hour; however, according to the engine manufacturer specifications the expected fuel consumption for the UL Power 350i engine is about 7.9 gallons per hour at 2,700 rpm.
Based on the recovered PFD data, the engine was operated a total of 1 hour 25 minutes on the day of the accident and, as such, the airplane would have used about 11 gallons of fuel. The pilot stated that most of the flight was flown using fuel from the left tank, and that the loss of engine power occurred a few minutes after he had switched to the right fuel tank. WRECKAGE AND IMPACT INFORMATIONThe 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. The airplane sustained substantial damage to the forward fuselage, left wing, and rudder. Flight control continuity was established from the cockpit controls to the individual flight control surfaces. Neither wing tank contained any recoverable fuel; however, the airplane sat inverted several days before it was recovered to an upright position. The wreckage recovery personnel reported there was no evidence of a fuel spill under either wing when the airplane was recovered. Fuel was present in the fuel lines within the engine compartment.
The fuel line between the fine fuel filter and the engine contained fuel, and there were no obstructions observed to the fuel line or the filter assembly. Fuel was also present in the return line to the fuel tanks.
Both fuel pumps functioned when connected to a 12-volt battery source. A continuity test confirmed that both fuel pumps were wired properly to the pump control switch panel. There was no evidence of any worn wires, burnt wires, or loose connections. The fuel shutoff valve was on, and the fuel tank selector switch was positioned to use fuel from the left tank. The fuel tank selector switch was wired correctly to an electric selector valve located under and behind the right seat, and a functional test confirmed the fuel selector valve operated correctly.
The engine crankshaft was rotated through the propeller to confirm internal engine and valve train continuity. The No. 3 cylinder exhaust rocker moved freely, but the 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. The No. 3 cylinder and head were removed to extract the stuck exhaust valve. The stuck exhaust valve had to be hammered out of the valve guide; however, after being extracted from the cylinder head, the No. 3 exhaust valve stem did not appear bent when rolled on a flat surface. A borescope inspection of the crankshaft and camshaft did not reveal any anomalies or mechanical damage. The fuel injectors were removed from each cylinder and ejected a clean spray pattern when tested with an injector test kit.