Kernville, CA, USA
N830AZ
ZENITH STOL CH750
The pilot reported that he was conducting a personal, cross-country flight in the experimental, amateur-built airplane with three flight legs planned. About 15 minutes after the pilot departed from a remote airstrip for the second flight leg and while the airplane was in cruise flight about 2,000 – 2,500 ft above ground level, the engine experienced a total loss of power. The pilot attempted to restart the engine to no avail. The terrain below and ahead of the airplane was mountainous, so he chose to turn toward terrain that was flatter to conduct an emergency landing, during which the airplane impacted dense brush and then nosed over. According to recorded engine data, when the airplane was about 7,425 mean sea level, the engine rpm began to decrease from about 3,000 to 1,100 over 10 seconds. Eight seconds later, the rpm had decreased to 0, corroborating the pilot’s statement regarding the loss of engine power. Examination and test run of the engine revealed no evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation. The fuel tank inlet was located on the inboard tank wall in the aft area about 0.25 inch above the tank’s bottom. The forward wall height was about 7.75 inches, and the aft wall height was about 4 inches. The fuel quantity was provided by a float-type sending unit attached to the inboard, forward portion of the tank wall. Because of the tank’s shape, the relationship between the fuel depth and fuel quantity for a given tank attitude and depth measurement location will not be constant. Further, if the airplane is pitched down, the fuel will be pushed forward in the tank, and the float on the fuel sender will rise, indicating a higher fuel quantity than what is actually in the tank. The engine data showed that the airplane departed with about 2 gallons and 9 gallons of fuel in the left and right wing tanks, respectively. The pilot reported that he thought the fuel selector was positioned to the right tank but that could not be verified from the recorded data. Throughout the first 5 minutes of the flight, the fuel quantity continually fluctuated with values between 0.4 and 2.1 gallons and 7.8 and 9.4 gallons in the left and right tanks, respectively. Just before the engine lost power, the fuel quantities in both tanks increased as the airplane assumed a nose-low pitch attitude. Therefore, the fluctuations in the fuel amount and the increase in the fuel amount when the airplane entered a nose-low pitch attitude were consistent with the fuel tank’s shape and the fuel quantity indicator’s position. Based on the tank’s dimensions, when the tank is level, about 0.5 gallon is required to have a 1-inch fuel level at the inlet port. Given that the engine data indicated that there was likely sufficient fuel to reach the ports in both tanks during the 5 minutes of flight before the engine power loss and that the fuel system’s design did not provide a means for air or fuel vapor to vent from the system, it is likely that excess air or fuel vapor built up and subsequently entered the fuel injectors, which prevented the fuel from reaching the engine cylinders and resulted in the total loss of engine power.
HISTORY OF FLIGHTOn April 14, 2018, about 1300 Pacific daylight time, an experimental, amateur-built Zenith CH-750 STOL airplane, N830AZ, was involved in an accident about 15 miles north of Kern Valley Airport, (L05), Kernville, California. The pilot sustained minor injuries, and the airplane sustained substantial damage. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot stated that, earlier in the day, he had departed from his home airport in Corona, California, with three flight legs planned. After taking off from a remote airstrip in Palmdale, California, for the second flight leg, the airplane continued north for sightseeing around a lake. About 15 minutes after takeoff, while in cruise flight about 7,000 – 7,500 ft mean sea level (2,000 – 2,500 ft above ground level), the engine experienced a total loss of power. The pilot attempted to restart the engine by turning on the auxiliary fuel pump and switching fuel tanks. The engine would crank, but the motor made no sound. The terrain below and ahead was mountainous, so the pilot chose to turn toward terrain that was flatter to conduct an emergency landing, during which the airplane impacted dense brush in the Sequoia National Forest and then nosed over. Data downloaded from the combination electronic flight instrument system and engine monitoring system indicated that, about 5 minutes after departure, when the airplane was about 7,425 mean sea level, the engine rpm began to decrease from about 3,000 to 1,100 over 10 seconds. Eight seconds later, the rpm had decreased to 0. The airplane crashed about 6.5 minutes after the engine rpm reduction. AIRCRAFT INFORMATIONFuel System Design The airplane was equipped with pressurized fuel tanks in the inboard sections of each wing. Each tank had a supply line and a return line routed to the fuel selector valve, which was electrically actuated from a two-position (“left” and “right”) switch on the instrument panel. Downstream of the shutoff valve, the fuel line penetrated the firewall where there was a 90° elbow, and the fuel would flow to either of the two firewall mounted fuel filters. Each respective filter was located just below the main and auxiliary fuel pumps, each of which were electrically driven, where the fuel lines then combined and continued to the main fuel filter. The fuel return line was then routed through a fuel flow transducer, then to the injectors on the left side (Nos. 2 and 4 cylinders), then to the injectors on the left side and over the top of the engine to the right side (Nos. 1 and 3 cylinders), and then routed to the pressure relief valve. The fuel then continued passing through another transducer and a check/shuttle valve before passing through the firewall. The fuel was then returned to the selector and routed to the tank selected on the fuel selector. According to the CH-750 Pilot’s Operating Handbook, the two fuel tanks had a total capacity of about 24 gallons, of which 22 gallons were usable. The fuel inlet was located on the inboard tank wall near the tank’s aft end. The inlet was about 0.25 inch in diameter and its bottom was located about 0.25 inch above the tank’s bottom. The fuel quantity was provided by a float-type sending unit with the arm attached to the inboard tank wall near the forward portion of the tank. The forward wall height was about 7.75 inches, and the aft wall height was about 4 inches. Because of the tanks’ shape, the relationship between the fuel depth and fuel quantity for a given tank attitude and depth measurement location will not be constant. With the bottom of the tank level (in both axes), the fuel depth measured at the tank’s front wall will increase about 0.5 inch per gallon until the depth reaches 4 inches. For depths above 4 inches, due to the sloped top of the tank, fuel depth will increase about 1.02 inch per gallon until the tank is full. If the airplane is pitched down, the fuel will be pushed forward in the tank, and the float on the fuel sender will rise, indicating a higher fuel quantity than what is actually in the tank. Due to wing incidence and dihedral angles, the tank bottom will not be level during cruise flight, and the actual depth-to-quantity ratios will differ slightly from the above-cited values. Further, the angles resulted in the fuel level being higher at the outlet. With a 2-inch rise, about 0.5 and 1 gallon would result in about a 1-inch and a 2-inch fuel level at the port, respectively. AIRPORT INFORMATIONFuel System Design The airplane was equipped with pressurized fuel tanks in the inboard sections of each wing. Each tank had a supply line and a return line routed to the fuel selector valve, which was electrically actuated from a two-position (“left” and “right”) switch on the instrument panel. Downstream of the shutoff valve, the fuel line penetrated the firewall where there was a 90° elbow, and the fuel would flow to either of the two firewall mounted fuel filters. Each respective filter was located just below the main and auxiliary fuel pumps, each of which were electrically driven, where the fuel lines then combined and continued to the main fuel filter. The fuel return line was then routed through a fuel flow transducer, then to the injectors on the left side (Nos. 2 and 4 cylinders), then to the injectors on the left side and over the top of the engine to the right side (Nos. 1 and 3 cylinders), and then routed to the pressure relief valve. The fuel then continued passing through another transducer and a check/shuttle valve before passing through the firewall. The fuel was then returned to the selector and routed to the tank selected on the fuel selector. According to the CH-750 Pilot’s Operating Handbook, the two fuel tanks had a total capacity of about 24 gallons, of which 22 gallons were usable. The fuel inlet was located on the inboard tank wall near the tank’s aft end. The inlet was about 0.25 inch in diameter and its bottom was located about 0.25 inch above the tank’s bottom. The fuel quantity was provided by a float-type sending unit with the arm attached to the inboard tank wall near the forward portion of the tank. The forward wall height was about 7.75 inches, and the aft wall height was about 4 inches. Because of the tanks’ shape, the relationship between the fuel depth and fuel quantity for a given tank attitude and depth measurement location will not be constant. With the bottom of the tank level (in both axes), the fuel depth measured at the tank’s front wall will increase about 0.5 inch per gallon until the depth reaches 4 inches. For depths above 4 inches, due to the sloped top of the tank, fuel depth will increase about 1.02 inch per gallon until the tank is full. If the airplane is pitched down, the fuel will be pushed forward in the tank, and the float on the fuel sender will rise, indicating a higher fuel quantity than what is actually in the tank. Due to wing incidence and dihedral angles, the tank bottom will not be level during cruise flight, and the actual depth-to-quantity ratios will differ slightly from the above-cited values. Further, the angles resulted in the fuel level being higher at the outlet. With a 2-inch rise, about 0.5 and 1 gallon would result in about a 1-inch and a 2-inch fuel level at the port, respectively. WRECKAGE AND IMPACT INFORMATIONDuring postaccident examination, the fuel line between the fuel selector and gascolator was disconnected, and the fuel pumps activated, but no suction could be felt on the open line. The fuel line between the fuel shut-off and the gascolator was then disconnected, and the pumps activated, but no suction could be felt on the open line. The line was then disconnected at the 90° elbow forward of the firewall, the pumps were activated, and slight suction (as momentary surges) could be felt. The fuel line between the main filter and the transducer was then loosened, the pumps were activated, and strong suction could be felt, and fuel forcefully exited the line, consistent with air being trapped in the line. The remainder of the fuel system was examined, and no evidence of any preaccident mechanical malfunctions or failures was found that would have precluded normal operation. The fuel caps remained secured on the tanks. When air was forced from an air compressor nozzle into the outlet, the left tank pressurized, and when the fuel cap was removed, the pressure was released. The right tank could only be partially pressurized, and air could be heard escaping from the cap. Removal of the fuel caps revealed that they were vented with air passages from the center tab in the cap vented to the outer cap, but it could not be determined if they were clear. Two gallons of water were added to the left tank and the wing was positioned in an approximate wings-level attitude. The water reached the lower lip of the outlet. Changes in tank attitude (for example, tilting nose down or simulating a descent or a right turn) prevented any liquid from exiting via the tank outlet. The engine was test run twice, and it successfully started both times and ran for over 3 minutes at an idle power setting from 800 to 1,000 rpm with no anomalies noted. Examination of the engine revealed no evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation. ADDITIONAL INFORMATIONPreaccident Kit Manufacturer Service Letter (SL) The kit manufacturer issued a SL in February 2018 regarding CH model designs. It stated that some owners/builders think that the "standard vented cap is marginal…[and] that the tanks do not drain evenly. Some have also said that with one tank empty, fuel flow stops on the other tank." It further stated the following: Completing in-flight tests during Phase 1 should determine if the fuel system is functioning properly. In the event that additional fuel flow is desired, you can install a simple gooseneck tube to the fuel cap(s)...This will force air into the tank to increase fuel flow. Or, if you notice that fuel is not flowing evenly from both tanks with a simple ON/OFF valve, your fuel lines and valve can be changed so that you can select fuel from just LEFT, RIGHT or BOTH. Postaccident Engine Manufacturer Service Bulletin (SB) About a year after the accident, the engine manufacturer issued an SB recommending that owners install a restricted bypass fuel line orifice between the pressurized fuel supply and the fuel return line to, in part, “improve…recovery from air/vapour ingestion” and to allow “fuel/air/vapour to flow in small quantities from the supply line on the pressure side of the fuel pump to the low pressure return line,” and result in a “much quicker return to normal engine operations.” NTSB Accident Database A search of the National Transportation Safety Board’s accident/incident database revealed three previous accidents involving CH-750 airplanes that had probable causes related to the fuel system. ERA14LA183: The fuel system's inadequate design, which resulted in negative pressure in the right fuel tank and a total loss of engine power during cruise flight due to fuel starvation. CEN17LA092: A failure of the right fuel tank to supply fuel to the engine for reasons that could not be determined, which resulted in a total loss of engine power due to fuel starvation. CEN17LA021: A total loss of engine power due to fuel vapor lock.
A total loss of engine power due to air or fuel vapor entering the fuel system, which prevented fuel from reaching the engine cylinders, which necessitated an emergency landing, during which the airplane nosed over.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
