History of Flight
On April 22, 2020, about 1110 mountain daylight time, a Czech Sport Aircraft SportCruiser, N802PS, was substantially damaged during an instructional flight at Logan-Cache Airport, Logan, Utah. The pilot was not injured. The student pilot was conducting his second solo cross-country flight. He departed South Valley Regional Airport, Salt Lake City, Utah about 0900, completed two touch-and-go landings at Ogden, Utah, then continued to Logan for a full stop landing. After a short time, he started the engine and performed a magneto check during run-up; the engine momentarily lost all power. He shut down and spoke with his instructor by cell phone. On subsequent starts, the electric fuel pump sounded louder than normal, and fuel pressure fluctuated within normal range but dropped to 1.8 psi when avionics were turned on, later recovering. He completed a normal run-up and taxied to runway 28. During departure, at about 50 to 100 ft above ground level, the engine surged and fuel pressure decreased to 1.0 psi. He retarded the throttle and landed straight ahead, but the airplane touched down hard, resulting in substantial damage to the fuselage.
Wreckage and Impact Information
The airplane was equipped with a Dynon Avionics Flight DEK-D120 EFIS and engine monitoring system. Downloaded data contained several erroneous parameters. Recorded data showed the accident flight started with about 9.4 gallons in each fuel tank. Engine rpm increased to 4,954 rpm when fuel pressure suddenly decreased to 1.9 psi, then to 1.1 psi, with fuel flow at 9.8 gph. Subsequently, rpm decreased, and fuel flow oscillated between 0.9 and 4.6 psi. Just before the accident, fuel flow was below 2.2 psi and oil temperature rose to 180°F.
A mechanic disconnected fuel lines from each carburetor and activated the electric fuel pump; fuel flow appeared normal with no debris. A postaccident examination by an NTSB investigator found no impact damage to the engine exterior. A fuel source was connected and the engine ran multiple times with no anomalies. Examination of the fuel system after flow testing revealed the fuel return line was plumbed to the right wing, and the fuel line from the pump drain was routed to the slipstream. The Facet Automotive Cube electric fuel pump was plumbed in parallel with the BDC Corona engine-driven pump; there was no check valve. When activated, the electric pump made a loud noise that stopped once primed, and fuel pressure went from 1.0 to 3.5 psi for about 1 minute, then fluctuated between 1.6 and 1.7 psi.
The Rotax Installation Manual specified the electric fuel pump should be in series with and before the engine-driven pump, required for malfunction or defect of the engine-driven pump and to preclude vapor lock. The installation diagram depicted a fuel bypass circuit, but the accident airplane was not plumbed accordingly. Section 14.2 of the Rotax IM cited minimum, normal, and maximum fuel pressure limits as 2.2, 4.4, and 5.8 psi, respectively, measured at the fuel manifold. A warning stated that fuel pressure exceeding limits can override the float valve and cause engine stop. The IM also noted that an auxiliary pump must ensure operation within specified pressure limits and cautioned that auxiliary pump pressure should not exceed 4.4 psi.
The airplane had a fuel flow transducer with an internal rotor. According to a Rotax technical representative, introduction of air into fuel lines can cause higher-than-normal fuel flow readings (over 8 to 9 gph) because air spins the rotor faster, and vapor lock can appear as spikes in fuel flow. Air pulses can cause the rotor to spin back and forth, resulting in jumping fuel flow readings.
Additional Information
The Pilot's Handbook of Aeronautical Knowledge defines vapor lock as a problem affecting gasoline-fueled engines when fuel changes from liquid to gas in the delivery system, disrupting pump operation and causing loss of feed pressure, transient power loss, or stalling. Restarting may be difficult. Fuel can vaporize due to engine heat, climate, or lower boiling point at altitude.