Czech Sport Aircraft As PIPER SPORT N422PS accident near Addison, Texas

Probable cause

The operator’s use of improper fuel, which resulted in partial loss of engine power due to vapor lock.

NTSB analysis

According the operator, before the flight, the airplane was serviced with winter-grade 93-octane automotive gas. The flight instructor and one passenger were conducting a local introductory instructional flight; the outside ambient temperature was 81°F. The airplane's recorded data showed that the engine was running for about 18 minutes during engine start-up, taxi, engine run-up, and while holding short of the runway. During the engine run-up, the fuel flow was about 6 gallons per hour (gph), and the fuel pressure was between about 6 and 7 pounds per square inch (psi) at an engine speed of 4,000 rpm.

Shortly after departure, the pilot stated to an air traffic controller that "we're having vapor lock, we need to come back and land." During the initial climb, the fuel flow increased to a peak of 9 gph, and the fuel pressure decreased to 3.3 psi at 5,025 rpm. The airplane made a climbing left turn to the southeast, continued circling left as the engine surged, and then descended toward the end of the departure runway before the flight track data ended. An airport surveillance video showed that, the airplane subsequently continued descending in a wings-level attitude near the end of the runway before it entered a sharp left bank and then impacted the ground.

Examination of the airplane revealed no evidence of any preexisting mechanical malfunctions or failures that would have precluded normal operation. Although the engine manufacturer recommended the use of mogas; however its Engine Operating Manual (EOM) warned to only use “fuel suitable for the respective climatic zone” and that there was a “risk of vapor formation if using winter [grade] fuel for summer operation.”

The EOM stated that the fuel flow at takeoff is 7.1 gph. When an engine experiences vapor lock, air enters the fuel flow sensor and causes the unit to spin at a higher rate than if only fuel was passing through, and it is often represented as spikes in fuel flow. Therefore, the fuel flow reaching 9 gph during the initial climb was consistent with vapor lock. It is likely that, due to the use of a winter-grade fuel and the high ambient outside temperature, fuel vapor formed in the fuel system, which resulted in vapor lock and a partial loss of engine power.

Text from the NTSB. Open the NTSB docket for CEN18LA119 for the full record.