Piper PA28 N4403Z accident near Marana, Arizona

Probable cause

Excessive fuel pressure and temperature throughout the fuel system due to heat soaking, which resulted in an engine fire during engine start.

NTSB analysis

During engine start about 30 minutes following the previous shutdown, the engine caught fire, which resulted in substantial damage to the airplane. The operator reported that six of their airplanes experienced flooding and subsequent fires during the engine start process within the previous 6 months. The airplanes that experienced flooding and fires had all previously flown and been parked between 5 and 28 minutes before the engines were restarted and the flooding occurred. During these events, the daily average temperatures were between 85° and 98°F.

Testing conducted to simulate the conditions of the engine fire events revealed that, in the minutes following engine shutdown, the temperature of the fuel system components and fuel pressure to the carburetor would rise, most likely as a result of heat soaking from the combination of the hot engine and high outside air temperature. The tests showed that during the engine restart, about 6 to 12 ounces of fuel and vapor emanated from the carburetor intakes. During some of the tests, a whistling sound was heard originating from the carburetor inlet, which is likely from the buildup of fuel vapor as a result of the heat soaking and escaping through the internal passages of the carburetor before exiting out the carburetor intake.

The carburetor from the accident airplane was disassembled, and the accelerator pump needle valve showed wear at the valve contact area. This wear was likely a result of the high fuel pressure vapor and fuel escaping past the valve, causing it to chatter excessively. The operator had experienced recent growth in its student population, which resulted in increased frequency of flights during the day. This most likely prevented the engine and fuel system from cooling between flights.

Text from the NTSB. Open the NTSB docket for WPR19LA259 for the full record.