Accident Overview
On September 13, 2019, at about 1730 mountain standard time, a Piper PA-28-181 airplane, registration N4403Z, was substantially damaged in a fire after an unsuccessful engine start attempt at Marana Regional Airport (AVQ), Marana, Arizona. The aircraft was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor, student pilot, and passenger were not injured.
Sequence of Events
The flight instructor reported that after landing, the occupants exited the airplane, used airport facilities, and had the airplane refueled. Approximately 30 minutes later, the instructor completed a preflight inspection and observed no signs of fuel or liquid leaking from the airframe or engine. The occupants then boarded and performed the engine start checklist. The instructor attempted to start the engine but was unsuccessful. During a second unsuccessful start attempt, the instructor and the two other occupants smelled smoke. They exited the airplane and observed smoke coming from the nosewheel well. The airplane subsequently caught fire and burned for about 15 minutes before the fire department arrived.
Postaccident Examination
Examination of the airplane revealed fire damage to the forward fuselage and cowling. Blue staining was found at the airbox. The carburetor was disassembled, and the accelerator pump needle valve showed wear at the valve contact area. However, examination of the engine and airframe found no evidence of preaccident mechanical malfunctions or anomalies that would have precluded normal operation.
Fleet-Wide Issues
The operator reported that within a 6-month span, six of their PA-28-181 airplanes experienced flooding and subsequent fires during the engine start process. Five of these airplanes, including the accident airplane, were manufactured in 2013, and one was manufactured in 2019. All of the airplanes that experienced flooding and fires had been previously flown and then parked for times ranging between 5 and 28 minutes before the engines were restarted. The daily average temperatures at the time of each event were between 85° and 98°F.
The operator had been operating for 7 years without any engine fires during engine start. They noted a recent significant increase in student population from about 225 to over 700 students, with an increased frequency of flights per day. Other flight schools in the area operating the PA-28-181 did not report similar flooding or engine fires during engine start.
Testing Results
Testing was performed on the operator’s fleet of PA-28-181 airplanes during hot climate conditions. Engine runs were conducted to establish oil temperatures above 200°F, then the engine was shut down. Following shutdown, the temperature of fuel system components increased, and fuel pressure throughout the system to the carburetor rose. After a certain time, the engines were restarted, and approximately 6 to 12 ounces of fuel and vapor were observed emanating from the carburetor intakes. In some tests, a whistling sound was heard originating from the carburetor inlet while vapor was escaping.
100 low lead aviation fuel begins to vaporize at approximately 106°F. For a given volume, the higher the fuel temperature, the greater the ratio of vapor-to-liquid formation.
A PA-28-181 manufactured in 1976, equipped with a Piper fuel cooling modification per Service Letter (SL) 427, was also tested. During the engine run, the engine oil temperature was increased to 230°F and the engine was shut down. The testing could not duplicate the increase in fuel pressure.
Piper issued SL 427 in 1964, specifying installation of the fuel cooling kit at the owner’s discretion “in areas where high atmospheric temperatures may be encountered, and additional cooling is desirable.” This modification was discontinued for unknown reasons. None of the operator’s airplanes were equipped with the Piper fuel cooling modification.
The operator reported that after installing an ignition modification that increased spark energy during engine start on all of their airplanes, no additional flooding events or engine fires occurred.