On September 13, 2019, about 1730 mountain standard time, a Piper PA-28-181 airplane, N4403Z, was substantially damaged when it was involved in an accident near Marana, Arizona. The flight instructor, student pilot, and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. According to the flight instructor, after landing at Marana Regional Airport (AVQ), Marana, Arizona, they exited the airplane, used the airport facilities, and had the airplane refueled. About 30 minutes later, the instructor completed a preflight inspection, during which he did not see any fuel or liquid leaking from the airframe or engine. They boarded the airplane and performed the engine start checklist. The instructor stated that he attempted to start the engine but was unsuccessful. During a second unsuccessful engine start attempt, the instructor and the two other occupants smelled smoke. They exited the airplane and saw smoke coming from the nosewheel well. The airplane subsequently caught fire and continued to burn for about 15 minutes before the fire department arrived. Postaccident 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. Examination of the engine and airframe revealed no evidence of preaccident mechanical malfunctions or anomalies that would have precluded normal operation. 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 the airplanes, including the accident airplane, were manufactured in 2013 and one was manufactured in 2019. All of the airplanes that experienced the flooding and subsequent fires had been previously flown 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 occurring during the engine start process. The operator also reported that they recently had a significant increase in the student population from about 225 to an excess of 700 students, with an increased frequency of flights per day. Other flight schools in the area that operated the PA-28-181 did not report similar flooding or engine fires during engine start. 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 in excess of 200° F then the engine was shut down. Following shutdown, the temperature of the fuel system components increased and fuel pressure throughout the fuel system to the carburetor rose. After a certain amount of time, the engines were restarted and about 6 to 12 ounces of fuel and vapor were observed emanating from the carburetor intakes. In some of the tests, while vapor was escaping, a whistling sound was heard originating from the carburetor inlet. 100 low lead aviation fuel begins to vaporize at approximately 106°F, and for a given volume, the higher the temperature of the fuel, the greater the ratio of vapor-to-liquid formation. A PA-28-181 manufactured in 1976, which was equipped with a Piper fuel cooling modification installed per Service Letter (SL) 427, Fuel Cooling Kits, 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 SL427 “Fuel cooling kits” in 1964, which specifies installation of the 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.