HISTORY OF FLIGHTOn August 25, 2021, about 1822 eastern daylight time, an Acro Sport II experimental airplane, N169BF, was substantially damaged when it was involved in an accident near Fishers, Indiana. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the purpose of the flight was to “break-in” two engine cylinders that were replaced on the day before the accident. According to the pilot’s statement and fueling documentation, the airplane’s 23-gallon fuel tank was topped off before the flight. The pilot reported that the engine was operated at maximum speed (2,800 rpm) with a full rich mixture setting throughout the flight and that the airplane lost total engine power about 1 hour after takeoff. During the forced landing, the forward right side of the fuselage impacted a street signpost, resulting in substantial damage to a structural tube in the fuselage. AIRCRAFT INFORMATIONThe experimental biplane was designed for aerobatic flight, and its engine was modified with oversized cylinders and pistons. The engine had no data plate or serial number. During the airplane’s last condition inspection, the tachometer was reset to 0 hours. At that time, the airframe total time and engine time since overhaul were 641 and 60 hours, respectively. On August 24, 2021, cylinder Nos. 3 and 4 were replaced after being refurbished due to low compression. The mechanic, who was also the pilot/owner, reported no issues during his engine runup after the cylinder replacements. At that time, the airframe total time and engine time since overhaul were 669 and 88 hours, respectively. Air traffic control radar track data showed that the pilot flew the airplane about 1 hour 15 minutes (1.25 hours) during the accident flight. Thus, at the time of the accident, the airframe total time and engine time since overhaul would have been 670 and 89 hours, respectively. AIRPORT INFORMATIONThe experimental biplane was designed for aerobatic flight, and its engine was modified with oversized cylinders and pistons. The engine had no data plate or serial number. During the airplane’s last condition inspection, the tachometer was reset to 0 hours. At that time, the airframe total time and engine time since overhaul were 641 and 60 hours, respectively. On August 24, 2021, cylinder Nos. 3 and 4 were replaced after being refurbished due to low compression. The mechanic, who was also the pilot/owner, reported no issues during his engine runup after the cylinder replacements. At that time, the airframe total time and engine time since overhaul were 669 and 88 hours, respectively. Air traffic control radar track data showed that the pilot flew the airplane about 1 hour 15 minutes (1.25 hours) during the accident flight. Thus, at the time of the accident, the airframe total time and engine time since overhaul would have been 670 and 89 hours, respectively. WRECKAGE AND IMPACT INFORMATIONPostaccident examination of the airplane found that its fuel tank was empty, with 2 to 3 fluid ounces of fuel recovered when the fuel system was drained during the examination. Evidence of a fuel leak was found within the engine compartment and on the exterior lower fuselage skins aft of the firewall. This evidence included areas of fuel pooling, droplets of fuel dripping from control cable clamps, a fuel sheen on the firewall, and fuel streaking on the exterior lower fuselage skins aft of the firewall. Engine control continuity was confirmed from the cockpit controls to the throttle arm and mixture control at the fuel servo. The fuel inlet fitting to the fuel control unit was finger tight. In preparation for an engine test the loose fuel inlet fitting was tightened (to determine the fuel consumption at the maximum static engine rpm), and the fuel system was pressurized at least twice, which confirmed that no other leak sources existed. During the test, the engine started and ran normally without any hesitation, stumbling, or interruption in power. No evidence of a fuel system leak was found after the engine run. The fuel consumption rate at maximum engine power on the ground (static power) was about 13.69 gallons per hour at 2,260 rpm. On the basis of the engine test run results, the estimated fuel consumption rate while in flight at maximum engine power (2,800 rpm) was about 16.96 gallons per hour. With a full fuel tank (23 gallons) at departure, the airplane’s fuel endurance would have been about 1 hour 21 minutes (1.35 hours) if the engine was operated at maximum power with a full rich mixture setting. Other than the loose fuel inlet fitting, the engine examination and test run revealed no mechanical malfunctions that would have prevented normal engine operation during the flight.