On May 14, 2019, about 0919 mountain daylight time, a Vought F4U-7 airplane, N965CV, was substantially damaged when it was involved in an accident near Jerome, Idaho. 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 airplane had just finished undergoing an annual inspection that spanned 8 months. After arriving to the airport, he looked over the airplane and asked his mechanic if he had flown the airplane. The mechanic responded that he “couldn’t get it running;” however, “he adjusted everything, and got things working.” The pilot said that his mechanic told him that the engine was “loading up” on the ground and that the engine would have to be manually leaned instead of using the mixture in the auto-lean position; however, it should not be a problem in the auto-rich position.
The pilot conducted a preflight inspection and taxied the airplane to the end of the runway, where he placed the mixture in the auto-lean position. He subsequently saw black smoke originate from the exhaust, and the engine sputtered. The pilot manually leaned the engine, and “it cleared up.” The pilot placed the mixture in the auto-rich position, performed a magneto check, then taxied onto the runway, where the engine “loaded up” a second time. The pilot stated that he manually leaned the engine, then moved the mixture to the auto-rich position while he advanced the throttle to 30 inches of manifold pressure. He said that he held this power setting for about 1 minute, and, after detecting no anomalies, he initiated the takeoff roll.
The pilot stated that, as the airplane accelerated beyond 100 to 110 knots, the engine “coughed” and he reduced power to idle and aborted the takeoff but “ran out of runway.” The airplane exited the right side of the runway and impacted a dirt berm and fence before it came to rest upright.
The mechanic reported that, while watching the airplane during the takeoff, the engine gradually came up to power, and appeared to reach full power. The mechanic said that, at the time he would have expected the airplane to become airborne, the airplane remained on the runway in a tail-high attitude. The mechanic said that about that time, the airplane was passing midfield, and the tail of the airplane appeared very high shortly before the pilot aborted the takeoff.
A video of the takeoff and accident sequence was captured by a witness located on the airport property. The video showed the airplane in a slight tail-low attitude during the takeoff roll. Shortly after the airplane passed the windsock and airport weather reporting station, a reduction in engine power was heard (refer to the following figure). Shortly thereafter, a streak of smoke originated from the left main landing gear. About 3 seconds later, as the tail of the airplane transitioned onto the ground, another streak of smoke originated from the left main landing gear. The airplane was observed exiting the right side of the runway and became airborne briefly before it impacted the ground and cartwheeled.
Figure 1: Diagram showing the area where a reduction of power was heard along with a screen shot from the captured video.
The mechanic reported that, in the days before the accident, he secured the airplane and conducted an engine run at full power. The mechanic said that he made fuel enrichment changes on the ground and told the pilot that the engine would run rich on the ground and that he should manually lean it but to use auto-rich at full power. The mechanic added that the engine ran well for him during all previous engine runs. Additionally, the mechanic said that he did not test fly the airplane before the pilot’s arrival due to weather conditions and was later told by the pilot that he would fly it.
The pilot stated that, in hindsight, he should not have flown the airplane but that he did because he wanted to get the airplane to an airshow and relied on the information provided by the mechanic. The pilot stated that he should have removed the carburetor for overhaul instead.
Postaccident examination of the airplane revealed that the left-wing aft spar was structurally damaged. Examination of the engine by a mechanic under the supervision of a Federal Aviation Administration inspector revealed that magneto-to-engine timing was correct. Thumb compression was obtained on all cylinders. Borescope examination of the cylinders revealed no evidence of detonation or preignition. The magnetos were removed and installed on a test bench. When tested, both magnetos functioned normally. The carburetor was undamaged and was removed for functional testing.
The carburetor was further inspected at the facilities of Vintage Carburetors, Tehachapi, California, under the supervision of a National Transportation Safety Board investigator. The carburetor was installed on a test bench and functionally tested at various settings. During the flow tests, the fuel flow rates for higher power settings were below, or leaner, than the specified fuel flow limits. At lower power settings, the fuel flow rates were either within or slightly higher than specified limits.
The carburetor was removed from the test bench and disassembled and inspected. During the disassembly, the enrichment valve diaphragm was found intact; however, the diaphragm was noted to be stiff. According to representatives from Vintage Carburetors, the stiff enrichment valve diaphragm would cause the fuel flow at the higher settings to be lower than required and would affect the fuel flow in both the auto-lean and auto-rich settings. The company representative further stated that, when the diaphragm of the enrichment valve begins to stiffen, power settings from idle to high cruise power settings will run rich with the mixture lever in the auto-rich and auto-lean positions; however, at higher power settings, such as takeoff power, the fuel flow would be lean if the mixture lever is in the auto-rich position, inducing the possibility of detonation.