Mooney M20C accident near College Park, Maryland, April 3, 2021

The NTSB's narrative final · quoted from the NTSB record

The pilot planned for a recreational trip of 2 hours or less. During his preflight inspection, he found, “a bunch of water” in the left fuel tank, which he said he continued to sample until it was free of water. He then dried his collection tube and sampled the tank twice more reporting the sample was clear with no water. Before departure, with the fuel selector positioned to the left fuel tank, he performed an engine run-up with no engine or engine systems discrepancies reported. During takeoff he rotated at 83 mph and when over the runway the engine began to “cough/sputter.” He continued straight ahead, but review of airport surveillance video revealed that the airplane likely stalled and landed hard resulting in substantial damage to the left wing. A postaccident examination of the airplane found that there was water in the flexible fuel hose from the firewall fitting to the engine-driven fuel pump inlet, and about 5 ounces of fluid drained from the carburetor bowl contained equal parts of fuel and water. On-wing pressure testing of the fuel caps revealed extensive leakage from both and a subsequent examination revealed evidence of unapproved or missing parts from both caps. The correct parts were then installed on both caps and they passed testing to 25 psi, but leakage was noted during on-wing pressure testing. Additional testing identified leakage between the adapter and the doubler of each assembly at low pressures, which increased in intensity as the pressure increased. Prior to the accident flight, the co-owners of the airplane replaced the seals on both fuel caps which did not eliminate the water intrusion. In addition, they purchased wing covers in an attempt to further reduce the potential for water intrusion; however, they were not installed on the airplane when the pilot arrived at the airport for the accident flight. They did not alert their mechanic of the issue. Although the pilot reportedly sampled the left fuel tank until water was no longer present, he likely did not drain out all the water from the tank nor did he drain the selector valve sump as part of his inspection. The results of the extensive postaccident testing revealed that even if the fuel caps were repaired properly, additional issues were present with each fuel tank filler cap assembly that were not addressed by the owners. Had the owners consulted with their maintenance provider about the water intrusion, it is likely that the root cause of the water intrusion could have been identified.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.