On February 9, 2021, at 1453 eastern standard time, an experimental, amateur-built Vans RV7, N611E, was substantially damaged when it was involved in an accident near Thonotosassa, Florida. The private pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 test flight.
The pilot, who was the builder of the airplane, was completing Phase 1 flight testing of the airplane. He stated that, two days before the accident, the airplane experienced a loss of fuel pressure and a total loss of engine power. He completed a successful forced landing uninjured, and during the subsequent inspection of the airplane, no damage was found. Examination of the engine revealed that the fuel pressure adapter “t”-fitting, used to attach the fuel pressure line, had disconnected from the fuel pump elbow fitting.
According to the pilot, the adapter fitting was adopted during the airplane’s construction for fitment in a confined space. Reattachment of the fitting was accomplished with a modified and shortened 5/8” spanner wrench for the 11/16” b-nut. The pilot departed the forced landing site and landed successfully at his destination.
Minutes after departing on the accident flight, the airplane once again experienced a loss of fuel pressure and a total loss of engine power. During the subsequent forced landing, the pilot sustained minor injuries and the airplane was substantially damaged. Post-accident examination of the airplane revealed that the same fuel pump elbow fitting was disconnected from the adapter fitting.
The airplane’s airworthiness certificate was issued January 21, 2021. According to the pilot, the first loss of engine power occurred about 3.5 total aircraft hours. The second total power loss occurred about 4.1 total aircraft hours.
A Federal Aviation Administration aviation safety inspector recovered the adapter fitting and performed some rudimentary, comparative, hand-tightened torque applications with the shortened, modified wrench, and a standard 11/16” spanner and recorded those tests.
According to the pilot, the aluminum adapter fitting attached to the steel fuel pump elbow fitting was purchased at an automotive “speed shop.”
In the pilot’s NTSB Form 6120.1 Pilot/Operator Accident Report Form, the pilot stated, “Fuel line 'T' fitting disconnected from engine fuel pump fitting elbow most likely due to improper torquing.”
The fitting and associated parts were examined at the NTSB Materials Laboratory. Examination and research revealed material hardness below an Air Force Navy (AN) military specification for the aluminum adapter fitting. The manufacturer of this automotive fitting stated that there was no specified torque value, but that the fitting should be tightened to a “snug” condition. Multiple sources cited a 150 to 195-inch-pound torque value for the fitting or the “flats” method (1 to 1.5 flats past finger tight). When the flats method was attempted in the lab, two to three times the torque value specified was required to rotate one flat and, once the nut was removed, the shoulder of the adaptor body displayed damage. Further, the body of the adaptor fitting rotated when torque was applied to the nut. The movement of the adapter fitting created the potential for stresses in the fuel lines to apply force in the loosening direction.