Thonotosassa, FL, USA
N611E
VANS RV7
During the previous flight, the experimental, amateur-built airplane lost total engine power, and the pilot, who was the builder of the airplane, completed a successful forced landing with no damage to the airplane. He found that the fuel pressure adapter “t”-fitting, used to attach the fuel pressure line, had disconnected from the fuel pump elbow fitting. The automotive adapter fitting was adopted during the airplane’s construction for fitment in a confined space and did not allow for measured torque application. The pilot reattached the fitting 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. The accident flight was the airplane’s next flight. Minutes after takeoff, 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. Examination of the adapter revealed that its material was soft relative to materials used in Air Force-Navy (AN) standard fittings. As a result, the shoulder on the adapter body could have been deformed under the pressure of the retaining ring even when applying minimum torque values suggested for AN standard fittings made of aluminum alloys specified in military specification. This deformation may have inhibited the proper frictional resistance in the threads to prevent the nut from backing off in service. Additionally, the body of the adapter fitting tended to rotate when applying torque to the nut. If the body was not properly secured while torque was applied to the nut during field repairs, the twisting of the adapter fitting body could have produced stresses in the attached hoses in the assembly that may have worked to rotate the adapter fitting and nut in the loosening direction during operation.
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.
The pilot’s failure to apply adequate torque on the fuel pressure line aluminum adaptor fitting, which resulted in the loosening of the adapter, a loss of fuel flow, and a total loss of engine power. Contributing to the accident was the pilot’s choice of a soft aluminum automotive fitting, and the design, which required hand-tightening with a shortened, modified wrench that prevented a measured torque application.
Source: NTSB Aviation Accident Database
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