Amateur-built SUPER GLASAIR TT N400MC accident near Madera, California

Probable cause

The loosening of a B-nut in the fuel system which resulted in a rapid, undetected fuel leak and subsequent fuel exhaustion.

NTSB analysis

The commercial pilot, who was also the builder of the experimental airplane, departed with full fuel for the planned 2.5-hour cross-country flight. The airplane was equipped with two independent means of determining fuel quantity: a capacitance-based fuel quantity indication system that provided a direct measurement of the fuel quantity and a fuel totalizer that provided a calculated value using a pilot-entered initial fuel quantity and a sensed fuel flow rate. The pilot reported that both quantity indication systems were reliable and normally registered similar values. While in cruise, after he had been airborne for about 1.5 hours, the pilot noted that each system indicated a remaining fuel quantity of about 38 gallons, which was ample fuel for the completion of the trip. A few minutes later, the engine lost all power. The pilot noticed that the totalizer indicated about 37 gallons remaining but that the capacitance system indicated minimal or no fuel remaining. The pilot determined that he would be unable to reach the nearest airport and set up for a forced landing on a road. Due to motor vehicle traffic, the pilot landed on the side of the road, and the airplane sustained substantial damage due to impact with a bordering fence and vegetation.

Postaccident examination of the airplane revealed that a B-nut in the fuel system had backed off. The loose B-nut was just downstream of a fuel pump, which allowed fuel to leak overboard at a location that could not be seen from the cockpit by the pilot. Because of the leak location, once the leak began, the totalizer-indicated fuel quantity became erroneous, but the capacitance-based system quantity remained accurate. The evidence indicated that once the leak began, the fuel was depleted rapidly. The pilot reported that when he was building the airplane, he installed the B-nut, torqued it to prevent loosening, and marked it with a torque stripe. The airplane had been flown about 56 hours since the B-nut was installed. Although maintenance activity conducted to repair an oil leak near the B-nut could have resulted in inadvertent and undetected loosening of the B-nut, the investigation was unable to determine a specific cause for the loosening of the B-nut.

Text from the NTSB. Open the NTSB docket for WPR14LA237 for the full record.