Background
A maintenance discrepancy on the number one fuel tank quantity indicator had been present on Navy Hercules 151890 for over three months prior to the accident. Before the mishap flight, maintenance personnel attempted to repair solder connections in the cannon plug but could not complete the repair in time. They hastily reassembled the plug and verbally instructed the oncoming flight engineer to keep the number one fuel tank quantity indicator system circuit breaker pulled to prevent the indicator motor from running and damaging the internal clutch.
The aircraft subsequently flew four flights before the accident. The verbal warning was relayed between flight engineers for the first three flights but was not passed along for the fourth. During preflight for the fourth flight, the engineer noticed the circuit breaker was out and reset it. The breaker tripped within seconds and was left out for the return flight. Between landing and the following day's preflight by the mishap crew, an unknown person reset the circuit breaker.
Flight Events
Shortly after takeoff from Patuxent River Naval Air Station, Maryland, while climbing through 7,500 feet, an explosion occurred in the outboard section of the left wing. A fire immediately erupted. The number one engine was shut down and the fire extinguisher bottles were discharged. However, the crew was unable to control the fire. Control difficulties necessitated a forced landing. The aircraft performed an emergency descent at 270 knots indicated airspeed and executed a forced landing in a cornfield. After the aircraft came to rest, the fire continued burning for 36 minutes, spreading aft and inboard, before being extinguished by a local volunteer fire department.
Investigation Findings
The official investigation determined that the explosion occurred in the number one main fuel tank in the forward center area beneath the outboard access plate, near the number 10 fuel quantity probe. The explosion tore open the upper forward wing surface from wing station 369 to 576. The fuel instantly ignited into a continuous fire. Fifteen feet of the outer left wing was consumed by the fire.
The cause of the explosion was the introduction of 115-volt, single-phase, 400 Hz power into the fuel quantity indicating system for the number one main fuel tank. This resulted in an electrical arc in the fuel tank's airspace. The arc likely originated from either the number 10 fuel quantity probe to an unknown ground, or from the coaxial cable in the immediate vicinity of that probe to an unknown ground.