Loss Of Power Of The No. 1 Engine For Reasons That Could Not Be Determined

Location: Fairbanks, Alaska Accident Number: ANC24FA029
Date & Time: April 23, 2024, 10:03 Local Registration: N3054V
Aircraft: Douglas C54D Aircraft Damage: Destroyed
Defining Event: Explosion (non-impact) Injuries: 2 Fatal
Flight Conducted Under: Part 91: General aviation - Other work use

Analysis: The purpose of the flight was to transport fuel and propane tanks in the airplane, which was powered by four radial engines. About three minutes after takeoff, the pilots reported to air traffic control that there was a fire onboard and began a left turn back to the airport. 

Eyewitnesses who saw the airplane shortly after takeoff reported that the outboard left (No. 1) engine was not running, and that the engine was trailing a small, white plume of smoke, followed shortly thereafter by visible flames. Surveillance video captured the white smoke and flames emanating from the No. 1 engine. Seconds after flames appeared, a bright white explosion could be seen from aft of the No. 1 engine. The airplane then entered an uncontrolled, descending left turn into terrain.

Review of the airplane’s maintenance records revealed the No.1 engine was replaced with an overhauled engine one week before the accident. According to the director of operations, the outboard left fuel tank, located near the No. 1 engine, was reported in the days before the accident to leak when full. The fuel leaked into a space behind the No.1 engine and dripped out of the wing behind the engine. Although a repair had been made, this repair did not fix the leak. 

For several days before the accident, the airplane was parked with full fuel tanks, dripping fuel into the wing space behind the No. 1 engine. The examination of the No.1 engine and portions of the recovered airframe revealed an AN-8 hose resting in the exhaust heat shield area. The hose had burned from the fitting and the threads were in good condition with no apparent mechanical damage. A portion of the firewall, which was separated from the engine, engine mount, and remaining portion of the airframe, had a 90° elbow -8 sized AN bulkhead fitting installed. One end of the  fitting had a B-nut and metal hard line installed. The opposing end did not have a B-nut or hose attached. The threads 
of the fittings were intact and undamaged. The fitting was part of the propeller feathering system. Oil residue was observed throughout the entire external area of the exhaust system. The outer structure exhibited a light residue of  oil on the external side.

Based on the witness statements of the No. 1 engine not running and trailing smoke, it is likely that the No. 1 engine lost power shortly after takeoff; however, due to the extensive thermal and impact damage to the engine, the reason for the loss of power could not be determined based on the available information. Following the loss of engine power, the pilots would have attempted to feather the propeller. It is likely that, when the feathering pump system was activated, the incorrectly installed B-nut near the engine firewall would have produced a spray of high-pressure oil around the hot exhaust system. This would be consistent with the initial white smoke and fire seen in the video, and the oil residue found in the area of the exhaust system. The fuel that had leaked from the outboard left fuel tank into a compartment behind the No. 1 engine subsequently ignited, resulting in the explosion that separated the airplane’s aileron bell housing and resulted in the pilots’ inability to control the airplane and subsequent impact with terrain.

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this accident to be -- A loss of power of the No. 1 engine for reasons that could not be determined, and the incorrect installation of a B-nut fitting in the propeller feathering system, which allowed engine oil to spray onto the exhaust system when the propeller was feathered following the loss of engine power. Contributing to the accident was an incorrectly repaired fuel leak, which resulted in an explosion that separated the aileron bell housing that resulted in a loss of control and subsequent impact with terrain. 

FMI: www.ntsb.gov