Accident at Miami, Florida, 20 Dec 1957

Accident narrative

On December 20, 1957, a Riddle Airlines C-46R developed an uncontrollable powerplant fire shortly after takeoff from Miami Airport in Miami, Florida. The crew returned to the airport and landed safely nine minutes after departure. The two pilots evacuated uninjured as ground equipment smothered the fire, but the aircraft sustained extensive damage.

### The flight

Flight 2 of the 20th was a scheduled cargo flight from Miami to New York, New York, with a planned stop in Stuart, Florida. It carried no cargo out of Miami.

Captain Otis E. McLendon had over 10,000 flight hours, including 5,000 in C-46 aircraft. First Officer Sheldon A. Crocker had about 2,900 hours, with 2,400 in the C-46. The aircraft's gross weight and center of gravity were within prescribed limits, and excellent weather prevailed.

### Sequence of events

Takeoff from runway 9L was normal at 1510. At an altitude of 900 to 1,000 feet, an estimated two minutes after takeoff, the right engine's BMEP gauge suddenly dropped to zero. The aircraft did not yaw. When the captain closed the right throttle, the BMEP gauge returned to normal except for fluctuations. The right manifold pressure gauge dropped to about 30 inches (atmospheric) and did not respond to subsequent throttle movement.

A few seconds later, the right tachometer and right fuel flowmeter started fluctuating. Almost immediately, the right engine fire warning lights for zones 1, 2, and 3 illuminated. The first officer reported a fire in the right nacelle, and the captain closed the right throttle.

Captain McLendon actuated the firewall shutoff valve switch, feathered the right propeller, and actuated the No. 2 fire extinguisher switch. The first officer informed the captain that the fire seemed unaffected from the cockpit. The captain ordered the use of the second extinguisher charge. The first officer actuated the No. 1 switch, but there was no apparent effect.

Meanwhile, the tower observed smoke from the right engine, advised the flight, and cleared the aircraft to land on any runway. The flight landed on runway 17 with the landing gear down and wing flaps extended. Tower-alerted airport fire apparatus met the aircraft and quickly brought the fire under control.

### What the investigation found

Ground witnesses observed heavy black smoke trailing the aircraft, and one witness saw a 5- to 10-foot flame coming from the engine's right side for about 20 seconds. The heavy smoke temporarily disappeared—which the Board believed followed the application of the fire extinguisher—before returning as heavy white smoke on the downwind leg. The right engine became enveloped in flame after the aircraft was on the runway.

Examination of the right engine revealed no evidence of malfunction, leakage, induction fire, or electrical arcing in the engine proper, which operated normally when subsequently tested. Fire damage was general throughout zone 2. It penetrated zone 3 by melting a dural support channel, allowing the stainless steel firewall to give way, and penetrated zone 1 around the carburetor.

Investigators found that the No. 2 fire extinguisher bottle had discharged, but the No. 1 bottle had not, although its actuating switch safety wire was broken. A CAA-approved drawing had, through oversight, omitted the grounding of the bottles to complete their electrical circuits, and the installing mechanic had not connected ground wires. The No. 2 bottle functioned only because its paint had chafed against supports, creating an accidental ground. The No. 1 bottle, installed about a week before the accident, had new, nonconductive paint and failed to function.

The fuel, oil, and hydraulic shutoff valves were found open. The circuit from the cockpit switch to the firewall shutoff valves shared an automatic reset circuit breaker with the generator air blast tube door motor. The wire to this motor was found burned. The Board noted it was likely the circuit breaker continued to cycle on and off without delivering sufficient current to close the valves.

Three lines in zone 2 were ruptured or melted: the hydraulic flexible pressure line, the main fuel inlet flexible hose, and the fuel pressure gauge flexible hose. After ruling out the vacuum pump system and the fuel lines as the initial source, the Board believed the hydraulic pressure line ruptured, spraying fluid under 1,500 pounds of pressure. This fluid vaporized and was likely ignited by the exhaust tail pipe or exhaust flame, which the vapor could have reached through a 1-1/2 inch hole in the cowling where a fabric shield had torn away.

The Board outlined a logical chain of events: flames burned the BMEP and tachometer wiring, which were routed in the same bundle, causing the gauge fluctuations. The fire burned through the manifold pressure gauge hose and melted the main fuel line's dural B-nut, releasing fuel. Because the firewall shutoff valves failed to close, fuel continued to flow and feed the fire even after the right propeller was feathered and the hydraulic pump stopped. The fire penetrated the firewall into zone 3, and it was not until the aircraft was rolling on the runway that the fuel boost pump wiring shorted due to fire damage, opening the circuit breaker and finally shutting off the fuel.

The investigation noted that the aircraft, engine, and propeller times had been zeroed in September 1955 during modification. The Board stated this was an irregularity, and possibly a violation of Civil Air Regulations regarding maintenance records, but found it was not a contributing factor to the accident.

### Probable cause

The Board determines that the probable cause of the accident was "the failure of a hydraulic fluid hose and the subsequent ignition of its vaporized fluid. Contributing to the seriousness and the uncontrollability of the fire was an inadequate emergency fuel shutoff system, an inadequate fire barrier between zone 2 and zone 3, and an inoperable fire bottle."