Douglas C-54 accident at Washington, D.C., 12 Sept 1946 (NX-91068)

Accident narrative

On September 12, 1946, a Pennsylvania-Central Airlines Douglas C-54, registration NX-91068, landed safely at Washington National Airport following an in-flight fire in the No. 4 engine. After the aircraft was brought to a stop, the fire spread throughout the structure, causing extensive damage. The four crew members sustained minor injuries during the evacuation.

### The flight

The aircraft departed Washington National Airport at 1310 for routine transitional training maneuvers. It was certificated for restricted operation and utilized by the company for crew training only. At the time of takeoff, the aircraft's weight was approximately 16,000 pounds less than the maximum authorized gross.

Captain William Ralph Sewell, acting as instructor, was in command. He had accumulated 7,272 total flight hours, of which 3,765 had been obtained in DC-4 equipment. The student crew consisted of First Officer Edwin B. Gately, acting as co-pilot, and First Officers Charles W. Nason and Albert A. Anderson.

### The emergency

At approximately 4,500 feet in the vicinity of Hyattsville, Maryland, a student pilot observed the No. 4 fire warning light illuminate and advised the captain. Flames were seen streaming from the cowl flap opening at the lower cylinder position of the No. 4 engine and back along the oil cooler scoop.

The captain immediately initiated emergency fire extinguishing procedures and notified the Washington control tower. The co-pilot discharged the No. 2 bottle of the carbon dioxide fire extinguisher. When this had no effect, the No. 1 bottle was discharged shortly thereafter. Aside from momentarily retarding the flames, the extinguishing system produced no apparent effect.

With the No. 4 propeller feathered, the aircraft descended in a long, shallow right turn over Anacostia and the northern boundary of Bolling Field. An approach was made to Runway 33 at Washington National Airport, and the landing was made without difficulty.

Immediately after the aircraft made contact with the runway, the No. 4 engine fell from the wing. The aircraft was brought to a stop near the end of the runway. Fire continued to burn in the wing, fed primarily by gasoline running from broken fuel lines. Mobile fire fighting equipment was waiting, but the airport fire truck exhausted its foam and carbon dioxide supply without extinguishing the flames. Subsequent attempts by an Army mobile high-pressure water system were unsuccessful. Driven by a 15 to 20 mph northeast wind, the fire spread to the cabin. It burned for two hours, completely destroying the interior, the major portion of the right wing, the center section, and the fuselage.

### What the investigation found

Inspection of the No. 4 engine disclosed a series of failures. Due to metallic fatigue, the exhaust valve stem of the No. 2 cylinder had failed in flight, leaving the valve loose in the combustion chamber. The piston battered the valve against the cylinder head, eventually splitting the head between the cooling fins and battering a four-square-inch hole in the piston. Oil was forced into the cylinder head on the piston's down strokes and out into Zone 1 (the power section of the nacelle) on the up strokes. A considerable amount of oil streamed back over the exhaust collector ring and accumulated in the bottom of the cowl. Until the propeller was feathered, gasoline vapors continued to feed into the No. 2 cylinder and escape into Zone 1.

The investigation further disclosed that the exhaust connection between the No. 1 cylinder exhaust port and the collector ring joint was unfastened. A stud had backed out prior to the fire, exposing approximately 1-1/2 square inches of the exhaust port and allowing hot exhaust gases to escape. Normal airflow caused the escaping oil and exhaust gases to merge in Zone 1. The Board noted it is probable that this was the origin of the fire.

Evidence indicated that oil accumulated on the bottom of the cowl ignited, and flames traveled aft along the outside of the nacelle. The fire penetrated Zone 2 (the accessory section) through poorly sealed openings between the cowl and oil cooler shroud, and between individual cowl panels. Another flame path likely entered Zone 2 at the junction of the air scoop with the top of the nacelle.

Once in Zone 2, the flame attacked rubber and dural oil lines and burned away the rubber seals between the firewall and the cowling. Fire consumed micarta plates covering engine control cable openings, a large portion of the dural fittings in the firewall, and a dural junction box, leaving substantial openings through which the fire penetrated into Zone 3 (the nacelle area aft of the firewall). The investigation clearly indicated that the most severe fire occurred in Zone 3, where it consumed gasoline and hydraulic lines as well as the dural oil tank.

The aircraft's fire extinguishing system consisted of two 15-pound carbon dioxide bottles directed only to Zone 2, where the fire detectors were also located. It provided no protection for Zones 1 or 3. Although the crew followed company procedure by pulling the cut-off valve handle before feathering the propeller, the fire had already progressed aft of the firewall and consumed the lines behind the cut-off valve, allowing gasoline to continue draining into Zone 3.

Regarding the post-landing fire, the Board noted that the inadequacy of ground fire fighting equipment and techniques was clearly indicated, as both water and carbon dioxide proved completely ineffective for a fire of this intensity.

### Probable cause

The Board determines that the probable cause of this accident was the failure in the No. 4 engine of the exhaust valve of the No. 2 cylinder and the exhaust connection of the No. 1 cylinder which resulted in fire in flight. A contributing factor was the inadequacy of the seal between the firewall and nacelle skin and between individual cowl panels which permitted the fire eventually to reach Zone 3 and to become uncontrollable.