N861TA

On January 2, 1998, at 1526 Alaska standard time, a Douglas DC-6B airplane, N861TA, departed the edge of runway 34 during an aborted takeoff from the Nixon Fork Mine landing strip, thirty miles north of McGrath, Alaska. The transport category airplane was destroyed by a postcrash fire. The airline transport certificated pilot, and the other two flight crewmembers, were uninjured. The airplane was operated by Woods Air Fuel, Inc., of Palmer, Alaska. The flight was operated under 14 CFR Part 125 as a cargo flight transporting ore concentrate from Nixon Fork Mine. The flight originated from Palmer at 1230, and was on the takeoff roll of the return leg. Visual meteorological condition prevailed at the time of the accident, and an IFR flight plan was on file.

The captain reported that at an indicated airspeed of 45 knots during the takeoff roll, ice formed between the inner and outer windshield panes, with the icing following the flow of heated air through the windshield. The crew's forward visibility was obscured, and the takeoff was aborted. The captain reported that the airplane drifted left into snow berms on the side of the 4,200 feet long by 85 feet wide runway and caught fire. The three crewmen evacuated the airplane without injury.

The three crewmen relayed to the NTSB investigator that after arrival at the mine strip, the airplane and windshield cold soaked for about one hour in the -10 degree Fahrenheit temperature. After loading the airplane, they taxied for only a few minutes in light, powdery snow, before beginning the takeoff roll. The crewmen stated that the windshield anti-ice system was activated, but because of the short taxi, did not have time to warm completely. All crewmembers indicated that during the taxi, snow was circulated around the airplane and the wing scoops.

Research revealed that the windshield anti-ice system was modified as part of installing Supplemental Type Certificate (STC) number 981SO on June 26, 1987. This STC eliminated the superchargers on engines numbers 3 and 4 from providing conditioned air to the airplane's cabin. The modification was part of a conversion to a cargo configuration.

The modified cockpit heating and windshield anti-ice system receives supply air from one of two sources. On the ground, a blower fan draws air from the underside of the fuselage for a combustion heater. As the airplane transitions to flight, ram air pressure from two wing leading edge scoops becomes greater than ground blower discharge air, and becomes the heater air supply source. Air and any particulate matter in the wing scoops and ducting passes directly to the heater. Once airborne, the blower fan is deactivated by a weight on wheels switch.

From the heater the warm air is ducted directly to the windshield, where it flows between the outer glass pane and the inner vinyl pane, and is then discharged into the cockpit.

The "DC-6 Airplane Operating Manual" used by two other companies states "...Certain combinations of temperature and humidity will cause moisture to condense from the air between the windshield panels and settle on the inner surfaces of the glass, obscuring vision. Once formed, anti-icing airflow with heater operation will be required to clear the panels." This language is not included in the STC literature, nor in the flight handbook for the company operating the accident airplane.