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The gear segment on the end of each blade of each propeller was broken between the fourth and fifth teeth, indicating that the propellers were in low pitch at the time the breakage occurred.
Careful inquiry was made of persons who arrived at the scene of the accident shortly after it occurred in order to determine, if possible, the extent of ice accumulation on the airplane at that time. Mr. Henry Olson, a mechanic for Northwest, stationed at Fargo, was found to be the first person to have made a careful observation of ice accumulation. He arrived there at approximately 2:30 a.m. and observed that there was a coating of rough ice on the de-icer boot of the right wing ranging from ½ to 2 inches thick and extending from a point about 8 feet outboard of the landing light to the wing tip. He described the ice as "white and snowy". He also observed some ice of the same type lying beneath the right aileron but was unable to testify with respect to any ice which may have been accumulated on the stabilizer. Mr. Olson did not believe that the ice he observed had been affected by the fire since it had been on the windward side. Mr. Olson's testimony was corroborated by Mr. Floyd Berglund, an apprentice mechanic of Northwest at Fargo. During the period from the time of the accident until the arrival of these witnesses at the scene the temperature was about 32 degrees.
Weather Conditions
Reinhart C. Schmidt, Supervising Forecaster at the United States Weather Bureau Station in Chicago, and B. C. Haynes, Air Safety Specialist in Meteorology for the Board, testified with respect to analyses they had made of the weather conditions existing in the area traversed by the flight about the time of the accident. From this testimony it appears that the cold front which had been moving eastward at 15 to 20 m.p.h. had reached St. Cloud, Minnesota, about the time of the accident with the frontal surface sloping back westward so that over Fargo it lay at an altitude of approximately 3000 feet above the ground at this time. In advance of the front, the wind direction was south-southwest and to the rear of the front northwest, Fargo reporting a northwest wind of 17 m.p.h. about the time of the accident. In advance of the front the temperatures were above freezing up to approximately 4000 feet above sea level. At Fargo, in the rear of the front, the ground temperature at 1:30 a.m. was 33 degrees, lowering to about 32 degrees at 300 feet above the ground, 30 degrees at 900 feet, and to a minimum of 28 or 29 degrees between 1500 feet and the frontal boundary, where the temperature probably increased to above freezing.
These witnesses agreed that the atmospheric conditions near Fargo, as above described, were favorable to the formation of ice just above the base of the overcast, since the temperature was below freezing and, by reason of the relatively strong northwest wind, some turbulence could be expected. However, neither could express an opinion as to the severity of the icing conditions to be encountered there.
Mr. Haynes testified that icing is usually greatest in a freezing rain area or in a long and active cold front and that the high moisture content of the clouds near the base of the overcast would render conditions there similar to those encountered in a freezing rain. Mr. Haynes was of the opinion that the ice accumulated in this area would be a soft type of clear ice but would accumulate in a jagged form.
Flight Tests
The behavior of the airplane, as described by Captain Bates, suggested stalling of the inner sections of the wing, with buffeting as a result, but it gave no clue to the reason20635
