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flight and checked the leading edges of the wings frequently by the use of a flashlight. He turned on the propeller anti-icers long before reaching the Forgo area because, as he explained, they must be turned on before encountering ice in order to be completely effective. He did not turn on his de-icer boots at this time.
When he had reached a point about 25 miles southeast of Fargo, he noticed that the airplane was picking up very light ice "but not any amount to even be bothered about". He stated that about this time the air-speed indicators "started letting up and didn't read accurately". The air-speed indicator needles indicated about 130 m.p.h. Although he did not know what the exact reading had been prior to that time he said that it had been somewhere around 165 m.p.h. and that at any rate the reading had been normal for the altitude involved. He concluded that the pitot tube had iced up and decided that he would disregard his airspeed indicators from that point on.
As is shown by the radio log of Northwest's Fargo station and by Captain Bates' testimony, the trip arrived over the Fargo radio range station at 1:47 a.m. at an altitude of 2700 feet above sea level. Since Fargo is 890 feet above sea level, the airplane was approximately 1800 feet above the ground at this time. A short time prior to his arrival over the range station, Captain Bates had received the report of the current Fargo weather observation which stated that the ceiling was 500 feet and the visibility 8 or 9 miles. Thus, at this time the ceiling was 100 feet below Northwest's authorized ceiling minimum at Fargo. Nevertheless, Captain Bates began a standard let-down-through procedure. He explained this action by saying in effect that in view of the constant improvement, which had taken place during the past few hours in weather conditions at Fargo, he decided to let down to his authorized minimums to see if the ceiling had risen in the interim sufficiently to permit him to land.
After passing over the range station at 1:47 a.m., he made a left turn to go out on the west leg of the Fargo radio range in accordance with standard procedure. A procedure turn was made on the west leg, the ship again passed over the station, and proceeded out the east leg. A gradual descent was begun after passing over the station a second time and was continued for about 3 or 4 minutes, according to Captain Bates' estimate, until he had reached an altitude of 2000 feet above sea level. During this descent he encountered light ice. He made a procedure turn to the right at this altitude and just as he was returning to the leg at the completion of this turn, he again began descending. At this point he directed First Officer Onsgard to let the landing gear down and turn the de-icer boots on. While the rate of ice accumulation had increased to some extent at the 2000-foot altitude level, Captain Bates still did not consider the icing conditions existing to be serious enough to cause any great concern. He stated that the air temperature was about 30 degrees and identified the ice as rime ice, "the type that forms in irregular chunks and irregular chips, little extensions out on the windshield".
He continued his descent from 2000 feet above sea level at about 400 feet per minute proceeding toward the station in the "A twilight" of the east leg until he reached an indicated altitude of 1500 feet above sea level, when, according to Captain Bates, "we did start to pick up quite a lot more ice". However, having on previous flights experienced what he considered heavier ice accumulation, he still was not unduly concerned.
Upon reaching 1500 feet above sea level, he had not broken out of the overcast, and since he had reached his minimum altitude of 600 feet above the ground, he leveled out intending to proceed over the station at this altitude and if he did not break out of the overcast, to proceed to Bismarck. However, as Captain Bates described his experience, just20635
