Page:CAB Accident Report, United Airlines Flight 21.pdf/71

 by giving to Captain Scott readings which were erroneously high, thus permitting him inadvertently to stall the airplane. However, the testimony of pilots shows that the heating device now used by United is quite effective in keeping the air speed pitot tubed clear of ice. A test was conducted by Captain Thompson of United with a simulated build-up of ice on the mast supporting the pitot tubes, which is not protected by the heating device. Very little, if any, effect upon the reading of the air speed indicator was observed during this test. Although there are reports of interference with the pitot operation due to a build-up of ice on the mast supporting the pitot tubes, the effect of this ice would be, ordinarily, to make the air speed indicator read low. A low reading of the air speed indicator, according to pilots, may ordinarily be expected from the effect of ice on the air speed pitot tubes themselves with the effect of causing the pilot to maintain a higher speed than is actually indicated, thus operating as a safety factor.

It is definitely established that Captain Scott's windshield was covered with ice when he first broke out under the overcast, since he reported to the Control Tower that it would be necessary for him to circle the field before landing in order to get the ice off of his windshield. The record indicates that the hot air blower windshield de-icing equipment is not always entirely effective during heavy icing conditions in keeping the windshield clear of ice. Moreover, even if he had been successful in removing the ice from his windshield, his visibility might still have been interfered with by mist or wet snow. Other pilots who landed during this period testified that the visibility from the cockpit was poor after the windshield ice had melted due to the mist which collected. Therefore, at