Page:CAB Accident Report, Northwest Airlines Flight 5.pdf/10

 for the apparent development of violent buffeting after the power was applied, or to the failure of the application of power to check the descent of the aircraft. With throttles fully open, an aircraft with the general characteristics of the DC-3 would ordinarily be expected not only to maintain altitude but to climb, even at a speed very near the stall, and it is the common characteristic of multi-engine aircraft of this general form that they are much less susceptible to buffeting with power on than with power off. It is for that very reason, indeed, that such aircraft as the DC-3 are more liable to stalling of the wing tips, with resultant uncontrollable dropping of the wing, in the power-on condition than in that with power off. In the latter case, buffeting due to stalling of the inner part of the wing gives the pilot a warning which is ordinarily much less apparent, if not entirely lacking, when the aircraft approaches the stall with power on.

Captain Shank, check pilot for Northwest, testified that he had encountered similar conditions in his experience with Douglas aircraft. Captain Shank has logged a total of approximately 13,500 hours solo flight time. He has had between 2000 and 2500 hours in DC-3 airplanes, of which approximately 1600 hours were spent checking pilots. He has checked between 50 and 60 pilots in stalls and recoveries, the check for each pilot including a minimum of 48 stalls and recoveries and for most of them a check in about 75 stalls and recoveries.

Captain Shank testified that in this pilot checking experience he had observed that, in attempting to reduce loss of altitude to a minimum, occasionally a pilot would apply full power when the airplane had reached a nearly stalled condition and rapid descent had begun. Occasionally in these instances, at an air-speed indicator reading of from 55 to 90 m.p.h., a terrific buffeting occurred about concurrently with application of full power. During this lateral and directional control were retained, although with difficulty, but an uncontrolled descent took place. In order to avoid a prolonged and severe strain on the airplane, this condition was not permitted by Captain Shank to continue for any great length of time, recovery being effected by dropping the nose of the airplane, thereby increasing air speed. Little had been known about this tendency or characteristic of the airplane, according to Captain Shank, Mr. Marshall and Mr. Ferguson, and it had been believed that the application of power would pull the airplane out of the condition described without it being necessary to drop the nose.

A series of flight tests were accordingly planned, with the object of simulating the behavior of NC 21712 immediately prior to the accident, as described by Captain Bates, and discovering any relation which might exist between the behavior during the accident and that which had previously been encountered in test and check flights by Captain Shank. The tests were made on November 28 and December 2, 1941, at Minneapolis, Minnesota, and were witnessed and participated in by Frank E. Caldwell, Chief, Investigation Division of the Safety Bureau, and the following personnel of Northwest: Captain Clarence F. Bates, Captain Eugene S. Shank, W. Fiske Marshall, Division Superintendent, Kenneth R. Ferguson, Assistant to the Vice President in Charge of Operations, Karl Larson, Chief Engineer, and Don Jones, First Officer. The data accumulated in these tests and testimony of the participating witnesses were incorporated in depositions taken at St Paul, Minnesota, on December 1 and 3, 1941, which thereafter were made a part of the record of the investigation. The tests have not been fully worked up as yet, and are to be amplified by further studies, but certain general conclusions can be drawn, and will be disclosed herein.

The airplanes used were DC-3's, owned by Northwest, and they were piloted by Captain Shank. The gross weight and C. G. Location were kept as nearly as possible identical with those of the airplane involved in the accident. The general procedure in the tests was to 20635