Page:CAB Accident Report, Northwest Airlines Flight 5.pdf/11

 bring the airplane up gradually toward the stall, while flying with reduced power (20 to 29 in. manifold pressure), until slight buffeting became apparent. The landing gear was then retracted and full power applied to both engines, the controls being used to maintain approximately a level attitude with respect to the horizon.

Following the application of power, the buffeting increased sharply, and in some cases became quite violent. Instead of flattering out the flight path and beginning to climb, as would normally have been expected, the airplane lost altitude rapidly, the rate of descent seeming typically to stabilize at about 1500 feet per minute, though rising at times to 1800. The airplane could be kept in a constant attitude with respect to the horizon, and on its course, by normal use of the three controls, although the maintenance of lateral equilibrium required constant alertness and much rapid and extreme movement of the ailerons, and the elevator also required constant manipulation to hold a constant attitude.

Subject to this necessity for constant use of the controls, it appeared that the airplane could be kept for an indefinite length of time in the attitude that produced the buffeting and the rapid descent despite the use of full power. The readings of the airspeed indicator during the descent varied between approximately 65 m.p.h. and slightly over 80 m.p.h., and the artificial horizon maintained an indication of approximately level flight. To regain normal flight it was found necessary to push the control column forward enough to change the attitude of the airplane with respect to the horizon by approximately 4 to 5 degrees.

Captain Shank identified the maneuver as the one that he had previously experienced in checking pilots on stalls. Captain Bates, who took the controls during one of the tests, testified that the feeling during the tests was quite like that shortly before the accident. "In the accident at Fargo," he said, "I do not remember this severe fighting of the controls to keep the airplane level, but I do remember the similarity between the way the airplane felt according to my last recollection in the accident at Fargo and in the way the airplane felt in our test the other day. That buffeting was very similar to what I experienced at Fargo." He stated also that the approach to the stall and the beginning of the buffeting in the tests were very similar to the beginning of the condition which he experienced preceding the accident.

The tests included trials with the de-icer boats in operation, and with power applied on one engine only. The results in those cases did not differ, in any substantial respect, from those previously recorded.

As the failure to realize the anticipated checking of the descent by the application of power suggested a failure to secure the expected increase in propeller thrust when the throttles were open, the propeller performance was made the subject of special investigation. The propeller pitch was measured during the various stages of the maneuver of stall and recovery, and with various settings of the propeller pitch control. Certain of the test data are tabulated in an appendix to this report, and indicate a substantial increase in the pitch angle of the propeller after power was applied. 20635