Page:CAB Accident Report, Northwest Airlines Flight 5.pdf/14

 by the airplane was sufficient to affect materially the flight characteristics of the plane. The effect of such ice is to reduce air speed and increase the stalling speed.

Considering all the evidence, it is our conclusion that the uncontrollable descent of the aircraft was primarily caused by a stall of the type which was repeatedly produced in the tests made following the accident and described herein. Captain Bates, during the tests, noticed only two features for which he recalled no strict parallel during the experience at Fargo (1) the necessity for constant rapid movement of the aileron controls, and (2) the sensation of loss of speed. With respect to the first point, however, Captain Bates recalled a sensation of uncontrollability of the airplane immediately prior to the accident and that his "main idea was to keep it level". His absence of any specific recollection of having made vigorous use of the controls for that purpose does not seem significant in view of the obvious difficulty of recalling every detail from a time of such stress, and immediately prior to the pilot's having suffered a staggering blow on the head.

With respect to the loss of speed, the pilot's sensations alone, while flying in a closed cockpit, could hardly be a reliable guide, and Captain Bates, having concluded that his air-speed indicators had been disabled by ice, was disregarding their indications.

The increase of power, from the amount ordinarily used during an approach to the maximum possible, would ordinarily be expected to change the rate of vertical motion of a DC-3 airplane, at a constant air speed, by at least 800 feet per minute 5. The attempt to fly straight ahead, using full power to cheek the descent, was consistent with the normal practice of highly skilled pilots, although the violent shaking of the airplane, typically characteristic of close approach to a stall, might well have suggested, without any reference whatever to the conditions under which the propellers may have been operating, that the rate of descent could have been reduced by increasing the speed to a value nearer that corresponding to the best angle of climb (at least 25 percent above the stalling speed). It must be admitted, however, that it is not easy even for a skilled pilot when already at a low altitude and approaching the ground rapidly, to overcome the instinctive tendency to try to pull the ices of the airplane up away from the ground, and follow instead the correct course of pushing the control forward and nosing the airplane down with respect to the horizon in order that the flight path may be brought nearer to the horizontal.

III

CONCLUSION

Findings

We find upon all of the evidence available to the Board at this time that the facts relating to the accident involving NC 21712 which occurred approximately 1-1/4 miles north of Moorhead, Minnesota, on October 30, 1941, are as follows:

1. The accident which occurred at approximately 2:04 a.m. on October 30, 1941, to Northwest Airlines' Trip 5 resulted in complete destruction to aircraft NC 21712, fatal injury to the 12 passengers and 2 members of the crew, and injury to one member of the crew.

2. At the time of the accident, Northwest Airlines held a currently effective certifi

5. e.g., to convert a rate of descent of 500 feet per minute into a rate of climb of 300 feet per minute.