Page:CAB Accident Report, Northwest Airlines Flight 5.pdf/12

 The analysis of the problem, as previously indicated, has not proceeded to the point of making any final conclusion possible, but two alternative explanations of the behavior of the aircraft are immediately apparent. The explanation might lie either in that (1) the airplane was so completely stalled, at such a large angle of attack, that the power required to maintain level flight would have been more than the power plant could supply even with the engine and propellers functioning perfectly, or (2) the particular conditions of the maneuver may have so adversely affected the functioning of the propellers as to prevent them from developing the thrust which would normally be anticipated.

Each of these alternative explanations presents certain difficulties. If the wing had been stalled to the degree that the first explanation would have required, it is hard to conceive of the airplane having been kept continuously under control during the descent. The normal consequence of such a complete stall would be the dropping of one wing tip, with the recovery of normal attitude only after a dive to gain speed and escape the stalling attitude. Also, the first of the alternative explanations would fail to account for the violent increase in buffeting as power was applied.

The second explanation, that related to the propeller, presents the difficulty that if the propeller blade sections had been stalled and the propeller efficiency correspondingly lowered during flight at stalling speed, at full power, and with the r.p.m. of the engine equal to the r.p.m. normally used in a take-off, the same phenomenon would be expected to appear in an even more severe form during the take-off itself, and it would become almost impossible to gain the speed necessary for take-off. There is no immediately apparent explanation for a loss of propeller thrust at an intermediate speed, i.e., at the stalling speed without any corresponding difficulty being encountered either at lower or at higher speeds. In view of these difficulties of explanation, we arrive at no definite conclusion here, but merely present the facts as so far encountered.

As indicated previously in this report, we have determined that the aircraft had been properly maintained and inspected in accordance with the requirements of the Civil Air Regulations. The evidence has disclosed no indication of structural or mechanical failure of the aircraft until it crashed with power on in the vicinity of Moorhead, Minnesota, and we conclude that it was in an airworthy condition prior to that time. It has been determined, also, that the aircraft was properly loaded and it appears that it had ample fuel for the flight. It is clear from the evidence that the radio range and other radio facilities involved, operated and maintained by the Civil Aeronautics Administration, were operating normally during the flight.

In seeking to determine the cause of the accident, we must consider first whether, under the weather conditions which existed at the time, it was proper to dispatch the flight from Minneapolis to Bismarck with clearance to land at Fargo in the event that weather improved to permit a landing under ceiling and visibility conditions at or above prescribed minimums. It appears from the evidence that the observed weather information and current weather forecasts gave sufficient indication at the time of clearance that the ceiling and visibility at Fargo probably would be at or above the prescribed minimums when the flight should arrive at that point. We conclude, therefore, that the clearance was accomplished in accordance with Civil Air Regulations and that it is not subject to criticism.

It remains to be determined whether the dispatcher, in failing to direct the pilot to proceed to his alternate prior to the arrival of Trip 5 in the vicinity of Fargo, and Captain