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stability of the aircraft was sufficient to overcome the upsetting force in each instance.
The training records for the captain of Flight 705 indicate that he received a rating in the Boeing 720B following check flights on November 9 and 13, 1962. The initial check flight was discontinued after three hours and 50 minutes, but prior to the successful completion of the proscribed maneuvers, because of mechanical difficulties with the aircraft. A two-hour training flight was flown on November 12. The rescheduled check flight was then accomplished on November 13, and lasted one hour and 24 minutes. The FAA Air Carrier Operations Inspector issuing this type rating gave grades[1] of 4 on the Dutch Roll[2], jammed stabilizer, electrical emergency and engine fire, and five additional items of the 22 graded. The captain of Flight 705 had accumulated 150 hours of flight time in the Boeing 720B, and was off duty from January 13 until February 9, 1963. He was described by a fellow pilot as having no problems flying instruments or the 720B; also he was very speed consious in turbulence. The first officer had accumulated 1,093 hours in the 720B, the second officer and five stewardesses were all qualified.
Prior to the accident, Northwestern's turbulence penetration procedure referred to a chart which provided an airspeed range to maintain during flight in or near turbulent air. This was typical of the industry. Recovery from unusual attitudes, not exceeding 10 degrees in pitch with 45 degrees of bank, was a part of the initial checkout for 720B captains. However, once they received their type rating in the aircraft, this was never repeated in recurrent training nor was any flight simulator training provided.
Following the public hearing there were two areas which the Board believed required additional study. The first was the possibility of rain freezing in the balance bay area. Icing of the balance panel seals and the piano hinge, which connects the balance panel to the elevator, could restrict movement of the elevator, and consequently its effectiveness. There had been at least 13 occurrences of longitudinal control difficulty attributed to this icing problem. However, in these instances the difficulty was usually characterized by either a stiffness in the control column with poor aircraft response, or a cycling force in the column with a corresponding porpoising motion. In some cases more force was required to move the controls than the crew cared to exert, and the stabilizer trim was used to control the aircraft. In some cases descents to lower altitudes restored normal feel and response, in others greater than normal pilot forces alleviated the problem. In no case did the icing precipitate a loss of control.
Joint Northwestern-Boeing flight tests were performed in climatic conditions similar to those experienced by Flight 705 and the temperature within the balance
