Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/16

 tab hinge moment picture on the 720B. In this sense alone their work was a signifcant contribution to the Board's investigation, and the Board is indebted to NASA for their cooperation and assistance. without the horizontal tail hinge moment data from the full scale wind tunnel tests, some doubt regarding the validity of the calculated control forces in the negative angle of attack range would have remained since these original calculations were based on small—scale model wind tunnel tests and theoretical methods. Although the control forces derived from the full scale tests were not appreciably different from the earlier predicted values, the elevator control force did show the same lightening effect at large down elevator angles but did not reverse within the range of negative lift coefficients used in the NASA analysis. The analysis did note that any change in the conditions of the analysis which would allow control to larger negative lift coefficients would further reduce the push force as a result of the associated aerodynamic characteristics. Moreover, in quantitatively establishing the control force sensitivity both to small variations in cove gap clearance and SEE tab rigging, and qualitatively to aeroelastic wing bending effects, the analysis indicates to the Board that control force lightening to within the system friction hand or even mild force reversal is possible or service aircraft. The flight tests conducted by Boeing in October, 1963, to explore the high negative tail angle of attack and negative lift coefficient flight regime, produced elevator control force data which was in essential agreement with the NASA results in those instances where a direct correlation could be made However. the dangers involved precluded flight testing at high negative lift coefficients and full down elevator, the regime N724US was operating in Just before and after the noseover into its dive.

When questions arose regarding the possibility of making a successful recovery from a vertical dive below 20,000 feet, Boeing provided the Board in November, 1964, with the results of a study they had made in this area. Their study showed that with application of full up elevator the aircraft was recoverable from a 95-degree dive at 14,200 feet and 320 knots with full aircraft nosedown trim. The leveloff altitude would be about 5,000 feet. The airspeed at which the recovery is commenced is most important because zero dive angle must be reached before the speed in the dive exceeds 480 knots. Beyond this speed it is not possible to maintain 1G flight with full airplane nosedown stabilizer trim and full up elevator. Boeing also provided some load factor and control force data associated with the limiting recoverable condition. At the start of the recovery at 14,2OO feet, application of full up elevator would develop a ƒ4G airplane load factor and require 185 pounds of pull force on the control column. While maintaining full up elevator throughout the recovery, the developed airplane load factor would continuously decrease due to loss in elevator effectiveness with increasing airspeed until the maximum dive speed (472 knots) was reached however, in this same interval the elevator control column load would increase to a maxinum value of 320 pounds shortly before leveloff. The total time consumed in the recovery was found to be 31 seconds. The Board found these resuts extremely enlightening and indicative of the difficult problem confronting a pilot in such a recovery.

While the Board was still actively investigating this accident and, later, while awaiting the results of pertinent test, study, and research programs, several incidents and other accidents occurred under conditions bearing some similarity to the conditions associated with this accident. Not all of these cases involved the sane aircraft model family, and several of the cases were at