Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/10

 bay were measured at four points. These tests showed that the measured temperatures in all cases were equal to or warmer than the raw air temperature. When were correlated with the pertinent accident data Boeing determined that the balance bay ambient temperature of N724US was approximately 40°F. In such case the balance bay cavity walls would have been at least 50°F ad the piano hinge 60°F. Northwest also analyzed this data, and the results of their study were presented in a comprehensive report which detailed their views on all of the accident evidence. This analysis indicated that the pertinent temperatures in the balance bay area would have reached the freezing level shortly before the final maneuver.

The second area indicating a need for further study resulted from calculations of NASA aerodynamicists who provided technical assistance to the Board throughout the investigation. Their initial study of the 720B longitudinal control system indicated the possibility of control force lightening or even reversal at high down elevator deflection. However, a full scale wind tunnel test of the horizontal tail was required to resolve this possibility. Accordingly, the Board and FAA jointly requested NASA to conduct the necessary testing. Boeing volunteered the use of a half horizontal tail as well as personnel to help instrument it.

NASA conducted the test in the fall of 1963 at their 40 x 80-foot wind tunnel at the Ames Research Center, Moffett Field, California. Aerodynamic control tab hinge movements and elevator hinge movements were derived for a range of elevator angles and tail angles of attach. The data from the wind tunnel were then used to analyze the control forces which would be experienced in a pitching maneuver similar to Flight 705, at a series of elevator angles with the stabilizer at normal climb and full nosedown. Additionally, control forces were also calculated for +1G level flight at a series of stabilizer settings. All computations were based on an equivalent airspeed of 250 knots, 15,000 feet altitude, 173,000 pounds and a c.g. of 25 percent MAC, to closely approximate the parameters of Flight 705. The results indicated that for +1G level flight with varying stabilizer settings, the variation of control force with elevator angle was in the normal direction for all elevator angles. During pitching maneuvers with constant stabilizer settings, the push force to maintain down elevator angles reached a maximum at approximately 10 degrees down elevator, and then decreased as the down elevator angles increased. Positioning the stabilizer at full aircraft nosedown or normal climb settings did not appreciably alter the control forces. The push force to hold full down elevator during the pitching maneuvers with either of these stabilizer settings was about 15 pounds from aerodynamic loads and 15 pounds from the elevator centering spring. The analysis also included data on the control force sensitivity for variations in balance panel core gap clearances, and stabilizer actuated elevator (SAE) tab misrigging. The push forces in the pitching maneuvers studied was reduced 7.5 pounds for each 0.05 inch reduction in the cove gap and 8 pounds for each degree of misrigging of the SAE tab. A qualitative evaluation of aeroelastic effect indicated that these would be in a direction to receive the push force required for the negative load factors developed in nosedown pitching maneuvers.

N724US was airworthy for departure, and the crew was properly certificated. Flight 705 was dispatched in accordance with FAA regulations and company procedures