Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/24

 were within Boeing specifications; (b) Boeing be requested to make a detailed evaluation of aeroelastic effects on elevator control forces in the down elevator range at high negative load factors; and (c) Boeing be requested to assess the feasibility and advisibilityadvisability [sic] of modifying the SAE tab linkage as to preclude the lightening of control forces.

The FAA acknowledged the Board's letter on June 4, 1964, stating that our recommendation was being studied, and that we could expect a full report on the matter later. An interim letter from the FAA, dated July 10, 1964, indicated that they were taking action in line with our recommendations and would provide definitive comments in the near future. It was also noted that their Project CAPER flight tests should provide valuable information on the general problem and that this information would be considered in their assessment of the Boeing 720 airplane. In a lengthy, detailed reply, dated December 30, 1964, they advised that after a thorough study and evaluation of all available information it was their opinion that the data did not justify a requirement for modifying the longitudinal control system to preclude control force lightening during extreme conditions such as those experienced in the accident. In specific reply to the three points in the Board's May 27, 1964, letter, FAA advised that (a) an assessment of operational information obtained from eight operators regarding their ability to maintain the pertinent cove gap and SAE tab tolerances indicated no discrepancies were found which would indicate "out of tolerance" settings were probable; (b) Boeing was asked to provide information on the aeroelastic effects on control forces, and the information supplied showed the net aeroelastic effect would reduce the control force lightening and (c) they concurred with Boeing's conclusion that neither modification was justified because the SAE tab linkage would become too complex, and changing the cove gap to improve the down elevator characteristic would result in undesirable force characteristics for other important flight conditions. In summarizing their views on the general problem, FAA advised that current industry actions directed toward avoiding extreme regimes of flight beyond the aircraft design envelope will provide needed improvements in the level of safety for turbulence operation of this and other transport aircraft. Some of the current actions noted were improvements in attitude indicators and stabilizer trim setting displays, better turbulence penetration techniques, and flight and simulator studies of crew environment and airplane characteristics during turbulence penetration.

The Board determines that the probable cause of this accident was the unfavorable interaction of severe vertical air drafts and large longitudinal control displacements resulting in a longitudinal "upset" from which a successful recovery was not made.

BY THE CIVIL AERONAUTICS BOARD.

Adams, Member, did not take part in the adoption of this report.