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Data developed by an independent research agency, under government contract, revealed that the rate of response to elevator deflection has a profound effect on the behavior of the aircraft from the pilot viewpoint. This mode of motion, the longitudinal short period mode, has a one to five second period and is involved in the maneuvering of the aircraft. As the short-period frequency decreases, a slower response is experienced, and the initial motion is not a good indicator of what the final response will be. They reported that this leads the pilot to over-correct, and consequently produce a pilot-induced oscillation (PIO). Flight testing demonstrated that with the frequency adjusted between 0.2 and 0.3 cycles per second (c. p. s.), with a damping ratio of 0.4 to 0.7, which is quite heavy, the aircraft could be flown with no difficulty as long as the pilot flew gently, accepting the slow response. If he attempted to force a more rapid response, as might be done in a gust turbulence, a short-period PIO resulted. The pilots quickly found this characteristic could be overcome by smaller corrections. However, it was disturbing to fly because one was never certain when a quick response might be needed. The optimum longitudinal short period frequency from a pilot standpoint was found to be 0.6 to 0.7 c. p. s. The DC-8 has a longitudinal short period frequency of approximately 0.28 c. p. s. in cruise, with a damping ratio of 0.6. The first fuselage bending frequency is 3.78 c. p. s.
During the investigation the Board discovered several incidents of misrigging in the longitudinal control systems of other DC-8 aircraft, including some from other airlines. One incident involved an aircraft leased by EAL from January 15, 1964, through March 4, 1964. Pilot writeups on this aircraft resulted in the installation of a PTC actuator on February 13, and a new indicator on the following trip the same day. There was no further action by EAL in this area, and following 225 hours of accumulated flight time from this date, the aircraft was returned to the owner. The aircraft was then operated by the owner, from March 5 until March 31, when the pilot suggested that the elevator load-feel mechanism be checked for proper adjustment. He wrote that "It does not center control wheel with gust lock on and feel is not in proportion to other aircraft." Following a check of the load-feel mechanism and a visual inspection of the PTC the aircraft was cleared to continue. On April 1, the crew of a training flight reported that "At times it is necessary to crank trim to full nosedown and still necessary to hold forward control." A thorough examination of the various components of the system at this time revealed that.
- The actuator arm of the PTC was extended 1/2 inch too far (this displaces the control column neutral, introducing a noseup input at all times). This was installed with zero time since modification by the manufacturer.
- The pitch trim compensation spring (providing noseup control input when the PTC operates) was reset from 56 to 36 pounds. This adjustment bolt still retained the original DACO factory seal.
- The right elevator control tab was found 3/8 inch out of rig in the nosedown direction. This item had last been adjusted by the owner on November 11, 1963.
- The PTC indicator was found to have excessive play in the mechanical linkage which resulted in erroneous or no indication. The indicator neutral position had also been displaced to indicate PTC retraction with 1/2 inch of actuator input into the system.
