– 10 –
telegraphic alert recommending, "(1) Operators perform a one-time inspection of stabilizer adjusting mechanism to determine proper installation of bushing P/N 2652666 and shim P/N 2648310, (2) Operators maintenance procedures be reviewed regarding adequate assembly and installation instructions of stabilizer drive mechanism…." The DACO DC-8 Overhaul Manual instructions pertaining to the drive sprocket support bushing of the sprocket assembly were revised on August 1, 1964, to incorporate the additional information, "Make certain bushin (104) is installed with flange up."
During initial certification of the DC-8 satisfactory stability characteristics were demonstrated in high speed cruise (350 knots) configurations and also at the best rate of climb speed (220 knots). However, it was found in flight testing subsequent to the accident that with the aircraft trimmed at 300 knots in an aft c. g. climb configuration with maximum continuous thrust (MCT), and the PTC inoperative, the slope of the stick force curve remained essentially zero as the aircraft accelerated to 390 knots. This stick force relationship to airspeed conflicted with then existing regulations which specified that speed changes be perceptible to the pilot through a change in the stick force. The criterion generally used at that time was at least one pound of force/seven knots of airspeed change.
Further flight tests were conducted to evaluate the controllability of the aircraft with unprogrammed PTC extensions or retractions. The final determination was that adequate elevator control was available to overpower the PTC input even though the time delays for pilot response were actually longer than those used during autopilot "hardover" testing.
One of these later tests involved an aircraft loaded to an equivalent c. g. of 24 percent. The FAA test pilot stated that an interesting discovery was made, "…during maneuvering with a fully extended PTC at a velocity of approximately 220 knots and the airplane trimmed to its previous extreme of full AND (2.0 degrees), it was observed that any attempt at maneuvering the airplane with the elevator system resulted in sharp reversals in the airplane's maneuvering stability. This was true in applying either noseup or nosedown control. A pilot with this condition existing during turbulent atmosphere would be presented with a very difficult control problem. When the nosedown trim was adjusted to the new limit of 0.5 degrees AND the airplane demonstrated less tendency toward maneuvering instability."
Another test pilot reported that flight testing of the DC-8 handling characteristics under abnormal conditions, i. e., PTC extended to offset a 0.5 AND stabilizer setting, in a cruise configuration at 220 knots, revealed that, "…the aircraft exhibited no stick force stability. This lack of stick force is caused by a shifting of the stick neutral position to a very flat portion of the load feel spring when PTC is extended. The low gradient of the load feel spring in this area is masked by the control system friction which necessitates flying the aircraft by stick position only. The aircraft is neutrally stable at small airspeed increments about the trim point in any normal attitude, including 45 degrees turning flight, and would maintain a 45-degree coordinated turn hands off until the speed was changed. With a change of ± 10 knots, the aircraft exhibited classic instability and would continue to increase or decrease, whichever the case may be, until restrained."
The investigation also focused on the aerodynamic stability of large swept-wing jet aircraft, with particular emphasis on the longitudinal natural frequency.
