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trim against an unwanted AND stabilizer position by applying a stick pull force, or the control forces are in balance through the medium of using AND stabilizer to counteract unprogrammed PTC, or extension of the PTC by use of the "test" position to balance out unwanted AND stabilizer settings. Of course, if it becomes necessary for the pilot to hold a pull force against AND stabilizer, the magnitude of the force necessary does become a factor insofar as physical quality and pilot fatigue are concerned.
One very interesting aspect in the leased aircraft occurrence was the discovery that one elevator control tab had been rigged in such a manner that it partially offset the effect of the PTC extension, i. e., the compensating stabilizer deflection moved the controls into or toward Area B (Attachment C), and the tab rigging tended to shift them back toward Area A, but to a lesser degree. Tabs misrigged in the opposite direction, or for that matter correctly rigged, would have worsened to control difficulties of the aircraft.
The variation of stick force per g versus speed for the 2-degree AND case, shown in Figure 2 of Attachment C, is also significant. It should be noted that while the stick force per g is light but at a reasonable level at 310 knots, it degenerates to about 13 pounds at 220 knots. This level of force gradient is extremely light[1] and is to a large extent masked by the friction forces of the system which are about ±5 to 6 pounds. Thus, at 220 knots a pilot could maintain a 1.5-g maneuver without feeling any resistive force, or he could hold limit load (2.5-g) by feeling out only about 14 pounds, considerably less than required for a similar maneuver in a military fighter aircraft.
The report to his supervisors, and the testimony of a highly qualified FAA test pilot become significant here. He stated that under similar conditions he found the aircraft exhibiting maneuvering instability. In his testimony he described that test, "…we left the PTC extended and that was at approximately 220 knots and I trimmed… two degrees aircraft nosedown. I started doing some nominal maneuvering with the airplane in this configuration. I found that any time I attempted to depart from my trim point, either in noseup or nosedown direction, that I received reversals in the airplane's maneuvering stability. The rate of pitch would increase and my stick force would go to the opposite direction to check the maneuver." He also indicated that after a few maneuvers the tests were discontinued because of the "…nervousness of all the crew."
The Board recognizes that the reported test flight was made in a non-instrumented (for test work) aircraft and also, that all available data indicate a positive, though week, stick force gradient. The Board, however, submits that whether the aircraft under these conditions is in fact unstable or just feels that way to an experienced test pilot is a difference unworthy of further discussion.
- ↑ The Civil Air Regulations under which the DC-8 and other transport aircraft were certificated do not specify stick force per g values. For the superscribed statement the Board relies on general consensus of opinion and on MIL-F-8785 (ASC) which specifies maximum and minimum gradients by formula. Applying the formula to the DC-8, the stick force per g values are a maximum of 80 and a minimum of 30 pounds.
