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he will view the aircraft in his mind's eye as being in a less severe attitude than it really is, or he may allow the aircraft to stray farther in pitch Simply because his attitude instrument is presenting him with conservative data. If the attitude indicator presents "geared-down" pitch attitude information to the pilot, it likewise presents "geared-down" pitch rate information and could cause a degree of over-control when the pilot attempts to restore the aircraft to normal attitudes. Coupled with this, of course, is the small physical Size of the instrument face. The Board does not Wish to imply that, because of its small size, the instrument is unreadable, however, it must be accepted that it is more difficult to read and interpret than the larger indicators. Likewise, the solid black background does not display to the pilot the immediately interpretive picture of the two-colored instruments.
During the hearing in connection with this investigation, the Board heard testimony concerning the miscues presented to pilots by their flight instruments during turbulence flying. Additionally, there have been several papers written on the subject in the past several months. Generally Conceded is the fact that airspeed, rate-of-climb, and altitude presentations can lack accuracy and, even more, can present completely erroneous information as to longitudinal attitude, i. e., trends exactly opposite to that expected of a given attitude. Now the Board finds that the one remaining instrument, the primary one, the attitude indicator, presents to the pilot information which, while not illogical, is certainly not optimum.
Previously this report dealt with the subject of maneuvering stability. Additionally, the Board gathered information in the form of testimony and reports, on the subject of speed stability.[1] Flight tests have shown that the DC-8 speed stability in the climb configuration approaches neutral at speeds above 300 knots when the PTC fails to extend the programmed amount. The FAA Witness who testified on maneuvering stability also stated in regard to speed stability, "When I trimmed the airplane at 300 knots, I found that the static stick-free stability was positive when I departed to 85 percent of trim, but when I increased to 115 per-cent of trim, the static stability was within the friction band and, for all practical purposes, would be called neutral." Speed stability characteristics were explored with a research pilot who has considerable experience in experimentation with specially adapted variable stability aircraft. He indicated, as did other test pilots, that neutral speed stability in itself does not pose a serious problem to the pilot, and, in fact, under normal flying conditions "…it is actually quite a pleasant airplane to fly…." He further pointed out, however, "The thing that is dangerous about a situation like this is a distraction. If the pilot, for example, is distracted for any reason and allows the aircraft to start diverging from its trim condition, especially if he is in turbulence and he is faced with a fairly substantial change in his trim or his attitude, the tendency usually is to make a large input, and this is where the trouble begins."
The Board conducted studies pertaining to aircraft characteristics in turbulence. This information revealed that turbulence has known energies broad enough to excite aircraft natural frequencies between 0.2 cps and 4.0 cps. An example
- ↑ Speed or static stick-free stability is the measure of the aircraft's ability to return to trim speed if momentarily disturbed to a lesser or greater speed. An aircraft which has positive speed stability will likewise require pull forces to maintain altitude if the speed decreases and push forces if the speed increases from trim speed.
