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times, his use of reverse thrust, whether he attempts to retrim and perhaps the most important of all, how much load he is willing or able to place on the airframe in the pullout.
This last factor deserves some additional discussion since the failed condition of the stabilizer drive system of N8607 suggests problems in this regard. The failure of the stabilizer drive system at the full AND position automatically establishes a lower limit to the pilot's ability to recover from a diving condition, and here it should be recognized that it makes no difference whether the failure occurred during a PIO or earlier as previously hypothesized. The larger size of the stabilizer makes it approximately three times as powerful as the elevator, and therefore, about six degrees of up elevator is required to counteract the effect of an unwanted 2-degree AND stabilizer position. This amount of elevator deflection is lost insofar as recovery is concerned. Also, as speed increases, the ability to get any recovery action from the elevator diminishes, and disappears completely at about 470 knots. Other factors could also prolong the dive. The several PIO incidents established the fact, if not already know to pilots, that high stick forces (about 80 pounds or more) produce moments on the stabilizer which exceed the trim motor capability and that under these circumstances it is necessary to relax some of the pull force in order to reposition the stabilizer. The pilots in this case, if the drive system failed during a PIO rather than earlier, had no way of knowing the real reason for its failure to operate in the ANU direction. In the split seconds available to them for analysis they could easily have concluded that the failure was due to heavy stick forces. Reverse thrust, in addition to drag, produces a nose up pitching moment, a fact known to the first officer if not to the captain, and as indicated previously, they did employ this aid. It is also true that during the time, no matter how short, required to go from forward thrust to reverse, the noseup pitching moment of forward thrust has been removed and therefore contributes to the severity of the dive. Small as it may be, this factor becomes more significant at very low initiating altitudes.
There is an additional noteworthy element which is impossible to assess. It is most probable, based on voice identification and crew practices, that the first officer was at the controls during and following takeoff from New Orleans. Likewise, this same pilot was at the controls during the development of a longitudinal upset in another DC—8. There are still many unanswered questions concerning the exact mechanics of that earlier incident, but it is known that this pilot did not hesitate to apply full forward control column, and additional nosedown trim when faced with an unusual attitude in turbulence.[1] The result was a dive reaching about 40 degrees nosedown and from which about 13,000 feet were required for a recovery. The Board fully recognizes that what this pilot did in one situation at one time is not necessarily indicative of his actions in another, even similar, situation at another time. While the Board admits to the subjective nature of this information, it cannot ignore its existence.
Throughout this report the term "pilot-induced oscillation" has been used repeatedly, partially because it is reasonably descriptive but primarily because
- ↑ Based on crew testimony, the amount of trim used in that case was probably less than the 2-degree AND limit.
