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from a particular case, stated, "Here is a configuration in the short period, 0.2 cps and a damping ratio of 0.5. This particular pilot rated the aircraft a nine on the Cooper scale, which would be completely unacceptable. He says (quoting from the report) 'Trim ability extremely poor. Stick forces light initially, causes immediate response, wants to overshoot…. The general feel is very bad, almost dangerous.' I think that comment is fairly respresentative."
Based on the information available to the Board, the DC-8 exhibits very low speed-stability characteristics, particularly at higher climb speeds when the PTC does not operate as programmed. There was testimony at the hearing and depositions about whether these stability aspects were within the requirements of the Civil Air Regulations and much was made over the fact that the regulations do not address themselves to stability in the event of a mistrimmed condition or a system malfunction.[1] To the pilot the aircraft responds the same, whether or not it was required to meet any stability criteria for the condition in which he finds himself. What would be of primary interest to the pilot and is of primary interest in this report is the fact that at lower speeds (220 knots) the airplane can under certain mistrim conditions exhibit low to neutral stick force per g and stick force versus elevator deflection, and at the higher climb speeds (310 knots) it can have very low speed-stability.
Earlier the Board discussed the possibility of a partially or fully extended but inoperative PTC, and it should be pointed out here that this condition would Contribute to reduced maneuvering stability at lower Speeds but would improve to a small extent the Speed stability at the higher speeds. On the other hand a retracted, inoperative PTC would have no effect at lower speeds but would. produce marginal speed stability at speeds in excess of 300 knots. As stated before, the possibility exists that the PTC actuator was extended. This condition while it could worsen the situation, is not a necessary prerequisite to a PIO situation. The Board has investigated several PIO accidents and incidents in which the PTC was not involved, some involving aircraft which do not have this type of compensating system.
One element, however, common to almost all PIO occurrences has been the application of nosedown stabiliser trim at some point during the oscillatory cycles. The Board sees in this the results of forcing the aircraft, as described by the research pilot. In other words, the pilot, finding his aircraft in an excessively nose-high attitude, pushes the column forward and, when the aircraft does not respond to his satisfaction, he also actuates the trim switch. He then suddenly finds the aircraft responding more rapidly than he anticipated, and thus motion could also have been aggravated by a gust reversal which becomes additive to the elevator and stabilizer inputs. At this point, in all probability, the PIO conditions have ended for all practical purposes, and the aircraft is in a dive. The problem now becomes one of dive recovery.
Now a new set of factors comes into play, several of which are quite basic while others are more subtle and involved. As to whether or not the aircraft can be recovered to level flight, the basic considerations are dive angle and altitude, if the former is too great and the latter too small, recovery is obviously an unattainable goal. More subtle and difficult to assess, but greatly affecting the seriousness of angle and altitude, are pilot response
- ↑ Under such conditions, the regulations require only that the aircraft be safely controllable.
