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it has been adopted generally by the aviation industry. It does, however, imply that the pilot may be solely responsible, and as sometimes used means exactly that. The Board has not intended this latter interpretation. It realizes that at least part of the input in a PIO is provided by the pilot as an integral part of the "control loop," but the fault certainly should not rest solely with him. Man characteristically is more adapted to a short period frequency of response about 0.6 to 0.7 cps, yet the industry is asking him to control under all tolerable situations a machine the natural frequency of which is 0.2 to 0.3 cps. This, of course, is not an impossible task as well demonstrated over the past several years, but it does introduce a measure of increased difficulty equally well demonstrated.
2.2 Conclusions
Based on the limited information available to it, the Board concludes that, although the exact time of trim failure cannot be established, such failure did occur and either contributed to the introduction of a PIO in turbulence or was contributory to the failure to recover therefrom, and that the inoperative PTC also contributed whether retracted or extended, and that there is a strong possibility that it was at least partially extended. The exploration of the histories of this and other DC-8 aircraft suggests also that there could have existed some degree of control system misrigging which could have been additive to any other control difficulties.
At this point it would be appropriate to summarize the many factors with which the pilot may have been required to contend on the night of the accident. It should be noted that none of these factors in itself constitutes a hazard or even a serious situation, however, several or all of them in combination could create conditions under which control of the aircraft could be lost, partially or completely.
a. Findings
- Night, instrument conditions prevailed.
- Moderate to severe turbulence was encountered.
- The PTC was inoperative and may have been partially or fully extended.
- The stabilizer drive system failed in the 2-degree AND position at some time during the flight.
- The altitude indicator, which was small with a solid black background, was difficult to interpret at night.
- The pitch indication of the altitude indicator was "geared-down" but not indexed as to degrees.
- The aircraft exhibited marginal to non-existent speed stability and a stick force per g characteristic which test pilots have interpreted as unstable.
