– 20 –
By presuming an acceleration after takeoff to speeds where stick forces against a jammed AND stabilizer would become noticeable and further hypothesizing some continued acceleration while trying to reactivate the stabilizer, say to 260 or 270 knots, one can account for the speed element necessary to place the aircraft near the region of the accident. Further, if the pilot now reduces his airspeed, e. g. to 220 knots, to relieve the stick force necessary for trim and finds himself in a field of moderate to severe turbulence (as analysis of the weather has shown), he could conceivably, because of the low stick force gradient, overcontrol the aircraft to the extent that on one of the oscillations the aircraft reaches a nosedown attitude for which the altitude does not permit recovery.
The nosedown attitude necessary to establish a non-recoverable position is not as steep as one might first imagine. Attachment D has been constructed from available data, principally DC-8, by superimposing on the altitude required to recover from given dive angles (utilizing a 2-g recovery), the altitude lost in getting from level to these dive attitudes. Not included is altitude dissipated during during the time required for situation analysis, decision, and reaction, and the time necessary to apply the stick force for a 2-g maneuver. Examination of the graph shows that recovery becomes problematical if a pushover to 30 degrees is initiated at any altitude below 5,000 feet. If one considers the additional altitude losses referred to above, the limiting altitude would be considerably higher or, conversely, the maximum dive angle for recovery would be less. Calculations based on flight recorder data of the DC-8 turbulence upset which occurred with the same first officer at the controls showed that the aircraft reached a nosedown attitude of about 40 degrees. It is known that 13,000 feet was required to recover level flight.
Against the argument that the PTC actuator had been in the aircraft for 10 months, and therefore, any irregularities would have been detected much earlier, the Board offers two comments. First, the leased DC-8 was operated by EAL for 20 days and subsequently made over 100 scheduled flights after being returned to the owner before the condition was discovered. (There were two minor write-ups by pilots, but no corrective action was taken.) Second, it must be remembered that N8607 was involved in a turbulence upset[1] incident after the installation of the -3 actuator.
The Board has devoted the last several pages to a discussion of a set of circumstances and a possible causal area largely unprovable. This has been done for two very good reasons, first, the possibility of the above described situation cannot be completely discounted and, second, much of the foregoing development of handling characteristics are equally applicable to the next possible to be discussed. In fact, the previous possibility and the following one differ only in the manner in which the stabilizer operation and time of failure are introduced into the situation.
The Board, in its report of June 1, 1965, on the Northwest Airlines, Inc., Boeing 720B accident near Miami, Florida, discussed at some length the general nature of the man-machine-environment complex and the characteristic patterns shown in the simulation of turbulence flying. A pertinent portion of that report states: "While the Board was still actively investigating this accident and,
- ↑ EAL, DC-8, N8607, at Dulles International Airport, August 20, 1963.
