Page:CAB Accident Report, Eastern Air Lines Flight 304.pdf/18

 The data available indicate that for the 2-degree AND condition the stick forces necessary to hold the aircraft in steady-state 1-6 flight range from 33 pounds (PTC retracted) and zero pounds (PTC extended) at 242 knots, to 55 pounds (PTC retracted) and 22 pounds (PTC extended) at 320 knots. Accordingly, the force characteristics should have become noticeable to the crew at speed above 220-240 knots, depending on extent of PTC extension, and they would have not accelerated much beyond this speed band. Rather, they would have elected to return to New Orleans and would have made their intentions known to departure control. The facsimile airspeed trace in Attachment B shows that this speed range would have been reached at about 0201:36 to 0201:46 and yet a simple acknowledgement of "OK" was made about a minute and a half later when, if not in difficulty, the aircraft would have already reached en route climb speed. In this case, notwithstanding the unavailability of an operable PTC (the primary corrective measure for a jammed stabilizer), there were many avenues of trouble-shooting available to the crew, each and all of which would be time consuming: Operation of the trim switches on both control wheels, checking, pulling, and resetting circuit breakers; checking hydraulic quantity and pressure, trimming attempt with the secondary trimming devices, the "suitcase" handles, and use of the tertiary electric trim switch. The slow trim rate of 1/17 degree per second of the electric system could in itself consume as much as 15 seconds before giving the crew a positive indication of operating or not operating. There is, then, the possibility that the time interval between attainment of 220-240 knots and the radio transmission "OK" was a period of problem- and trouble-shooting, during which no decision had been reached as to whether the flight should continue or return to New Orleans. The difficulty could have degenerated to an emergency and, ultimately, to catastrophe after the final transmission.

However likely or remote the possibility, the Board found it difficult to conclude that this condition alone, PTC extension, and AND stabilizer, could precipitate the complete loss of longitudinal control so obviously manifested by the condition of the wreckage. Exploration of the aircraft manuals, the testimony, and investigative data shed some light in this regard. It was established that under any condition whereby the aircraft is placed in trim by using AND stabilizer to counteract unprogramed PTC actuation, the overall effect is to shift the zero-force point of the control column away from its normal position in relation to the dual rate feel spring to a point where the stick force per g becomes relatively light. This is depicted in Figure 1 of ActachmentAttachment [sic] C, wherein the characteristic force pattern is reflected. The values on the abscissa and ordinate will vary depending on speed, altitude, and c. g. location, but the shape of the curve does not reflect the pattern for any regime. Normally trimmed, the control column will be centered about Area A and any column displacement from that area will follow the curve shown so that reasonable and expected stick force per g or per degree of surface deflection will be felt by the pilot. Excessive nosedown stabilizer positions, on the other hand, require the elevator to keep the aircraft in trim. The new column center position is in Area B where pilot inputs in the pull direction are at a considerably lower gradient. It must be pointed out here that the primary concern is with the gradient and not the actual force itself. The gradient is the same whether the pilot hold the aircraft in