Portal:Civil Aeronautics Board

“ 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. ”

“ Data developed by an independent research agency, under government contract, revealed that the rate of response to elevator deflection has a profound effect on the behavior of the aircraft from the pilot viewpoint. This mode of motion, the longitudinal short period mode, has a one to five second period and is involved in the maneuvering of the aircraft. As the short-period frequency decreases, a slower response is experienced, and the initial motion is not a good indicator of what the final response will be. ”

“ 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. ”

“ Speed stability characteristics were explored with a research pilot who has considerable experience in experimentation with specially adapted variable stability aircraft. He indicated, as did other test pilots, that neutral speed stability in itself does not pose a serious problem to the pilot, and, in fact, under normal flying conditions "...it is actually quite a pleasant airplane to fly...." ”

“ Generalizing from a limited number of cases not fully evaluated or clearly understood is usually a technically unsound approach, yet it is still difficult to escape concluding that the phasing relationship between turbulence-induced aircraft motion with control inputs is at least a factor in these occurrences. ”

“ The inherent or augmented stability of the simulated aircraft provides the restoring forces necessary to maintain the trim condition. In most of the trials with a pilot control input, the simulator could be flown successfully through the "storm" and the extent of the g, airspeed, and altitude excursions depended largely on how close the pilot tried to maintain the desired pitch attitude. Some of the trials revealed oscillations quite large in amplitude, indicating pilot control input out-of-phase with the simulator motions induced by the imposed gust/draft history. ”

“ An analysis of the flight recorder of the jet which departed New Orleans immediately after Flight 304 substantiates the severity of the turbulence in the area. Accelerations to +0.2 and +1.9-g between 2,000 and 6,000 feet, recorded on this tape, indicate severe turbulence. Since known or forecast turbulence along the climb path is the prime criterion for selection of the climb speed, it is probable that the crew of Flight 304, unconcerned about turbulence below 14,000 feet, chose 310 knots rather than the lower rough airspeeds depicted in their flight manual. ”

“ 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. ”

“ It should be noted that while the stick force per g is light but at a reasonable level at 310 knots, it degenerates to about 13 pounds at 220 knots. This level of force gradient is extremely light [10] and is to a large extent masked by the friction forces of the system which are about ±5 to 6 pounds. Thus, at 220 knots a pilot could maintain a 1.5-g maneuver without feeling any resistive force, or he could hold limit load (2.5-g) by feeling out only about 14 pounds, considerably less than required for a similar maneuver in a military fighter aircraft. ”

“ The gradient is the same whether the pilot hold the aircraft in trim against an unwanted AND stabilizer position by applying a stick pull force, or the control forces are in balance through the medium of using AND stabilizer to counteract unprogrammed PTC, or extension of the PTC by use of the "test" position to balance out unwanted AND stabilizer settings. Of course, if it becomes necessary for the pilot to hold a pull force against AND stabilizer, the magnitude of the force necessary does become a factor insofar as physical quality and pilot fatigue are concerned. ”

“ 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. ”

“ The low gradient of the load feel spring in this area is masked by the control system friction which necessitates flying the aircraft by stick position only. The aircraft is neutrally stable at small airspeed increments about the trim point in any normal attitude, including 45 degrees turning flight, and would maintain a 45-degree coordinated turn hands off until the speed was changed. With a change of ± 10 knots, the aircraft exhibited classic instability and would continue to increase or decrease, whichever the case may be, until restrained. ”

“ The Board, however, submits that whether the aircraft under these conditions is in fact unstable or just feels that way to an experienced test pilot is a difference unworthy of further discussion.
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. ”

“ The nosedown pitching moment encountered in high speed flight is offset in the DC-8 by the Pitch Trim Compensator (PTC) system which applies noseup control through the elevator system. Operation of the PTC is also required in the low altitude, high speed regime below mach effect to improve stick force characteristics as speed increases. This system consists of an electrical computer, an electrical actuator, spring loaded linkages, and a mechanical indicator. ”

“ 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. ”

“ It is nothing more, really, than overcontrolling. You don't see the airplane respond immediately so you have the tendency to put a little more elevator in, and by this time the airplane has started to respond and you suddenly find the response is more than you wanted. So the tendency is to reverse the process.... I can see this situation can be quite critical in turbulence or possibly under IFR conditions plus turbulence where, let's say, you do have some large gusts which change your attitude appreciably if the pilot attempts to... ”

“ The computer senses mach effect at high altitude and dynamic pressure below 20,000 feet, and provides the electrical signals to the actuator which actually moves the copilot's control column. The actuation begins at either Mach 70 or 310 knots and increases in displacement and rate up to Mach 88 or 410 knots. The maximum input is 36 pounds of stick force. Actuation of the PTC is indicated by the extension of a plunger from a flexible cable housing attached to the left side of the copilot's control column. ”

“ 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. ”

“ Examination of the drive gear assembly of the stabilizer drive unit revealed normal wear patterns on the planetary gears and the male spline extension which transmits power down to the dual sprocket assembly. ”

“ The flight maintenance logs fro N8607 were recovered back through May 22, 1960, the date of delivery to EAL. They revealed that on August 20, 1963, the aircraft was subjected to abnormal flight conditions during flight in severe turbulence. A Severe Turbulence Mechanical Check of the aircraft at that time revealed only minor damage. On September 11, 1963, the stabilizer jammed in the full ANU position during a landing at San Juan, Puerto Rico. An inspection at that time revealed that power from the drive unit was not transmitted through the sprockets to the jackscrews. ”

“ Both motors provide power through differential gearing to a drive shaft on which a dual sprocket assembly is mounted. The sprockets are connected to the common drive shaft by shear rivets, and each transmits rotation of the drive unit through roller chains to an irreversible jackscrew. Failure of either set of shear rivets freezes the stabilizer in the last selected position, and further operation is impossible. ”

“ All 51 passengers and the crew of seven were fatally injured.
The flight, scheduled from Mexico City to New York City, with several intermediate stops, had just departed from New Orleans at 0200. Three minutes later the captain acknowledged a request to change radio frequencies, but no further communications were received from the flight. A 0205:40 the radar target associated with Flight 304 had disappeared from the scopes of both the radar controllers who were observing the flight. Moderate to severe turbulence existed in the area at the time of the accident. ”

“ 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. ”

“ A bearing seat at the top of the dual sprocket assembly also exhibited the abnormal wear from a 1/4 inch displacement, and oscillation of the assembly. The lower support bushing P/N2652666 for the sprocket shaft, which supports the sprocket assembly in the vertical plane for proper spline engagement and also restrains it in the lateral direction, was not recovered. ”

“ This search rapidly assumed enormous proportions as additional electronic and sonic underwater detection gear became available. Discovery of the wreckage was finally confirmed late in the afternoon, March 13. Salvage operation commenced immediately and continued on a 24-hour basis until April 16, at which time approximately 60 percent of the wreckage, by weight, had been recovered. The operation involved raising the pieces from deep in the mud and silt bottom of the lake, and placing them on a barge. ”