Failure - Interactions
Page by page Notes and Excerpts
From CAB Aircraft Accident Report:
March 1, 1962 (CAB Report Released Jan 15, 1963)
Pg 1 Synopsis: after takeoff, while turning during climbout through 1600 ft. msl, airplane rolled left through 90 degrees of bank, entered inverted nose low attitude, and plunged earthward in a nearly vertical dive. Crashed and burned in Jamaica Bay, three miles southwest of Idlewild Tower, at 10:08AM.
Pg. 2 CAB investigation determined probable cause: "...rudder control system malfunction producing yaw, sideslip, and roll leading to a loss of control from which recovery was not effective."
Pg 11 Flight Data Recorder, discussion of readout begins. "...absolutely precise time correlation of the four traces was impossible and errors of one to two seconds between them exist. An additional error is introduced by the friction and play in the recorder, resulting in short period (up to two seconds) aberrations of airspeed, altitude and heading traces from the smoothly varying changes made by the airplane. Characteristic of these are the numerous steps in the airspeed trace for the takeoff run and acceleration to 190 knots. Although times, speeds, altitudes and headings are usually expressed in terms of exact values throughout the flight recorder discussion, it must be understood that they are approximate but generally accurate to within plus or minus one second...."
Pg 14 Heading trace shows instantaneous tripling of change rate; at time 1008:07 heading changes from 250 to 243 degrees at rate of 12 degrees per second.
Pg 15 Gimbal error is indicated at 1008:31 when sharp reversal in recorded heading is noted. this error in heading trace originated within the directional gyro as aircraft bank angle approached 90 degrees left. "At time 1008:18, 1008:25, and 1008:29 airspeed and altitude traces indicate sharp simultaneous increases... a single static port low on the left side of the forward fuselage is connected to the airspeed and altitude sensors of the flight recorder. as a result, nose left sideslip (relative wind from the right) and high angles of attack cause appreciable plus errors in the recorded airspeed and altitude."
Pg 16 ". . . rapid peaking and decrease in the airspeed and altitude traces indicates pronounced sideslip effects coupled with increased drag resulting from prolonged heavy buffeting."
Pg 16+ ". . . A program of flight tests known as 'Project RACE' was originated by the Federal Aviation Agency in an effort to shed light on the cause of the accident. Organizations participating in this effort to varying degrees included the National Aeronautics and Space Administration, American Airlines, Boeing and the Civil Aeronautics Board. All tests were flown in an FAA-owned Boeing 720 with FAA pilots controlling all flights and performing the maneuvers in all instances. These tests had three main objectives including:
provision of flight recorder traces for comparison with those made by Flight One by attempting to simulate possible flight conditions of N7506A;
measurement of response of the airplane to various pilot opposed malfunctions, particularly of the rudder control system;
and measurement of the effects of slips and skids by means of extensive test instrumentation to make possible a more definitive study of the flight recorder traces from Flight One.
Pg 17 "As a result of this program the FAA concluded, in a
January 1963 draft of the Project RACE Report, that the
'data from autopilot hard-over rudder tests do not appear to resemble trace characteristics of the accident data. Data from Project RACE tests of pilot hard-over rudder, however, appear somewhat more severe than the CAB read-out of the accident flight recorder tape.'
[NOTE equally applicable to yaw damper system.]
"This draft report stated also,
'It was found that the automatic pilot [NOTE equally applicable to yaw damper] system could not, within the limitations of its force authority, displace the rudder control sufficiently to develop sideslip angles to the extent that would cause uncontrollable lateral roll.
'During our tests simulating American Airlines Flight 1 configuration it was found that the rudder boost system did have this capability. Should the rudder boost system command full rudder, within the limitations of hydraulic pressure and aerodynamic resistance, the resulting sideslip angle would cause lateral rolling that could only be arrested by reducing rudder boost pressure or assisting lateral control by deploying symmetrical speed brake handle or asymmetric thrust.'" [NOTE equally applicable to yaw damper system.]
"Project RACE provided the Board with much information that will prove helpful in future accident investigations. Insofar as investigation of this particular accident is concerned, the Project RACE test data are considered valuable principally as corroboration of more applicable Boeing flight test data in some respects. However, the Project RACE test were made in an airplane having lower thrust engines, a shorter fuselage, lower moments of inertia about the Y and Z axes, and lower gross weights than necessary for close simulation of the conditions of Flight One. The lower gross weights necessitated power reduction for comparable performance, which resulted in decreased thrust asymmetry with one engine idling, or too much thrust reduction to produce asymmetry equal to the loss of one outboard engine during Flight One. The special test by Boeing in a 707-131B were closer in these respects. In addition, the Boeing tests to simulate yaw damper and rudder power control malfunctions included more realistic rudder deflection-time histories than those of Project RACE. Both the Boeing and the Project RACE malfunction tests were essentially '1 g' maneuvers which did not approach the high vertical accelerations and lift coefficients experienced by Flight One."
Pg 17 No evidence of inflight engine failure.
Pg 18 Reconstruction of wreckage: "...right inboard aileron was 10 degrees up and the left 10 degrees down at impact." Right wing inboard spoilers 5 and 6, and outboard spoilers were up, while left wing spoilers were down.
Pg 19 "Both cockpit control wheels were determined to be slightly beyond the position for full right wing down control command."
Pg 21 Rudder hydraulic pressure control valve and rudder pressure shutoff valve disassembled: "...nickel plating had partially peeled or flaked away within both of these valves... tests by Boeing indicated...not adversely affect...due to a filter..."
"Rudder damper measurements corresponded to a rudder position of 17.5 degrees left. The piston of the hydraulic actuator of the rudder power control unit was extended 3/4 inch from neutral in a direction corresponding to a position of 9 or 10 degrees right rudder."
Pg 22-24 Yaw damper described: augments yaw stability. Sensing devices generate signals to energize an electric servo motor which is geared to the hydraulic actuator in the rudder power unit. As the rudder surface "...moves to the desired position a follow-up autosyn generates a signal in proportion to surface displacement which opposes the original initiating signal and stops the surface at the desired displacement. ...The motor, rate generator, follow-up autosyn, clutch and gearing are all enclosed in a cast aluminum alloy housing."
Pg 25 Description of wire bundle within servo assembly: contains 8 wires to servo motor and rate generator, with protective sleeving covering the wiring. Rudder servo from accident aircraft examined: rudder servo wiring failed continuity check. After removing protective sleeving covering wire bundle, investigator found two wires severed and a third wire nearly severed, held together by only one strand.
Pg 26 Microscopic examination of damaged area of wire bundle. Internal metal casing scratched/gouged adjacent to damaged wires.
Pg27 No marks found on this rudder servo assembly "...in the area of the wire damage which would indicate that the wire bundle had been struck by some object or otherwise damaged during the breakup."
"Eight spare servo unit motors from American Airlines stock were then examined, and six of these had the same type of scratching or gouging as found on the rudder servo from N7506A. Some of them also had similar indentations or imprints on the sleeving enclosing the wires... one of the scratched units.. had never been disassembled since last leaving the factory."
Pg 28 Investigators inspected servo units at manufacturer's plant production line: found units with damage previously described. "... damage had occurred as a result of improper use of tweezers when tying the wire bundles to the motor housing."
Pg 29 Bench test proved damaged wires produced a "yaw damper hard-over", and "insufficient servo damping".
Flight test in a Boeing 707-131B with such wire malfunction produced
"a left rudder deflection of 7 degrees at 210 KIAS, causing the airplane to sideslip and roll to the left."
Pg 30 The Boeing Company offered a study which concluded that
"...rudder boost hard-over..." should not cause loss of control.
AAL conducted study of FDR traces: sideslip history deduced.
Pg 32 Project RACE: NASA, AAL, Boeing, and CAB;
flight test data compiled for rudder control system malfunction,
pilot/aircraft response , measure effects of sideslip.
January '63, Project RACE Report conclusions:
1) Yaw Damper could not displace rudder sufficiently to develop sideslip angles large enough to cause uncontrollable roll.
2) Rudder boost system did have this capability.
Pg 33 "... Project RACE provided the Board with much information that will prove helpful in future accident investigations."
Pg 34 Begins Analysis and Conclusions Section.
Pg 35-36 Review of aerodynamics of sideslip, "... swept-wing airplanes are subject to a more pronounced roll - yaw coupling than straight-wing airplanes. When a swept-wing airplane with dihedral yaws, not only is the advancing wing at a higher angle of attack but it also presents a greater span to the airstream... lift differential... produces a greater rolling moment... roll due to yaw input of the rudder is much more pronounced...".
Pg 40-41 Consideration of possible jammed left outboard aileron during flap retraction.
Pg 42 Cites incident involving another B707, difficulty of rolling out of turn immediately after flap retraction.
Pg 43 Rudder control system described.
Pg 46-48 Consideration of possible maximum rate hard-over, full hydraulic flow rate to rudder power control unit. Not consistent with the deduced sideslip history.
Pg 48 Consideration of possible uncentering of the rudder control valve. Reduced rate rudder deflection hard-over would "... produce sideslip effects grossly consistent ... with flight recorder traces ...".
Pg 49 Malfunction of rudder servo unit considered: investigation disclosed previously described wire damage within the rudder servo unit, which was the only "... instance of unairworthiness of N7506A at the time of the accident ...".
Pg 50 "... damage to the rudder servo unit of N7506A was initiated by assembly or maintenance operations. Following the original damage it is believed that tensile strain in the securing of the wire bundle caused wires that were damaged but not completely severed to be necked down and weakened to the extent that vibration and other disturbances over a period of time caused their final separation."
Sequence stated: wire separation, shorting, then yaw damper hard-over.
Pg 51 Suspected hard-over rate of rudder deflection, and the resulting roll rate. Review of pilot response to slow rate rudder hard-over: flight test pilots expected input; where as the pilots of the accident aircraft were distracted by flap retraction, departure procedures, turbulence, and lack of visual horizon due to high nose-up attitude.
Review of past incidents "... of yaw damper malfunction ... crew was late in recognizing the yaw damper as being the source of the problem and were slow in initiating corrective action. In some cases, even after initiation of corrective action the dangerously steep banked attitudes increased and persisted well beyond flight test values before recovery was effected. In some instances of yaw damper mismanagement the crew never properly analyzed the difficulty and the flights were completed after application of additional lateral control such as use of speed brakes, flap extension, ect. ..."
Pg 52 Flight recorder traces matched with nose-left sideslip effect of yaw damper malfunction: defined point where pilot unable to overpower the rolling moments.
Pg 53 Flight recorder traces analyzed as higher G loading leads to stall buffet.
Pg 54 Flight recorder traces suggest yaw damper disengagement, followed by deactivation of rudder boost system prior to impact.
"The Board therefore concludes that a rudder servo malfunction due to shorted wires is the most likely abnormality to have produced the accident."
Pg 55 Recommendations: inspect servo rate generator motors for damaged wire bundles, and FAA ensure quality control during manufacture and overhaul. Second, AD to replace solenoid-operated valves in flight control hydraulic systems due to flaking of nickel plating. Third, reevaluate CAR's related to automatic flight control systems "... as specifically applied to the high speed swept-wing design turbojet aircraft, ... for the purpose of establishing realistic time allowances for recognition of abnormal airplane motions... and that necessary changes to the requirements be applied retroactively to turbojet aircraft...".
Signatures of five CAB members.
Attached Supplemental data included personal information on crew, diagram of track of aircraft, and graph of flight recorder traces with labeled events .