TWA 841

• The captain stated that he was flying the aircraft on autopilot with the Altitude-Hold mode selected. While he was sorting maps or charts, which were located in his flight bag on the left cockpit floor, he felt a buzzing sensation. Within 2 or 3 seconds, the buzzing became a light buffet, and he looked at the flight instruments. He noticed that the autopilot was commanding a turn to the left with the control wheel displaced accordingly, but he noticed that the attitude director indicator (ADI) showed the aircraft in a 20° to 30° bank to the right. The AD1 showed that the aircraft was continuing to bank to the right at a slightly faster than normal rate of roll, so he disconnected the autopilot and applied more left aileron control to stop the roll.
• According to the captain, the aircraft continued to roll to the right in spite of nearly full left aileron control, so he applied left rudder control in addition to the aileron control. He stated that in spite of the almost full deflection of the left aileron and full displacement of the left rudder pedal, the aircraft continued to roll to the right. He believed that the aircraft was going to roll inverted so he retarded the throttles to the flight idle position, and he stated "we're going over," or something to that effect. The aircraft rolled completely and entered a second roll with the nose down.
• The captain asked the first officer to "get them up," meaning that he wanted the first officer to extend the speed brakes. The first officer stated that he was not aware of the buffeting or the aircraft's attitude because he was in the process of calculating the aircraft's groundspeed; therefore, he did not understand the captain's command. The flight engineer was aware of the buffeting but was facing his panel and was not aware initially of the aircraft's attitude except that it seemed to be in a right descending turn. The captain stated that when the first officer did not react to his command, he moved the speed brake lever to the deployed position.
• After detecting no reaction to the speed brake extension, the captain moved the control handle to the retract position and back to the extend position. Meanwhile, the indicated airspeed needle was moving rapidly toward its limit and he could see only "black" on the AD1 and bright areas in the windshield which he perceived to be the lights of towns shining through the undercast. The altimeter indicated such a rapid descent that it was difficult to read. However, he estimated that the aircraft was near 15,000 feet and descending rapidly when he commanded extension of the landing gear. The first officer immediately moved the gear handle to the "extend" position, and the flightcrew heard very loud sound similar to the sound of an explosion.
• The captain stated that he applied full left aileron and full left rudder throughout the descent but the aircraft continued to roll to the right. Simultaneous with the gear extension, he relaxed some of the back pressure on the control column and some of the pressure on the aileron and rudder controls. The airspeed began to slow, and he was able to roll the aircraft to a near wings-level attitude and to stop the aircraft's descent, after which the aircraft pitched upward into a 30° to 50° climb. He saw the moon in the windscreen and used it as a visual reference to maneuver the aircraft. The airspeed slowed rapidly, and with guidance from the first and second officers, he leveled the aircraft near 13,000 feet.
• After regaining control of the aircraft, the flightcrew noticed a warning light announcing the failure of the 'A" hydraulic system and a warning flag indicating that the lower yaw damper was inoperative. The captain decided to land the aircraft at Metropolitan Airport, Detroit, Michigan. He instructed the first officer and flight engineer to perform emergency checklist procedures and to notify the flight attendants to prepare the passengers for an emergency landing.
• The captain stated that when the landing flaps were extended during the approach by means of the alternate extension system, the aircraft rolled sharply to the left. Therefore, he ordered the flaps retracted and planned for a landing without flaps. The two main landing gear indicators showed unsafe landing gear conditions, so the captain made a low altitude pass down the runway for a check of the landing gear. Control tower and crash rescue personnel reported that all three landing gears appeared to be extended. About 2231, the captain landed the aircraft on runway 3 without incident.
• The accident occurred at night (about 2148) near latitude 43°39'N and longitude 84°05'W.

Source: NSTB-AAR-81-8, pg. 2

• The No. 7 leading edge slat on the right wing was missing. The slat tracks remained on the aircraft; the outboard track was twisted and bent rearward about midspan, and the inboard track was bent rearward near the aft end of the track.
• The No. 7 leading edge slat, which had broken into two pieces, and the outboard trailing edge flap track canoe-shaped fairing were found about 7 miles north of Saginaw, Michigan.

Source: NSTB-AAR-81-8, ¶1.12

• In 1975, The Boeing Company conducted flight and wind tunnel tests to determine the effects of asymmetric extension of wing leading edge slats on the control characteristics of the B-727 while in cruise flight conditions. Because of reports of slat actuator lock ring failures, these tests were conducted to evaluate control characteristics associated with an unscheduled extension of a single leading edge slat. The wind tunnel tests involved slat extensions from 0.4 to 0.95 Mach; because of adverse buffeting, O.80 Mach was the highest speed tested in flight. From these tests, it was determined that the extension of either the No.2 or the No. 7 leading edge slat caused the most adverse control characteristics, but with a significant amount of lateral control applied, the aircraft was controllable at altitudes and speeds of up to and including 35,000 feet and 0.80 Mach.

Source: NSTB-AAR-81-8, ¶1.16.1

• At the request of the Safety Board, The Boeing Company programed a fixed-base engineering flight simulator with B-727-200 aerodynamic and control data and the data obtained from [1975] flight and wind tunnel tests. Also, the simulator was programed with Flight 841's gross weight and center of gravity conditions and the pertinent meteorological data associated with its flight.
• The simulator tests produced two flight maneuver situations in which the recorded time histories of indicated airspeed, altitude, and normal load factors most closely approximated those recorded by Flight 841's FDR.
• The flight simulator traces showed that the simulated aircraft could be returned to wings-level flight with relatively little loss of altitude provided corrective action was begun before the roll and airspeed were allowed to increase excessively. In the simulations, the pilot could delay reaction for about 16 seconds and regain control with an altitude loss of about 6,000 feet. However, when the pilot delayed corrective action for 17 seconds or more, a maneuver was entered that approximated Flight 841's airspeed, altitude, and g-traces. In this maneuver, the aircraft continued throughout the descent to roll to the right, in spite of full left aileron and rudder, until the slat was retracted to simulate its loss from the aircraft.

Source: NSTB-AAR-81-8, ¶1.16.2

• In 1978, one operator experienced an unintended extension of leading edge devices. While in cruise flight at 25,000 feet and about 350 knots (0.82 Mach.), the captain detected an airframe vibration which he attributed to a partially extended trailing edge flap. He attempted to retract the trailing edge flap by using the alternate flap system. However, either the retraction switch was moved inadvertently to the "down" position, rather than the "up" position, or the switch was wired backward. In any event, the leading edge devices were unintentionally extended. The leading edge devices were retracted by turning the alternate flap master switch off; however, the No. 6 and No. 7 leading edge slats on the right wing did not retract. The aircraft began to roll and turn to the right, but the captain returned the aircraft to level flight by using left aileron and rudder. The aircraft was kept upright by about 45' of control wheel deflection to the left and by a significant amount of left rudder. After the captain slowed the aircraft, the slats retracted. An unscheduled but normal landing was made as a precautionary measure. The No. 7 slat and the alternate flap retraction switch were changed, and the aircraft was returned to service.

Source: NSTB-AAR-81-8, ¶1.17.2

• Based on the physical evidence, aerodynamic data, and the flight simulations, the Safety Board concludes that an extended No. 7 leading edge slat on the aircraft's right wing caused lateral control problems which preceded the aircraft's rapid descent.
• According to the flightcrew, before and immediately after the buzzing began, they saw no lights in the cockpit that indicated an unlocked leading edge device or a failure of a hydraulic system, including the "A" system. Also, the captain stated that there was no inadvertent or deliberate movement of the flap control handle or other controls that would have caused leading edge devices to extend. Therefore, if the flightcrew's recollections are accurate, the No.7 leading edge slat would have had to extended as a consequence of defects or malfunctions in the No. 7 slat extension/retraction systems.
• During the investigation, after repairing and plugging ruptured "A" system hydraulic lines, both the normal and alternate flap control systems were tested. There was no evidence of any malfunction in these systems that might have caused an extension of one or more leading edge devices.
• The evidence involves a fundamental conflict between the flightcrew's statements and the possibilities and probabilities of an unscheduled extension of the No. 7 slat. Although portions of the slat actuator were not found, the evidence indicates that the possibility of a series of malfunctions and failures occurring which permitted the slat to extend aerodynamically or hydraulically is extremely remote. On the other hand, we recognize that if the No. 7 slat did not extend as the consequence of some series of failures and malfunctions in the slat system, then it must have been extended as a result of flightcrew action.
• The Safety Board concludes that the Nos. 2, 3, 6, and 7 slats were extended as a consequence of flightcrew action. Further, that when scheduled to retract by the flightcrew, the No. 7 slat failed to retract probably because tensile forces created by aerodynamic loads combined with friction and side forces on the piston rod, caused by misalignment of the slat, exceeded the available retraction force.

Source: NSTB-AAR-81-8, ¶2.4

• After the No. 7 slat was torn from the aircraft, lateral control was restored and the captain was able to roll the wings parallel to the horizontal and recover from the spiral dive.
• The flightcrew denied having moved any controls that would have caused extension of flaps or slats. Since there is no other available evidence of flightcrew activities in the cockpit, the Safety Board is not able to determine conclusively why the Nos. 2, 3, 6,and 7 leading edge would have been extended. However, we note that since the flap lever must be moved up and over a gate before it can be moved to a flap/slat extension position, it is not likely that the lever was moved accidentally. Further, since operation of the alternate flap system to extend leading edge devices results in random and initially unsymmetrical extension of leading edge flaps and slats, extension of only the Nos. 2, 3, 6, and 7 slats would not have been likely.
• In summary, the Safety Board concludes that the following sequence of actions and events probably occurred to cause Flight 841 to enter an uncontrollable spiral dive involving two 360° rolls and a loss of about 34,000 feet of altitude in about 63 seconds:
• While cruising at mach 0.816 and 39,000 feet pressure altitude and with the autopilot controlling the aircraft, an attempt was made to extend 2° of trailing edge flaps independently of the leading edge slats, probably in an effort to improve aircraft performance.
• The attempt to independently extend 2° of trailing edge flaps was not successful, and about 2147:32 the Nos. 2, 3, 6, and 7 leading edge slats began to extend. Two seconds later, the aircraft began to buffet and roll slowly to the right. Six to seven seconds later, the rate of roll began to increase due to increasing slat asymmetry as the Nos. 2, 3, and 6 slats retracted. The No. 7 slat failed to retract.
• About 2147:45, the aircraft reached about 35° of right bank where the captain disconnected the autopilot and rapidly rolled the aircraft to the left to a near wings-level attitude. The aircraft could have been stabilized in wings-level flight with appropriate deflection of the lateral controls.
• About 2147:47, the aircraft again began to roll to the right, probably while the captain was distracted by activities related to the isolation of the No. 7 slat in the extended position.
• Shortly before 2147:51, the captain recognized the rapid right roll, and he rapidly applied full deflection of the lateral controls to stop the roll. The roll was stopped near 35° of right bank for several seconds during which the captain removed his right hand from the control wheel, pulled the throttles to flight idle, and deflected full or nearly full left rudder.
• In response to the rapid and full or nearly full deflection of the flight controls, the aircraft entered a substantial right sideslip. The sideslip combined with the aircraft's mach number and angle of attack to reduce the lateral control margin to zero or less. The aircraft resumed the right roll and began to descend rapidly and uncontrollably. The captain extended speed brakes, detected no reaction, and retracted them.
• About 2148:25, the aircraft completed 360° of roll while descending to about 21,000 feet. Shortly thereafter, the captain commanded landing gear extension which was accomplished by the first officer. The aircraft continued to descend rapidly, and it continued to roll to the right until the No. 7 slat was torn from the wing and lateral control was restored. About 2148:58, the captain regained control of the aircraft at an altitude of about 5,000 feet.
• Since our weighing of the evidence involves a rejection of the possibility of an unscheduled extension of the No. 7 slat and a partial rejection of the captain's recollection of his actions following extension of the slats, the Safety Board believes that the following comments are appropriate: We believe the captain's erasure of the CVR is a factor we cannot ignore and cannot sanction. Although we recognize that habits can cause actions not desired or intended by the actor, we have difficulty accepting the fact that the captain's putative habit of routinely erasing the CVR after each flight was not restrainable after a flight in which disaster was only narrowly averted. Our skepticism persists even though the CVR would not have contained any contemporaneous information about the events that immediately preceded the loss of control because we believe it probable that the 25 minutes or more of recording which preceded the landing at Detroit could have provided clues about causal factors and might have served to refresh the flightcrew's memories about the whole matter.

Source: NSTB-AAR-81-8, ¶2.5

The Safety Board determines that the probable cause of this accident was the Isolation of the No. 7 leading edge slat in the fully or partially extended position after an extension of the Nos. 2, 3, 6, and 7 leading edge slats and the subsequent retraction of the Nos. 2, 3, and 6 slats, and the captain's untimely flight control inputs, to counter the roll resulting from the slat asymmetry. Contributing to the cause was a preexisting misalignment of the No. 7 slat which, when combined with the cruise condition airloads, precluded retraction of that slat. After eliminating all probable individual or combined mechanical failures or malfunctions which could lead to slat extension, the Safety Board determined that the extension of the slats was the result of the flightcrew's manipulation of the flap/slat controls. Contributing to the captain's untimely use of the flight controls was distraction due probably to his efforts to rectify the source of the control problem

Source: NSTB-AAR-81-8, ¶3.2