TWA Flight 841 was a scheduled passenger flight from John F. Kennedy International Airport, New York City, en route to Minneapolis-Saint Paul International Airport in Minneapolis, Minnesota. On the evening of April 4, 1979, while flying over Saginaw, Michigan, the Boeing 727-31 airliner began a sharp, uncommanded roll to the right, and subsequently went into a spiral dive. The pilots were able to regain control of the aircraft and made an emergency landing at Detroit Metropolitan Airport.
Background Aircraft The aircraft involved, manufactured in 1965, was a 14-year-old Boeing 727-31, registered as N840TW with serial number 18905. In its 14 years of service, it had accumulated about 35,412 hours and it was equipped with three Pratt & Whitney JT8D-7B engines.
Crew In command was Captain Harvey G. "Hoot" Gibson, aged 44, who had logged a total of 15,710 piloting hours, 2,597 of them on the 727. The day before he had returned to flying following a three month medical leave. The First Officer was Jess Scott Kennedy, aged 40, who had logged a total of 10,336 total piloting hours, 8,348 of them on the 727. The flight engineer was Second Officer Gary N. Banks, 37, who had logged a total of 4,186 piloting hours, 1,186 of them on the 727.
Accident At 8:25p.m. EST, TWA Flight 841 departed JFK International after a 45-minute delay due to traffic congestion and reached its initial cruising altitude of 35,000feet at 8:54p.m. Due to a headwind, the pilots requested to climb to 39,000feet at 9:25p.m., which was granted. At 9:47p.m., while cruising at near the city of Saginaw, Michigan, Captain Gibson said the aircraft was operating with the autopilot on "Altitude Hold" mode when he felt a high frequency vibration in the balls of his feet, followed by a buzzing sound and a light buffeting of the airframe. He looked at his instruments and saw the plane banking right before yawing sharply to the right, briefly pausing, and yawing right again into a skidding right roll with the autopilot moving the control wheel to the left to level the wings. Gibson disconnected the autopilot and applied full left aileron, which had no effect. He got on the rudder pedals and applied full left rudder. As he did so something didn't feel right but he couldn't place it. Although Captain Gibson's inputs briefly stopped the roll from increasing, the roll continued and the aircraft spiraled out of control. Gibson shouted something to the effect of "We're going over!" - Copy at Embry-Riddle Aeronautical University. After completing one 360° roll, Gibson shouted to the co-pilot "Get em' up!" referring him to deploy the speed brakes, but the co-pilot didn't understand what the Captain meant so Gibson deployed them, which also had no effect.
Flight 841 dove about in just 63seconds. During the course of the dive, the plane rolled through 360degrees at least twice and exceeded the Mach limit for the 727 airframe.
At about the pilots extended the landing gear in an attempt to slow the aircraft. A few seconds after extending the gear, Gibson managed to regain control and pulled the 727 out of its dive at about .Peterson, Iver. "Plane Passengers Prayed But Expected to Be Killed." The New York Times (April 7, 1979), 6.The Post-Standard (Syracuse, New York), June 10, 1981, page 11 The plane suffered substantial structural damage with the No.7 leading-edge slat and a flight spoiler having detached, the right outboard aileron hinge fitting broke due to metal fatigue resulting in free-play, and the System A hydraulics ruptured due to the right main gear overextending that also broke the sidebrace and actuator support beam; the Flight Engineer reported they had a fail flag for the lower rudder yaw damper but made an emergency landing at Detroit Metropolitan Airport in Michigan at 10:31p.m. EST without further trouble. After landing and shutting the engines down the pilots turned on the Auxiliary Power Unit (APU) as a mechanic plugged into an intercom to talk to the pilots and reported fuel was leaking from the left side. All 89 people deplaned via the aft airstair with only eight passengers suffering minor injuries.
Investigation The aircraft returned to service in May 1979. The flight crew would be questioned on three occasions during the course of the entire investigation: hours after landing in Detroit, on April 12, 1979, and on January 29, 1980. The investigators of the National Transportation Safety Board (NTSB) located the No. 7 slat missing from the leading edge of the right wing amongst other parts that had detached. They requested that Boeing, the manufacturer, inspect the remainder of the slat assembly, including a portion of the slat actuator cylinder (the motor that moves the slat and holds it in position). Boeing determined that the No. 7 slat had failed because the slats had been extended while the aircraft was flying at cruising speed. They also determined that it was "impossible" for the slat to extend without manipulating the controls.
The TWA Flight 841 investigation would be at the time the lengthiest accident investigation in the NTSB's history. After eliminating all known sources of mechanical failure, the NTSB concluded in June 1981 in its final report, that the probable cause of the accident was the isolated extension of the No. 7 leading-edge slat from the flight crew manipulating the flap/slat controls in an inappropriate manner leading to an uncommanded roll to the right and the captain's untimely action.Witkin, Richard. "Safety Board Hints Crew Errors May Have Led to Jet Dive Over Michigan." The New York Times (January 18, 1980), A10.
Investigators claimed that 727 pilots (in general, and this crew specifically) were enabling the trailing edge flaps to extend independently of the slats via the flap/slat lever by turning the Alternate flap switch on, pulling the leading-edge slats circuit breaker, turning the Alternate flap switch off and then setting the flaps to two degrees during high altitude cruise. This configuration was rumored to result in increased lift with no increase in drag, thus allowing more speed, higher elevation, or decreased fuel consumption. Flight tests showed that airspeed and performance decreased in this configuration. Flaps and slats are intended to only be deployed at low speeds during take-off and landing.
The crew, Capt. Harvey G. "Hoot" Gibson (1934–2015) https://www.starmortuary.com/obituaries/harvey-gibson, First Officer Jess Scott Kennedy (1939-2017) https://emiliocorsetti.com/the-passing-of-scott-kennedy/ and Second Officer Gary N. Banks (born 1942), denied that their actions had been the cause of the flaps' extension:
The flight crew testified that they had not engaged the flaps and instead suggested that the actuator to the No. 7 slat had failed, causing its inadvertent deployment. The NTSB concluded that "if the flight crew's recollections are accurate," the slat extension must have been caused by a mechanical failure or defect but ultimately rejected this as improbable and attributed the extension of the flaps to the deliberate actions of the crew. The crew maintained that such failures had happened on other 727s prior and subsequent to this incident.Boeing Operations Manual Bulletin, OMB 75-7, March 10, 1976; Subject: "Leading Edge Slat Actuator Lock Rings"; outlined specific conditions that had previously resulted in a Leading Edge Slat being pulled from the Retracted position (Mach > .8M, with failure of "A" System Hydraulic pressure to the slat actuator, with SpeedBrakes/Spoilers Extended). The NTSB report noted seven separate cases involving a single leading edge slat extension and separation were reported between 1970 and 1973, but none of these reports indicated whether or not the slat extension was due to flight crew involvement. Records after 1974 did include two report slat extensions between 1974 and towards the end of the NTSB's investigation in 1981, one of which was inadvertently caused by the flight crew. Nonetheless, in none of these previous cases did the flights experience significant problems.
Gibson and the Air Line Pilots Association (ALPA) appealed against the NTSB's findings, first to the NTSB itself in 1983, 1990, 1991, 1992, and 1995 and then to the U.S. Ninth Circuit Court of Appeals. Both petitions were rejected: the former for lack of new evidence, and the latter for lack of jurisdiction due to the NTSB's "unreviewable discretion".
Cockpit voice recorder The aircraft was equipped with a Fairchild Industries Model A-100 cockpit voice recorder (CVR). 21 minutes of the 30-minute tape were blank. Tests of the CVR in the aircraft revealed no discrepancies in the CVR's electrical and recording systems. The CVR tape can be erased by means of the bulk-erase feature on the CVR control panel in the cockpit. This feature can be activated only after the aircraft is on the ground with the engines turned off and its parking brake engaged. In a deposition taken by the Safety Board, the captain stated that he usually activated the bulk-erase feature on the CVR at the conclusion of each flight to preclude inappropriate use of recorded conversations. In this instance, however, he and the other two pilots did not recall having done so.