TWA Flight 841 (1979)

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.

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), First Officer Jess Scott Kennedy (1939-2017) 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. 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".