TWA Flight 841 (1979) - Aftermath

Aftermath

The National Transportation Safety Board investigated the accident and established after eliminating all individual and combined sources of mechanical failure, that the extension of the slats was due to the flight crew manipulating the flap/slat controls in an inappropriate manner. The rumor was that 727 pilots were setting the trailing edge flaps (which were normally only deployed at low speeds along with leading edge slats) to two degrees during high altitude cruise, while at the same time pulling the circuit breaker for the slats so that they would not activate. This configuration was rumored to result in increased lift with no increase in drag, thus allowing more speed, higher elevation, or decreased fuel consumption.

The crew, Capt. Harvey "Hoot" Gibson, first officer Jess Kennedy, and flight engineer Garry Banks, denied that their actions had been the cause.

At no time prior to the incident did I take any action within the cockpit either intentionally or inadvertently, that would have caused the extension of the leading edge slats or trailing edge flaps. Nor did I observe any other crew member take any action within the cockpit, either intentional or inadvertent, which would have caused the extension. —Capt. Gibson, April 12, 1979

The crew suggested instead that an actuator on the #7 slat had failed, causing its inadvertent deployment. The NTSB rejected this as improbable and attributed the extension of the flaps to the deliberate actions of the crew. The crew claimed that such failures had happened on other 727s prior and subsequent to this incident. The NTSB report notes seven such cases.

Despite the sworn testimony of the crew that they had not engaged the flaps, the NTSB argued that they probably were attempting to use 2 degrees of flaps at cruising speed.

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 2º of trailing edge flaps independently of the leading edge slats, probably in an effort to improve aircraft performance.

When retraction of the flaps was ordered, the Number 7 leading edge slat failed to retract, causing the uncommanded roll to the right.

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.

Captain Gibson appealed the NTSB's findings, first to the NTSB itself, 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".

The aircraft was repaired and returned to service in May 1979.