TWA Flight 841 (1979)
N840TW, the aircraft involved in the incident.
|Date||April 4, 1979|
|Aircraft type||Boeing 727-31|
On April 4, 1979, a Boeing 727-31 operating as TWA Flight 841 took off from John F. Kennedy International Airport in New York City, en route to Minneapolis-Saint Paul International Airport in Minneapolis, Minnesota. At around 9:48 p. m. local time, over Saginaw, Michigan, while the plane was cruising at 39,000 feet (11,887 m) and Mach 0.816, it began a sharp roll to the right. The roll continued despite the corrective measures taken by the pilot and autopilot. The aircraft went into a spiral dive, losing about 34,000 feet (10,363 m) in 63 seconds. During the course of the dive, the plane rolled through 360 degrees twice and crossed the Mach limit for the 727 airframe. Control was regained at about 5,000 feet (1,524 m) after the pilots extended the landing gear in an attempt to slow the aircraft and following the loss of the #7 slat from right wing. The plane suffered substantial structural damage, but made an emergency landing at Detroit Metropolitan Airport in Michigan at 10:31 p. m. local time without further trouble. No fatalities occurred among the 82 passengers and seven crew members. Eight passengers reported minor injuries related to high G forces.
The National Transportation Safety Board (NTSB) 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 of the flaps' extension:
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.
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 were probably attempting to use 2º 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.
Cockpit Voice Recorder
The aircraft was equipped with a Fairchild Industries Model A-100 cockpit voice recorder (CVR). However, 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 located in the cockpit. This feature can be activated only after the aircraft is on the ground with its parking brake engaged. In a deposition taken by the Safety Board, the captain stated that he usually activates the bulk-erase feature on the CVR at the conclusion of each flight to preclude inappropriate use of recorded conversations. However, in this instance, he could not recall having done so. The NTSB made the following statement in the accident report:
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.
- Peterson, Iver. "Plane Passengers Prayed But Expected to Be Killed." _New York Times_ (Apr 7, 1979), 6.
- The Post-Standard (Syracuse, New York), June 10, 1981, page 11
- NTSB Accident Report AAR81-08, 1.17.2.
- Witkin, Richard. "Safety Board Hints Crew Errors May Have Led to Jet Dive Over Michigan." _New York Times_ (Jan 18, 1980), A10.
- Aircraft Accident Report - Trans World Airlines, Inc., Boeing 727-31, N840TW, Near Saginaw, Michigan April 4, 1979. Washington, D. C.: National Transportation Safety Board. 1981-06-09. p. 52. NTSB-AAR-81-8.
- Witkin, Richard. "Crew Will Testify Today On Near-Fatal Jet Plunge." New York Times (Apr 12, 1979), B10.
- Lindsey, Robert. "Pilot Says Extending Landing Gear Was Near Last Resort to Stop Dive." New York Times (Apr 13, 1979), A16.
- Boeing Operations Manual Bulletin, OMB 75-7, March 10, 1976; Subject: "Leading Edge Slat Actuator Lock Rings"; outlined specific conditions which 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).
- ibid., section 2.5, p. 32.
- ibid., Abstract.
- ibid., section 1.11
- ibid., section 2.5, final paragraph, p. 33.
- AAR 81-08 Boeing 727 Saginaw Michigan Dive (NTSB report)
- Accident description at the Aviation Safety Network
- From the Flight Deck - TWA Flight 841
- NTSB Synopsis
- NTSB Denial of Petition for Reconsideration, May 4, 1995
- U.S. Ninth Circuit Court of Appeals, GIBSON v NTSB (appeal dismissed--lack of jurisdiction)