On April 4, 1979, a Boeing 727-31 (tail number N840TW) operating as TWA Flight 841 took off from JFK International Airport, New York City en route to Minneapolis-Saint Paul International Airport. Over Saginaw, Michigan, while the plane was cruising at 39,000 feet, its #7 slat extended, initiating a sharp roll to the right. The roll continued despite the corrective measures taken by the autopilot and the human pilot. The aircraft went into a spiral dive, losing about 34,000 feet in 63 seconds. (For comparison, a normal rate of descent for an airliner would be 1800 feet per minute). During the course of the dive, the plane rolled through 360 degrees twice, and crossed the Mach limit for the 727 airframe. It was later estimated from the flight data recorder that the plane was momentarily supersonic. Control was regained at about 8,000 feet, following the #7 slat being torn off from the aircraft and symmetry of lift being re-established. The plane suffered substantial structural damage, but made an emergency landing at Detroit, Michigan without further trouble. No fatalities occurred among the 82 passengers and seven crew members. Eight passengers reported minor injuries relating to high G forces.

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

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. When retraction of the flaps was ordered, the Number 7 leading edge flap 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.

References

External references

• AAR 81-08 Boeing 727 Saginaw Michigan Dive (NTSB report)
• Aviation Safety Network Accident Description
• 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)