Accident Overview

On June 2, 1983, Air Canada Flight 797, a McDonnell-Douglas DC-9-32 (S/N 47196) of Canadian Registry (C-FTLU), was on a regularly scheduled international flight from Dallas, Texas to Montréal, Quebec, Canada with an en route stop at Toronto, Ontario, Canada. The flight left Dallas at 1725 EDT.

Due to smoke from an uncontrollable fire in the lavatory, located aft of the passenger seating area, the aircraft made an emergency landing at 1920, at the Greater Cincinnati International Airport (now identified as Cincinnati/Northern Kentucky International Airport, CVG). Twenty-three of the 41 passengers were unable to exit the aircraft and succumbed to smoke and toxic gases prior to and/or after the flash fire that occurred 60 to 90 seconds after the emergency exit doors were opened.

History of Flight 797

Cockpit, First Indications

The NTSB reviewed six possible sources of the fire: incendiary or explosive device; deliberate ignition; burning cigarette; the toilet flush motor; the flush motor electrical harness; or arcing of the generator feeder cables. While extensive analysis was conducted, the origin of the fire was not conclusively identified.

A fire in the area of the flush motor was shown to have the capability to cause the flush motor circuit breakers to trip. The three-phase motor uses 115 Volt alternating current at 400 Hertz. The wiring to the motor is protected by three five-ampere phase-A, phase-B, and phase-C circuit breakers in the flight deck. Immediately prior to the circuit breakers tripping the Cockpit Voice Recorder (CVR) captured three instances of a sound similar to electrical arcing. The CVR wiring is near the generator feeder bundles, and voltage excursions can electromagnetically excite the cockpit area microphone. This excitation can be recorded by the CVR, but will not be audible to the pilots. This phenomenon is similar to listening to a transistor radio during a lightning storm. As lightning occurs near the receiver, the signal is disrupted and a short moment of static is heard. Following the circuit breaker trips, the pilot attempted to reset each one, but was unsuccessful. Nine minutes later, another unsuccessful attempt to reset the circuit breakers was made.

Cabin, First Indications

Aft lavatory – believed to have been the point of origin of the fire
Aft lavatory – believed to have been the point of origin of the fire

About the same time that the circuit breakers were reset for the second time, a passenger seated in the last row asked one of the flight attendants (FA, number 3, FA3) to identify a strange odor. The flight attendant, equipped with a portable CO2 fire extinguisher, looked inside the aft lavatory and saw light grey smoke filling the lavatory. No flames were seen. The door was closed, and the flight attendant informed the next senior flight attendant (FA2), who in turn informed the flight attendant in charge (FAIC). FA2 went forward to inform the captain and then assisted FA3 in moving the passengers forward. In the meantime, the FAIC, equipped with the fire extinguisher, went to assess the situation in the lavatory. The FAIC opened the lavatory door and observed thick curls of black smoke emanating from the seams of the lavatory walls, at the top of the wash basin, behind the vanity, and at the ceiling. The FAIC sprayed CO2 at the walls, seams, and the door of the trash bin. The trash chute and receptacle area for trash was equipped with a heat activated automatic Halon 1301 fire extinguisher. The NTSB concluded that the trash chute was not a factor or contributor to the fire.

Delayed Decision to Divert to Nearest Airport

Aft cabin area following fire
Aft cabin area following fire
Damage to forward cabin
Damage to forward cabin

At 1902:40 the flight attendant (FA2) informed the captain of the fire in the aft lavatory and that the FAIC had gone to "put it out." This flight attendant had not seen the condition of the lavatory, and was therefore unable to inform the captain where the fire was located inside the lavatory. The first officer went aft to investigate but did not take his smoke goggles with him. A flight attendant requested approval to move passengers forward after the first officer left the flight deck. The FAIC informed the first officer that CO2 had been discharged, and that he believed it was not a trash chute fire.

At 1904:07 the first officer returned to the cockpit to retrieve goggles, as smoke had engulfed the last three rows of the aircraft. He told the captain that he couldn't go back, as the smoke was too heavy. He further stated that while it didn't have to be accomplished immediately, he recommended that they begin a descent and emergency landing. Immediately following, the FAIC entered the flight deck and informed the crew that the passengers had been moved forward and " don't have to worry, I think it's gonna be easing up." The first officer agreed that the smoke was starting to clear.

At 1904:46 the first officer returned to the lavatory with smoke goggles and a smoke mask. A flight attendant informed the captain that the FAIC discharged CO2 in the washroom, and the fire seemed to be subsiding. The FAIC stated that CO2 was discharged inside the washroom, but the source of the fire could not be seen. The first officer, after donning smoke goggles, went aft to the lavatory a second time. He felt the lavatory door and, finding that it is hot, decided not to open it. He then returned to the flight deck.

At 1907:11 the first officer stated "...I think we better go down..." The captain agreed, and preparations for an emergency descent were made.


Cabin Safety

Flammability:  Under the basis of certification, the cabin interior materials were required to be flash- and flame-resistant, but did not provide compliance criteria. There were also no Advisory Circulars available to provide guidance. In June 1982, Air Canada had refurbished the passenger cabin. The right rear lavatory had been removed and replaced with a clothing storage area. Overhead luggage bins were installed, and the cabin walls and ceilings were replaced. The manufacturer of this new equipment had been required to demonstrate that the subject materials met the flammability standards of 14 CFR 25.853 (a)(b) as amended on May 1, 1972.

Cockpit Door:  Earlier in the flight, the louvered kick plate of the flight deck door came off. During the descent, the door was not closed, which allowed smoke to enter the flight deck.

Smoke Detectors:  Smoke detectors were not required in the passenger compartment or the lavatories.

Passenger cabin following the fire
Passenger cabin following the fire

Smoke in the Cabin:  It was estimated that the fire burned undetected for almost 15 minutes. An aluminum lavatory vent line was designed to remove lavatory air through a venturi, located forward of the lavatory in an access panel. A flex line for the waste tank flush and fill pipe was also located close to the inlet of the lavatory vent line. If this flex line melts the pipe will also become an overboard vent. Hot gases melted the aluminum vent tube and then impinged on the generator feeder cables. This high temperature resulted in faults on the left and right generators, and the protective circuits tripped them off line at just about the same time that the pilot declared an emergency at 1907.

During the descent and prior to landing the air conditioning and pressurizations packs were cycled on and off by the first officer in the belief that the airflow was feeding the fire. Although he did not know it, turning off the system accelerated the accumulation of smoke, heat and toxic gases in the cabin.

Almost all fires propagate upward and in the direction of surrounding airflow. The fire traveled behind the amenities section and the toilet shroud. As the fire consumed the lavatory structure, the smoke and burned and unburned gases moved up the lavatory frame channels and forward along the space between the airplane skin and ceiling panels. The smoke, fumes, and hot gases entered the cabin through the ceiling and sidewall panels, and began to collect in the upper portions of the cabin.

Passengers found relief from the smoke and fumes by breathing through wet napkins and towels passed out by the fight attendants, or by articles of clothing held over their nose and mouth. They all attempted to breathe in as shallow a manner as possible, as the smoke (which contained noxious by-products such as carbon monoxide, hydrogen chloride, hydrogen fluoride, hydrogen cyanide) hurt their noses, throats, chests, and caused their eyes to water.

Egress:  By the time the plane landed and was brought to a stop, the entire cabin, including the cockpit, had been engulfed with thick smoke. More than one or two feet above the cabin floor, visibility was non-existent. None of the passengers noticed if the emergency lights had been illuminated. The smoke was sufficiently thick that most of the passengers had to find the emergency exits by using seatbacks to feel their way along the aisle. Some passengers leaving through the over-wing doors found these exits because they had memorized the number of rows between their seats and the exits. Some passengers were able to breathe and see slightly better when on their hands and knees.

Photo of Air Canada flight 797 burning on runway
Photo of Air Canada flight 797 burning on runway
Photo Copyright Capt. Grieder - used with permission

Click here to view Air Canada DC-9 Flight Path Animation.

Several factors limited the success of the evacuation of the passengers. The flight attendants at the forward doors were not able to make themselves heard inside the cabin. The location of the fatalities in the cabin tends to confirm that those who succumbed either made no attempt to move toward an exit or started too late and were overcome as they attempted to move toward an exit. Studies indicate that in the absence of commands, some passengers will remain seated and await orders, a phenomenon known as "behavioral inaction." It is also possible that some of the passengers were incapacitated because of exposure to toxic gases and smoke during the descent and landing.

A flash fire occurred within 60 to 90 seconds after the doors were opened and the cabin environment became non-survivable within 20 to 30 seconds after the flash fire began.

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