Air Canada Flight 797 - NTSB Investigation

NTSB Investigation

Though the fuselage was nearly destroyed by the intensity of the fire, the cockpit voice recorder (CVR) and flight data recorder (FDR) for flight 797 were still in good condition and produced vital useful data for the subsequent National Transportation Safety Board (NTSB) investigation. On the CVR, NTSB investigators heard eight sounds of electrical arcing—likely inaudible to the crew—beginning at 18:48 CDT. Three minutes later, at 18:51, the popping sounds that Cameron and Ouimet would later identify as the left toilet circuit breaker tripping are audible on the CVR; Cameron attempts to reset the circuits twice over the next 60 seconds, but the CVR records the breakers immediately popping again after each reset attempt. Cameron would attempt once more to reset the breakers at 18:59, but the CVR records arcing sounds followed by the popping sound of the breakers continuing to trip again after each reset over the next 60 seconds. At 19:02, the CVR records Flight Attendant Judi Davidson entering the cockpit to deliver the first report of a possible fire in the lavatory. Though a number of wires in the lavatory section were later found with insulation stripped away, NTSB investigators were unable to determine whether this insulation damage was the cause of the fire or was caused by the fire.

This particular DC-9 had experienced a number of problems over the months leading up to the incident; 76 maintenance reports had been filed in the plane's logs in the previous year, and the CVR records Cameron telling Ouimet to "put in the book there" when the breakers fail to respond to the first reset attempt at 18:52. Nearly four years earlier, on September 17, 1979, the plane, then serving as Air Canada Flight 680 (Boston, Massachusetts to Yarmouth, Nova Scotia), had suffered an explosive decompression in the rear bulkhead that required rebuilding the tail section and replacing or splicing most of the wiring and hydraulic lines in the back of the plane; Cameron noted in an interview for the "Fire Flight" episode of Mayday that the Air Canada maintenance crew "did a heck of a job getting everything put back together" after the decompression incident. Investigators were unable to find signs of arcing in any of the wire splices from the repairs done four years earlier, though much of the wiring in the rear of the plane was severely damaged or destroyed by the fire itself.

Despite finding neither the specific wires that caused the short circuit (the usual cause of arcing sounds and the likely cause of the breaker trips) nor the origin point of the fire that later consumed the plane, investigators determined that a short circuit likely sparked and ignited surrounding materials (such as insulation blankets) that sustained and propagated a fire that burned behind the wall of the lavatory, with the plane's outer skin serving as a conduit for smoke to seep in through the seams in the interior panels and collect near the apex of the cabin.