McDonnell Douglas DC-9-32 accident at Boston, Massachusetts, 17 Sept 1979

Accident narrative

On September 17, 1979, Air Canada Flight 680, a McDonnell Douglas DC-9-32, experienced a rapid decompression about 14 minutes after takeoff from Boston, Massachusetts. At an altitude of about 25,000 feet, the tailcone, the aft cabin pressure access door, and a portion of the aft cabin pressure bulkhead separated from the aircraft. The flight crew conducted an emergency descent and landed safely at Logan International Airport. Of the 45 persons aboard, one flight attendant received minor injuries.

### The flight

Flight 680 was a scheduled passenger flight from Logan International Airport to Yarmouth, Nova Scotia, Canada. The aircraft, Canadian registration CF-TLU, departed at 1212 eastern daylight time with 40 passengers and 5 crewmembers aboard.

The captain had accumulated 13,864 total flight-hours, with 1,213 in the DC-9. The first officer had 6,434 flight-hours, including 753 in the DC-9. The flight was cleared in accordance with an instrument flight rules flight plan and assigned an en route altitude of flight level 250. Visual meteorological conditions existed throughout the flight.

### The sequence of events

The takeoff and climb were uneventful. At about 1226, shortly before leveling off at flight level 250, an extremely loud bang was heard, accompanied by a complete loss of cabin pressurization. The captain stated that the first officer, who was flying, placed the aircraft on autopilot. The crew donned their oxygen masks. The captain assumed control, initiated an emergency descent, and observed that the cockpit door was missing and blue sky was visible through the aft of the aircraft.

At 1226:08, the flight radioed Boston Center, reporting a rapid emergency descent. Boston Center cleared the flight to return directly to Boston and descend. At 1226:34, the crew advised they had experienced an explosive decompression, were passing through 20,000 feet, and requested 9,000 feet for level-off.

At 1228:24, the flight reported, "Roger, we are just leveling now and the back end of our tail is blown completely off. If you could have some emergency crews standing by." The captain noted that the passengers' oxygen masks had deployed, so he slowed the descent rate. The first officer checked the cabin and reported everyone was all right, except for one flight attendant. She had been picking up trays near row 18 when the decompression occurred, caught her leg in row 17, and fell to the floor. She was unconscious for about 15 seconds and sustained minor leg, head, and hand injuries.

During the descent, the captain found that the right throttle would not advance beyond 1.25 exhaust pressure ratio, though engine parameters and hydraulics appeared normal. The first officer noted the right pneumatic crossfeed valve lever had opened and could not be closed. The captain kept the aircraft high during the approach, using flaps and landing gear to reduce airspeed and altitude because of the limited use of the right engine. The flight landed safely on runway 33L at 1250. After touchdown, the left engine could not be reversed.

### What the investigation found

The rear portion of the fuselage sustained structural damage. The tailcone, aft cabin pressure bulkhead access door, a drink cart, and the lavatory water supply tank were missing. The internal sidewalls of the fuselage were displaced, notably at the overwing area, and the cabin floor immediately forward of the aft pressure bulkhead was 5 to 10 degrees low at the aft end.

Examination of the flight controls revealed that the right elevator trim, rudder trim, and horizontal stabilizer indicating control systems were inoperable. A cable pulley support bracket on the right aft side of the pressure bulkhead had been torn off, leaving the cables loose. The left thrust reverser control valve linkage was jammed against the displaced pressure bulkhead, preventing full reverse. The right pneumatic crossfeed shutoff valve was jammed open due to the bulkhead separation.

The Safety Board's metallurgical examination of the aft cabin pressure bulkhead found a large crack in the door jamb below the access door. The crack originated from mechanical damage in a lockbolt fastener hole, which the Board noted most likely occurred during the assembly of the bulkhead. The examination determined the crack was caused by low-load, high-cycle fatigue. Cyclic loading was produced by cabin pressure fluctuations, with each flight correlating to one cycle. The crack extended downward, and the web most probably tore incrementally to about 10 inches below the upper rivet hole before failing catastrophically.

The aircraft had recorded 26,816 landings and 28,425 flight-hours. Its last maintenance inspection, a C-check, was performed on May 5, 1979, roughly four months and 1,006 flight-hours before the accident. During this inspection, the aft bulkhead was x-rayed. The Board found that crack indications from the lockbolt fastener hole in the jamb below the door were discernible on the x-ray radiographs. However, nothing on the inspection record indicated that inspectors detected the crack when examining the x-rays.

Following the accident, fleetwide inspections mandated by the Federal Aviation Administration revealed similar cracks in 33 out of 119 DC-9 aircraft inspected by seven other air carriers. Air Canada also found cracks in two of its other DC-9s.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the accident was "a fatigue fracture of the aft cabin pressure bulkhead which resulted in a rapid decompression of the aircraft's cabin area. This fracture initiated from a crack below the aft bulkhead access door which was discernible on the X-rays taken during the aircraft's last maintenance inspection but was not detected by the inspectors."