American Airlines Flight 1291

After takeoff from Miami, the flight experienced a pressurization system malfunction, which the captain identified as the airplane depressurizing. The flight attendants and passengers complained of pain in their ears at that time. The flight crew placed the pressurization system into manual control, turned off the autopilot and autothrottle systems, and began a descent to return to Miami. During the return to Miami, several lavatory smoke alarms activated and the captain call light illuminated in the cabin; however, no evidence of fire or smoke was found. The flight crew did not complete the checklists for manual pressurization control and emergency landing during the return to Miami, both of which called for the airplane to be depressurized prior to landing. After landing and stopping on a taxiway, the captain also noticed an aft baggage compartment fire loop light illuminated, prompting him to evacuate the airplane. After the captain ordered the evacuation, the flight attendants attempted to open the doors. The doors would not open. The flight attendant/purser at the L1 (front left passenger) door continued to attempt to open the door, and the door explosively opened, ejecting the flight attendant/purser from the airplane to the ground, causing fatal injuries. The remainder of the doors opened and the airplane was evacuated. The emergency evacuation checklist did not call for the flight crew to check for depressurization of the airplane prior to commanding an evacuation. Post-accident examination of the airplane revealed that insulation blankets, which had been manufactured and replaced by the airplane operator's maintenance personnel, had not been properly secured per the airplane manufacturer's data. The blanket had migrated over to, and partially blocked, the forward and aft pressurization outflow valves, leading to the pressurization system malfunction. The forward outflow valve was found 3/8-open and the aft outflow valve was found fully closed. The lavatory smoke alarms were found to activate when subjected to abnormal pressure. There were no FAA technical standards for the lavatory smoke detectors. A sensor in the aft cargo compartment was found out of tolerance and also activated when subjected to abnormal pressure. The cabin doors were found to have no means for relieving pressure prior to opening the doors. The cabin altimeter in the cockpit did not have a mechanical stop in the negative direction, and under excessive pressure conditions, allowed the needle to move past the negative range into the high positive range. The aircraft manufacturer stated that when the pressurization system is in the manual mode, the outflow valves do not automatically open during landing and that a person cannot open a door if the airplane is pressurized above approximately 1.5 psi differential. As result of this investigation, the Safety Board previously issued 18 safety recommendations to the FAA.

Verbatim NTSB analysis and probable cause from the NTSB dataset