Incident Description
Following takeoff from Miami, the flight experienced a pressurization system malfunction, which the captain identified as the airplane depressurizing. Flight attendants and passengers reported ear pain. The flight crew switched the pressurization system to manual control, disengaged the autopilot and autothrottle, and began a descent to return to Miami. During the return, multiple 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, both of which required the airplane to be depressurized before landing.
After landing and stopping on a taxiway, the captain noticed an aft baggage compartment fire loop light illuminated, prompting him to order an evacuation. Flight attendants attempted to open the doors, but they would not open. The flight attendant/purser at the L1 (front left passenger) door continued trying, and the door explosively opened, ejecting her from the airplane to the ground, causing fatal injuries. The remaining doors opened and the airplane was evacuated. The emergency evacuation checklist did not call for the flight crew to check for depressurization before commanding an evacuation.
Investigation Findings
Post-accident examination of the airplane revealed that insulation blankets, manufactured and replaced by the operator's maintenance personnel, had not been properly secured per the airplane manufacturer's data. The blanket had migrated 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 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.
Aircraft Systems Issues
The aircraft manufacturer stated that when the pressurization system is in manual mode, the outflow valves do not automatically open during landing, and a person cannot open a door if the airplane is pressurized above approximately 1.5 psi differential.
Safety Recommendations
As a result of this investigation, the Safety Board previously issued 18 safety recommendations to the FAA.