Safety Board Calls for Emergency Exit Door Redesign

Air Safety Week, August 19, 2002

Doors prove deadly if opened when cabin remains pressurized

Exit doors intended to save lives during an emergency evacuation need to be designed so they do not inadvertently kill or injure door operators if the cabin remains pressurized. The doors need to feature a vent or gate so that they can be opened safely even if the cabin pressurization system is malfunctioning.

An August 2 recommendation from the National Transportation Safety Board (NTSB) cited the Nov. 20, 2000, accident in which flight attendant/purser Jose Chiu was hurled more than 40 feet out of the left main door when it suddenly blew open as he attempted to open it during an emergency evacuation of the American Airlines [AMR] twinjet at Miami. The accident involved an Airbus A300-605R with 133 passengers and crew on board. Chiu, who by one account was thrown beyond the wingtip, was killed on impact with the concrete tarmac. A similar case occurred almost a year later, on October 20, 2001, involving an A300-605R operated by TunisAir. While an air stair was being positioned for normal deplaning, engine bleed air kept the cabin pressurized. Excessive cabin air caused the door to burst open. The flight attendant attempting to open the door was ejected, sustaining serious injuries. A second flight attendant at the doorway also ejected was killed.

In the case of the American Airlines fatality, the airplane took off from Miami for a planned flight to Haiti. Climbing through 16,000 feet, the ECAM [electronic centralized aircraft monitoring] display indicated that the forward cabin outflow valve was fully open (actually, according to sources, insulation blankets had clogged the valve). The aircraft pressurization system may be described as akin to a leaky balloon. Bleed air from the engines pressurizes the cabin to an artificial altitude of 8,000 feet, and one or more outflow valves regulates the discharge of cabin air to maintain a constant "cabin altitude" and to prevent sudden pressure changes during climb/descent (which passengers may notice by a popping of the ears).

Just 11 minutes after departure, and with warnings sounding from lavatory smoke detectors and a warning light indicating a possible fire in the belly hold (both false, as it turned out) the captain determined to return to Miami. He ordered an emergency evacuation upon landing.

The cabin pressure actually increased after landing, and the panoply of warnings persisted. With the cabin pressurized, none of the emergency exit doors could be opened. That is, until Door 1L suddenly blew open, ejecting flight attendant Chiu. Cabin overpressure was relieved, and all other doors with handles in the open position opened and their escape slides deployed.

Given the likelihood of future failures to outflow valves, the NTSB believes this type of overpressurization event "could occur again."

Emergency door design

Exit doors are designed with seals, latching mechanisms and structural restraints to contain the pressure of the cabin's artificial altitude during flight and yet to open smoothly on the ground. That is, when the fuselage pressure has equalized to that of ambient air. The A300 exit door opens upward over fuselage stop fittings, outward and forward parallel to the fuselage. Pressurized gas assists in the smooth opening of the door. Similar stop fittings are featured on Boeing [BA] doors.

However, the safety board is concerned that A300 emergency exit doors "do not have built-in systems to relieve pressure before the door is opened."

On all Boeing aircraft, from the B717 to the B747-400, the doors feature venting to relieve cabin pressure to a safe level before the emergency door can be opened. The NTSB voiced its "concern" that airplanes like the A300 do not have such a pressure relief system integral to the door, adding:

"The Board recognizes that pressure relief systems for emergency exit doors would depressurize the airplane slower than the opening of the outflow valves ... However, even this slower rate of depressurization would provide some protection against injury or death associated with opening the door while the airplane is still pressurized. Further, if the November 20, 2000, accident airplane had pressure relief systems for its emergency exit doors, they may have depressurized the airplane at a faster rate than the flight crew's opening of the outflow valves, which were partially blocked."

Education and training

Probing into why the cockpit crew did not alert the cabin crew to the fact that the airplane was overpressurized, the Safety Board noted that neither the carrier's flight crew operating manual [FCOM] nor its training program included information about how to recognize the signs of an overpressurized airplane. The pilots told investigators they were unaware that the airplane was still pressurized after rolling to a stop.

A similar shortcoming was found in the flight attendant safety manual. This document cautions flight attendants to evacuate the airplane immediately upon a signal from the cockpit, and to assess conditions outside the respective exits before opening them. However, the NTSB observed, "The manual does not address a situation in which all of the emergency exit doors fail to open during an evacuation and does not instruct flight attendants on recognizing the signs of an over-pressurized cabin." American Airlines was not alone - the same lack of explanatory information was found in the flight attendant safety manuals at 11 other airlines.