A strong door for tough times

DN Staff

November 5, 2001

6 Min Read
A strong door for tough times

Beefing up flimsy cockpit doors won't solve all the airline security problems that contributed to the attacks of September 11th-but it may help. This mechanical response to airborne terrorism received support from President George Bush just days after attack, when he announced a $500 million fund to finance strengthened cockpit doors and other security modifications for commercial airplanes.

TTF's amored flight deck door pairs security features, like the integrated armor and beefed up latch, with features designed to meet FAA regulations for quick removal and decompression.

The airlines began installing short-term reinforcements-such as metal locking bars- soon after the attacks. But the long term goal of making cockpit doors impenetrable to would-be attackers will require much more engineering work. Doors that once served primarily as privacy screens for the flight crew will have to be redesigned, according to a report from the U.S. Secretary of Transportation's Rapid Response Team. At the same time, the report stresses that the new level of cockpit security shouldn't come at the expense of safety in emergency situations.

One possible solution to this balancing act between protection and safety comes from TTF Aerospace LLC (Tukwilla, WA). The company, which designs and fabricates airline interior components, has come up with an armored flight deck door that's built to stop bullet, blade, and ramming attacks. The patent-pending door, which comes in single hinged and bi-fold versions, also addresses FAA and airline requirements for flammability, emergency decompression, and quick escape. "Our biggest engineering challenge was creating a secure door that maintains compliance to existing FAA regulations," says Tim Morgan, TTF's president and chief designer.

Shoot out. Outwardly, the TTF door resembles current flight deck doors. At one inch thick, it features a composite panel construction that sandwiches a phenolic honeycomb core between phenolic-impregnated fiberglass face sheets. The door's stopping power derives from something hidden-more than 35 lbs of armor imbedded in its core.

To combat both handgun and blade attacks, the door relies on two distinct types of armor. A rigid glass weave takes aim at the handgun attacks. Measuring 3/8-inch thick and weighing 0.37 lb/ft2, this bullet-resistant layer can stop a 9-mm full-metal-jacketed slug moving at more than 1,400 feet/sec, according to Morgan. "This material is actually superior to Kevlar in both ballistic performance and weight," he says. To thwart blade attacks, the door also has two 1/16-inch thick layers of metallic mesh, one on either side of the ballistic layer. "Our design goal was not just to delay destruction of the door," he says. "We wanted to eliminate the possibility of penetration."

In picking the materials for the armored door, Morgan had to juggle not only the technical requirements imposed by the FAA but also the airlines' weight and cost targets. Flammability requirements, for instance, dictated the use of phenolic-based materials for the core and face sheets, rather than tougher epoxy systems. "Epoxy-based Kevlar generally does not meet the FAA's smoke and toxicity requirements," Morgan notes. He rejected metal for the armor because it could have pushed the door and frame over a 165-lb limit imposed by some airlines. "Weight considerations required a composite solution instead of metal," he says. And to keep costs low, Morgan picked off-the-shelf materials for the armor. "There is no new materials technology in the door," says Morgan, who won't name the source of the armor layers.

At the left, while it outwardly resembles a conventional door, the TTF door panel has two kinds of built-in armor - one layer to stop bullets and two more to stop axe blades. At the right TTF has integrated a decompression mechanism into the door jamb, eliminating the need for blowout panels.

Lock out. A security door is only as good as the components that keep it shut, so the TTF door features a strengthened latching mechanism and frame. "The ability to repel the ramming action of a 300-lb man entered into the design equation," Morgan says.

To stop any linebacker-style charges into the cockpit, the TTF door starts with hardened steel bolt rather than the aluminum bolts often used today. Other security provisions can be found in the latch's mechanical design. The latch body, machined from treated aluminum billet, has already been tested to withstand a 9-mm slug at point blank range, Morgan says. And the latch features a dummy knob on the cabin side. "Only the flight deck side of the door is mechanically attached to the latch bolt," Morgan says. TTF can supply the latch with a traditional key entry, but Morgan predicts that the September 11 attacks will pave the way for higher-tech locking mechanisms. "We're looking at other ways to lock out the flight deck," he says, citing an encrypted, keyless entry as one option.

TTF also beefed up the area around the door-creating a frame from aluminum profiles and armored panels like those used in the door-to prevent the possibility of an attacker sawing out the lock.

Play by the rules. TTF also had to come up with a door that complies with a host of FAA standards, including those related to pressurized cabin loads, pilot compartment doors, general emergencies, and emergency landings. "FAA regulations drive the whole design," Morgan says. For quick removal, for example, the TTF door features a spring-loaded removable hinge inside the door.

And it has provisions to handle decompression. For passenger cabin decompressions, TTF has developed a system that equalizes any resulting pressure differentials with the flight deck-much like a pressure-relief valve. Integrated into the door jamb, the system takes its cues from sensors that pick up pressure differences between cockpit and cabin. "Once delta pressures reach a pre-set level, the system activates and allows the door to open," Morgan says. For rapid flight deck decompressions, which put much larger dynamic loads on the door than a cabin decompression, the TTF system can also tie into an electronically actuated door latch, which disengages so that the door can blow back into the flight deck.

Intended as a retrofit for existing cockpit doors, the TTF door still has to clear regulatory hurdles before it can take to the skies. But in the weeks following the attacks, more than ten airlines have contacted Morgan about the door. "There hasn't been a great interest in cockpit security-until now," he says.

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