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The cockpit door of a modern airliner is there to keep the pax from seeing into the cockpit and becoming alarmed by the many warning lights that come on during a perfectly normal flight. While somewhat cynical, this statement is mostly true. By Federal Aviation Regulation [FAR] a cockpit door must be weak enough to be kicked open by a normal person in the event of an emergency. It must also have allowances to open by itself in the event of a depressurization.
Despite media reports to the contrary, there are airline aircraft currently flying that have no door at all. Small, 19-seat commuter aircraft typically have nothing more than a curtain to keep the pax from seeing the aforementioned lights. In these tiny planes, which also lack a Flight Attendant, a galley, and a bathroom, there is room for little else. I should know, I used to fly one...
FAR requires that aircraft with 20 passenger seats or more have a Flight Attendant. The regs also mandate a solid cockpit door that can be locked. The door used to remain unlocked and in some cases open, dependant on the whim of the Captain. These were the days when kids could come up and visit with the pilots while enroute and curious adult passengers could be treated to a brief lecture on the miracle of modern aviation and make some small talk with the guys up front.
The rash of non-terrorist hijackings in the late 70's by disgruntled office workers, tax protesters, and the like put an end to all that. By FAR, the cockpit door must be closed and locked prior to takeoff and remain so thorough the flight, unless it was opened so a FA could provide food service to the pilots, or if one of the pilots had to leave the cockpit for physiological reasons. However, the security was mostly a ruse. The door remained weak but it was assumed--in some cases correctly--that a nonprofessional hijacker could be convinced that there was no way into the cockpit and therefore kept out.
The door also remained weak as it was thought--also correctly--that pilots or rescue personnel might need to kick the door open to evacuate in an emergency. During an emergency landing deformation of the doorframe could make opening the door by normal means impossible so the pilot would be required to simply force his way out. There are other ways out. The cockpit windows of all modern transport category aircraft can be either pulled out and removed or cranked far enough back to allow a pilot to crawl out. Escape ropes are provided for the pilots to use to shimmy down to the ground and in some cases (Airbus) the ropes are even equipped with inertia reels so the pilots can execute spectacular bungee-jump style rappels where they just grab the end of the rope and leap. (Yes, there have been cases where pilots tried this on aircraft not so equipped by mistake, with spectacularly painful results...) Of course, the main problem with all of this is that it requires your average out of shape and slightly rotund airline Captain like me to stuff himself through a window roughly 2' by 3'. If the flames were rising, I have no doubt that I could accomplish this feat, but throughout the airline world the window exits are regarded as tools of ultimate last resort.
In addition, the cockpit door is not secure because it must allow for a rapid depressurization. Most depressurizations, despite the movies, are not violent events where hapless passengers are sucked out through gaping holes. Yes, there have been spectacular cases, such as the United over Hawaii, but most are more annoying than dangerous. In a depressurization, the pressure in the entire cabin must equalize with the lower atmospheric pressure. Cockpit doors are designed to open completely or have pressure fuses (small doors built into the main door) that will open to allow the pressure to equalize. If the door did not, 12 psi of pressure would quickly cause the bulkhead the door was mounted in to fail.
However, in light of recent events, it is imperative that something be done to increase the security of the flight deck. There are several possible strategies to accomplish this, each with some attendant problems.
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It has been widely suggested to just seal the cockpit completely, removing the door and replacing it with a solid bulkhead. This would make hijacker access totally impossible but has a host of problems.
Any pressure relief fuse in the cockpit/cabin bulkhead would render it non-secure so it would have to be designed to withstand the pressure of a decompression. If it did not, than the cabin/cockpit bulkhead would fail, possibly crushing the pilots, severing control cables, or deforming the fuselage to such a degree that it would fail. This would require the instillation of a pressure bulkhead and the division of the aircraft into two separate pressurized compartments. This would require total rebuilding of the aircraft and in many cases would be simply impossible.
Modern transport aircraft have maximum pressure differentials ranging up to 12 psi. The fuselage is designed to support this pressure and to withstand thousands of cycles, which is defined as one takeoff and one landing, without failure. The best possible shape for the fuselage would therefore be a sphere, which is the strongest possible shape. However, there are *problems* with a spherical airliner such as an awful coefficient of drag, a tendency to be highly unstable in crosswind landings and ground maneuvering, and the fact that it would be damn ugly. The cigar shape of an airliner is a good compromise that allows for an aerodynamic shape with a low center of gravity that still has spherical end caps.
With this in mind, it is easy to see the difficulties of installing a pressure bulkhead between the cockpit and the passenger cabin. This bulkhead would require spherical, or at least partially spherical, pressure walls. The sacrifice of precious internal volume would be exacerbated by the massive void space between the opposing spherical end caps. In addition, the fuselage structure is not designed to support these two additional pressure walls and would have to massively strengthened. Even more structural mass would be required as the aircraft must be designed to not fail even with one of the pressure vessels depressurized and the other at maximum differential. This is even more complex than it seems because a pressure vessel expands when pressurized. Having one "blown up" and the other at unpressurized size would generate massive forces on the structure.
This is all quite possible with modern construction techniques, however, the addition of such massive structure would completely destroy the weight distribution of the aircraft, rendering it unable to fly. To rebalance, weight would have to be added to the tail. This would add even more weight to the airframe. This weight would require a stronger wing box (the fuselage/wing root complex that supports the weight of the aircraft inflight) and stronger landing gear which would also add weight. Every extra pound of fuselage and internal structure means one less pound of fuel and revenue cargo that can be carried. It is quite possible that the installation of the pressure bulkhead and the tail ballast to rebalance would completely displace the useful load of the aircraft, making it unable to fly with any passengers at all.
Therefore, walling off the cockpit could require the replacing the entire commercial aviation fleet with new designs. This is simply unacceptable. Transport aircraft have a design and certification cycle of at least five years and the cost would be almost unimaginable. In the United States alone it would require a government subsidy of some 2-3 trillion--yes, with a T--dollars, which is a substantial percentage of the gross domestic product.
Of course, there are other problems with a solid cockpit/cabin bulkhead, such as egress in an emergency, pilot incapacitation, and most importantly, the fact that pilots are people too and need to occasionally use the restroom. These difficulties, while important, however pale in comparison to the necessity to replace the aircraft.
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A much more workable solution would be to armor the cockpit door yet still allow pressure relief. Such schemes are already underway. The aim in this case is not to totally deny cockpit access to a terrorist or unbalanced passenger but to delay it.
While it would be possible to break into a cockpit secured with locking bars and/or massive deadbolts, such an effort would generate a great deal of noise and take a while. During this time, the pilots could alert ATC and/or violently maneuver the aircraft to make the terrorist's task more difficult if not impossible. After the events of September 11th, many passengers could also be convinced to rise up and attack those trying to gain access to the cockpit. The pilots would also have time to draw and prepare any weapons they may have.
This is by far the better method to secure a cockpit.
I will not go into great detail on strategies to armor the cockpit door, as it could pose a security risk and I implore commenters and other posters to do likewise.
Thank you.
-- EdFox
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