Sunday, 25 January 2009

Fear of Doors

If you ask most people what the single most dicey bit of equipment is on an airplane, you'll get a range of answers.
The obvious answer is the wings. But actually, wings are pretty stable things. They don't flap or do anything fancy, and the airframe is fitted to them pretty solidly. Wings tend not to fall off. Okay, so the control surfaces sometimes do fall off (some years back, a large flap landed in Richmond golf course that no airline wanted to claim), but this isn't as big a deal as you might think. The tailfin is important for stability, and you certainly don't want to lose that, but they're usually overengineered, too. Wheels are handy if you're planning on landing, but as long as you know that your undercarriage has failed, you can take appropriate steps. So maybe the little undercarriage warning light is one of the most important pieces of kit on the plane.

Engines are another matter. Jet engines are beautiful pieces of kit, but they're somewhat exposed. You have delicately-engineered compressor blades whose outer tips shear a helical scream through the air at more than the speed of sound, and they operate without safety covers. They're damned strong, but they're only designed to cut and chop air at supersonic speeds. Not water. Or ice. Or large items of poultry.

Try to convince a cruising jet engine that it ought to suddenly try being a food processor and attempt to make some albatross pate, and it gets unhappy. Blades can break and fly off. Control lines can get severed, and fuel tanks punctured. If you're lucky, when the engine tries to inhale a flock of geese, it'll just stop.

So bird strikes are dangerous. But they happen far more often than most people think, often to freight aircraft, where the incidental loss of an engine or an altered flightplan doesn't result in any scared passengers or a news story. Airliners have multiple engines, and unless you lose all of them, the pilot can usually try to do something with what's left.

If you're an airliner passenger, perhaps you should be rather more worried about geese than about the idea that the wings might fall off the plane.

So-o ... back to engineering. What's the most dangerous bit of equipment that the passenger is allowed to see? Perhaps it might actually be the aircraft's doors.
People don't think of doors as being dangerous. Doors are not supposed to be difficult pieces of engineering. But airliner doors are slightly different. See, there's no “safe failure mode” for an airliner door. If you need it to open in an emergency, it HAS to open, but while airborne, the door mustn't fly open when someone trips over and accidentally falls against it. It has to be really easy to open (or everyone dies), but not too easy (or everyone dies).

The door and its housing also have to operate under extremes of temperature, when the plane is parked in baking Saudi desert heat, or on a frozen Moscow runway with maybe a quarter-inch of ice on the seals. It has to cope elegantly with thermal expansion and contraction. If the door sticks, you do NOT want to damage the mechanism by forcing it too hard. If it's damaged, you can't fly.

Added to this was the problem that in older aircraft designs, the doors were liable to be one of the last things to be designed. And in the older “tapered” planes, you couldn't necessarily design a door in advance and place it anywhere you wanted ... oooooh no, the fuselage curvature was different all over the plane, sometimes a door designed for one part of the plane simply wouldn't fit anywhere else. And what happened to this mission-critical component whose failure could bring down a plane? It got slammed. It got shoved. People banged their luggage against it.

On recent airliners these things are better. Passenger doors now tend to be inward-opening, with a “bath-plug” taper to their cross-section that means that even if the locks fail, the internal air-pressure pushes the door tightly against it's frame. The doors are relatively small, rigid, and thick, and the higher you fly, the harder the doors are pushed into their mounts by the pressure-differential. The Airbus range (and the later Boeings) also have noticeably cylindrical fuselages, and while this may sometimes look like an ugly design cop-out, it helps with some aspects of the engineering. A “cylindrical” plane can be produced more easily in a range of “stretched” or “squashed” versions by adjusting the number of standard prefabricated sections. And it means that ... magically ... the doors work better. A door can start to be be designed and extensively tested as soon as the aircraft dimensions are decided, and then the resulting design can be deployed and duplicated at any point along the cylindrical section of the fuselage.

But for cargo-bay doors, things aren't so great. Cargo doors have to be big for loading, and while passenger doors can be designed with a “plug” taper so that they can't be forced open by air pressure when unlocked, cargo doors often open outwards. They also have a larger surface-area-to-edge ratio, so they're less rigid. They flex more. And the guys who load are using motorised equipment to move multi-ton pallets about, so you might expect the occasional accidental prang. And they aren't necessarily experts in aircraft safety.

As a result, serious cargo door failures in airliners do happen from time to time, sometimes leading to some of the passengers experiencing an unscheduled exit from the plane. In the worst-case scenario, in a pressurised aircraft at high altitude, the resulting explosive decompression can rip apart the plane's innards, destroying the control systems and bringing down the aircraft.

See, people just don't RESPECT doors.

Happy flying!

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