A bit of tech

2007-12-22 00:00

To have the engine fall out of your car would be not only outrageous but obviously impossible, its being solidly bolted within the framework of the thing, a structural part ot it. Okay, then, let’s take a lesser case of say now a wheel falling off, which is only slightly less impossible but still utterly outrageous — you wouldn’t expect even your bicycle to come to pieces beneath your buttocks in the middle of the traffic, would you? There must be a designer or a mechanic or SOMEBODY to blame, hey, so let’s get out there and have his guts. Similarly, you’d expect the flying public to bristle at mention of that ill-fated plane which shed an engine on its landing approach in Cape Town a couple of weeks ago, wouldn’t you? I mean it simply fell off. Never mind two weeks, they’ll still be bristling in 20 years, won’t they? Bad bad image for Boeing, Airbus, whomever, and let’s haul the airline before some International Body and have it struck off the roll of Something or Other and find a mechanic and an inspector and get them stuck in jail. I mean these folks are quite right to bristle, aren’t they?

Well, no, they’re not. Engines falling off are a design feature of most jet aircraft today. In the old days of reciprocating engines and propellors the pilot had the option of immobilising an engine in flight in the case of a bird strike or in the case of some unlikely heavy object hitting the works, he could shut off the fuel and switch off the ignition and feather the prop and everything would come to a standstill. Feathering means changing the pitch of the propellor blade so it lines up with the slipstream and causes no drag. But you can’t feather the compressor or turbine blades on a jet engine, there are just too many of them so there just isn’t room for a whole lot of variable pitch mechanisms, and anyway that would make for weak spots at the blade roots because the blades are under such extreme stress at 20 000 revolutions per minute. Furthermore, there are no spark plugs in a jet engine, it just keeps itself alight like a Primus stove, so there’s no ignition to switch off, and if you shut down the fuel supply it simply goes on windmilling at 15 000 rpm or so because, however heavy, it is so finely balanced on its bearings.

So then. If a nice heavy chicken-size bird like a hadedah or a couple of seagulls or pigeons get sucked into such a fast-turning piece of machinery you can imagine the damage it will do. Break off a couple of compressor blades and you’ve got intolerable vibration that few structures can withstand. If your engines are tucked in close to the fuselage as with the old DH Comet you’ll be all right, the fuselage will take the vibrations and you’ll have an uncomfortable ride home, but in the case of an engine mounted on a pylon beneath the wing the vibration is quite likely to break the main spar and the wing will fall off. So what the aero engineers do is design a thing called a shear pin, a metal cylinder 12 cm or so in diameter and of suitable length, an extremely carefully calculated alloy immensely strong and not given to metal fatigue, but to break straight across at a certain phase of vibration, and they build this into the main mounting point of each engine so it will fall cleanly off the wing at such a phase, and design an aircraft that can safely fly on its remaining engines. To get certified, even the biggest four-engined plane must be able to fly on only one engine. But it’s still one hell of a crisis, whatever the engineering.

Now if this drastic thing were to happen at 10 km altitude the aircraft would dip a wing on the opposite side and the autopilot would try to pick it up, though perhaps manual control would be needed. But there are no birds at 30 000 feet and no half-bricks, only meteors and a few angels, so there’s time and more importantly, altitude for emergency action. Mostly the crisis is near the ground, and mostly it’s about birds, and to lose an engine and pick up a dropped wing on landing approach as in this Cape Town crisis, that’s as skilled a piece of flying as you’re likely to witness in all your hours in the air. It’s about good maintenance, bloody good aircrew training, quick reflexes and cool, man, cool. So next time you go flying ask for a window seat where you can get a good view of one of the engines, and take a little notebook along and a ballpoint, and if this engine falls off anywhere along the way you can call a flight attendant and ask her to touch the pilot on his shoulder and hand him a small note of congratulations on his airmanship.

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