[A post for curious non-pilots...]
"What happens if the engine quits?" Most pilots have been asked this question at some point during their flying career. It seems that society has been the victim of substantial conditioning on this subject. I wonder if survey statistics have ever been compiled to indicate the leading fear of the average airline passenger. This has got to be the winner. Many people believe certain doom is the only possible result after an airplane loses power in its engine(s). And why wouldn't they? Consider how many movies depict horrifying scenes of airplane crashes caused by engine trouble. Passengers are under the impression that if the engine (or engines) quits, the airplane will fall from the sky like a rock. What a dreadful misapprehension these nervous flyers must live with. Luckily, they're wrong.
Remember making paper airplanes as a kid? Did you ever attach an engine to one of them? Your airplane was nothing more than a folded up piece of paper, yet when you threw it, it didn't fall from your hand like a rock, right? That's because your airplane was aerodynamic. Perhaps its lift-to-drag ratio wasn't quite up to par with the modern airline jet, but it was still aerodynamic and that's what allowed it to travel through the air differently than a rock, pencil, pillow or anything else you could throw across the room. Essentially, you created a glider. Human-flown gliders like the Schweizer 1-26 are flown by pilots everyday without engines. This is because it is not the engine that makes an airplane fly, it's the wings.
As a wing, or airfoil, moves forward through the air it creates lift. This lift makes an airplane fly. The engine (or engines, in the case of large airplanes) is only there to propel the wing forward through the air so that it can produce the lift needed to fly. Once you're off the ground, though, gravity may be substituted to keep the airplane moving forward if the airplane loses engine power. The airplane will come down, yes, but it will do so in a shallow controlled glide in just the same way as it would if the engine was still running but at idle power. Most jets descend with their engines at idle power. The jet is gliding under the influence of gravity. With the help of gravity the momentum of the airplane carries it forward which keeps the wings producing lift. In this way, gravity is acting as a friend. The only possible way for an airplane to drop vertically like a rock is if the wings fall off. In fact, many airplanes are incapable of descending downward at ninety degrees toward the earth with the wings still attached. An airplane is capable of maintaining that angle only for a short time before its airspeed increases to such an extent that the excess lift being produced by the wings either forces the nose back up and/or rips the wings off. Airplanes are designed to fly so well that you can't make one (excluding military jets and aerobatic airplanes) come down steeply without destroying it before impact, engines running or not. When an airplane's engines fail it simply reverts to being a glider.
"Sure, but doesn't one of those big, heavy jets still come down fast because they're so heavy?" No. In fact, the heavier, the better. The heavier the airplane, the greater its momentum and the better the glide performance. Competition glider pilots know this. These pilots load water ballast onto their gliders before takeoff because it allows them to glide farther due to the increased momentum the weight provides. Now, here's something that will blow your mind. The average airline jet can out-glide a hang-glider. In fact, these jets enjoy similar glide performance as some gliders and even birds. This is because jets are so heavy (added momentum) and so slippery (pilot talk meaning aerodynamically "clean" or efficient). A typical wide body airline jet can glide somewhere in the neighborhood of one hundred miles across the surface from a typical cruising altitude. That means if a jet's engines all fail at once (which is incredibly remote) during cruise the airplane can glide one hundred miles before contacting the earth, sometimes farther depending on wind. That leaves lots of room for options. The airplane might be within reach of a suitable runway for a power-off (known as a dead-stick) landing. One hundred miles of glide range is a mere fantasy to a light airplane pilot. Most light airplane pilots use a rule-of-thumb for calculating glide range during an emergency: one mile across the ground for every thousand feet of altitude above the surface. Light airplanes often cruise at altitudes only four or five thousand feet above the surface. That means we typically only count on four or five miles of glide range following an engine failure. Most light airplanes will glide a bit farther than this, but good pilots use this conservative calculation to prevent turning in an unsafe direction to attempt to make a landing site which may end up being out of reach. Light airplanes tend to be inefficient gliders because they're lightweight (less momentum) and dirty (pilot talk meaning aerodynamically inefficient).
Passenger jets are far superior gliders. However, passenger jets in Hollywood movies are notoriously bad gliders. The physics of the cinematic world differ greatly from those of the real world. They're far more unforgiving, havoc-creating, and revenue-generating. Luckily, the real world of flying isn't as exciting. So before you get nervous about an airplane's engines failing, just think about a Boeing 747 gliding from Philadelphia all the way to Washington D.C. with all four engines shut down. I've never seen a rock do that.
