If the engine fails, will the plane fall straight down?

Worried frequent flyers may fear that the plane will crash when the engines fail, or worse, when all the machinery fails.

Patrick Smith, a former pilot, revealed in his book Cockpit Confidential that most passengers have actually flown on planes with their engines turned off at some point. "This may surprise you, but it's quite common on descending aircraft - pilots call it idle flight when the engines are not producing thrust," Smith wrote.

At this point, the plane is still running and providing power to vital systems, but the engines on the wings are not producing thrust. The plane can be in this state multiple times per flight without passengers knowing.

The nature of "idle flight" is different from a complete engine failure - when the pilot loses control of the machinery - but the plane's condition in these two cases is almost the same. Smith compares planes in the sky to cars coasting with their engines off: a car keeps going, and so does a plane.

In fact, larger planes will perform better with their engines off than lighter aircraft like a Piper or Cessna. Planes need to reach significant speeds to glide, but that depends on the rate of descent and distance.

"From 30,000 feet, you can plan on an idle flight of about 100 miles," Smith says, reassuring that engine failure and stalling are unlikely - though not unheard of. Possible causes include fuel exhaustion, volcanic ash, or bird strikes.

In some cases, crews have been able to land with engine failure, with no fatalities or injuries. In some cases, one or more engines were restarted before the plane hit the ground.