What Happens When The Engine Quits

I’m still sticking to the idea of writing shorter articles with more plain language rather than my usual 4 to 5 page descriptions of obscure aerodynamic theories. I know, I skipped the month of July…it happens when you’re as forgetful as I.

Think we can make the runway from here?

Comedian Mitch Hedburg had a joke that escalators never break, they can only become stairs. The same holds true for airplanes that lose an engine, they simply become gliders.

A lot of movies show what happens to an airplane when the engine fails. With very few exceptions, they’re all wrong. Hollywood tends to overdramatize some parts of aviation and underdramatize others. Airplanes do not plummet from the sky, the controls don’t lock up and pilots don’t ask ATC to tell their wife that he loves her (insert multiple alimony payment joke here).

Aircraft are designed to fly, not to fall. Air moving over the wings provides lift which keeps the airplane in the air. Since air isn’t going to move itself, something has to push the airplane fast enough for lift to be effective. That’s the job of the engine. By creating thrust, the airplane is able to move forward, generate lift and do that thing we like to call flying.

But say for example that the worst luck has occurred and the engine decides to take an early retirement. Now what happens to our airplane? The answer is very simple…it glides. Needless to say, the glide characteristics of airplanes are as varied as their shapes, but all airplanes from the smallest private plane to the largest commercial airliners will glide. Whether or not they glide to an airport depends on a few things.

In physics, there are two energy states that are important to a gliding airplane. You have kinetic energy and potential energy. Kinetic simply means energy stored due to speed. This is the force that causes injury in car accidents…the faster you go, the more it hurts when you stop suddenly. There is also potential energy, which is the energy that can be created by allowing an object to fall. For this state, the higher you are, the more it hurts when you stop suddenly (picture a bellyflop from a 3 foot diving board vs a 30 foot diving board).

If an airplane has at least one of these states with a high value, it will be able to glide somewhere without an engine. If it has both of these states fully charged up, it can really glide somewhere. If by chance it is low on both states, the gliding range will be very poor and in some cases, nil. The Air France Concorde accident is an example of what happens when airspeed is still relatively low and there is no altitude to trade for velocity. The Air Canada “Gimli Glider” 767 incident shows what happens when you have airspeed and altitude in your pocket, plus pilots who know how to manage energy.

For decades, the Space Shuttle was the world’s fastest and heaviest glider. Returning from space at 25 times the speed of sound, it would make a powerless landing at just over 200mph. It goes without saying that Shuttle pilots were well trained in managing energy, and had tons of potential and kinetic energy to work with. For practice, they would go up in modified Gulfstream II business jets, reverse the engines and do approach after approach at the same angles and rates that they’d experience in the final stages of a Shuttle landing.

When pilots don’t know how to manage energy, the results are sadly predictable. Pinnacle Airlines 3701 experienced an double engine failure at high altitude. From 41,000 feet, the CRJ200 aircraft could have easily glided 50 miles or more in any direction and landed at one of several adequate fields. But the pilots focused so much on restarting the engines that they ran out of altitude (potential), airspeed (kinetic) and ideas at the same time. The result was the loss of both pilots and the airplane.

As for how airplanes fly without power, just pay attention on landing. Every airplane touches down at or near idle power. Many commercial jets go to idle around 50 feet, while smaller general aviation aircraft might be at idle power for the entire approach (a notable exception is any Navy aircraft, as they go to takeoff power the second they hit the deck just in case the hook misses the wires). You’ll notice that the airplane doesn’t shake, the controls don’t vibrate, and you don’t just drop straight down to the ground. Airshow pilot Bob Hoover used to shut off both engines in his Shrike Commander twin and THEN go into a big looping barrel roll just to show how managing energy works when you know what you’re doing.

So now you know what really goes on the next time you see a movie with an airplane emergency. Not to say that engine failures don’t cause the pilot’s heartrate, breathing rate and sweatrate to increase, but it is not always the wrestle-the-controls-call-control-tower-I-love-my-wife-and-kids-and-goldfish situation that it’s portrayed to be.