Tom Székely, P.E., LEED AP

Home | Background and Experience | Services | Affiliations | Projects | Newsletter Archives | Newsletter Sources | Contact Us

Last Issue

September 12, 2007

Next Issue

Of Airplanes and Bricks, or, The Gimli Glider vs. The Space Shuttle


OK, so I’ve been on an aviation kick lately, but lately I‘ve kept finding myself in situations where nonpilots would utter some accepted “truth” which is so far from reality, that I just couldn’t let it go.  The latest has been how certain aircraft glide like a brick.

The reality is that no aircraft glides like a brick, not even the Space Shuttle .  Hell, even parachutes have become gliders with maximum L/D (lift/drag) ratios of 3:1 for sport parachutes, and as high as 9:1 for modern paragliders.

This last is about the L/D of the Cessna 150s in which I learned to fly, but to translate into English, the max L/D is also referred to as the glide ratio and tells us how many feet (or furlongs, or miles or whatever) an aircraft will “coast” horizontally for every foot (or furlong or mile or whatever) of altitude it loses.

The key to understand here is that just as an automobile will coast downhill if it’s out of gas, with one still able to steer and brake the car, so does an airplane remain controllable sans engine power, becoming, in effect, a sailplane. Not a terribly efficient sailplane, but a sailplane - more on this later.

As I tell first time passengers in lightplanes, even if the engine were to fall clean off the airplane at an altitude of, say 5000 feet or so above the ground (about a mile), you’d be able to glide for (about) 9 miles or so before having to land.  So how much real estate do you then have in which to look for a suitable landing place? 

Lemme see . . . the area of a circle with a radius of 9 miles is equal to pi times the square of the radius, 3.14 x 9 x 9, or 3.14 x 81 = 254.34 square miles (about). 


Not only are major oceans or mountain ranges the only two places I can think of where this wouldn’t be enough area in which to find a large enough flat spot in which to land, there are so bloody many airports in the US that the chances are good the forced landing  would in fact be at an airport.

Don’t believe me?  In 2003/04 US scheduled air carriers (including regional airlines) served  about 1380 airports out of a total of about 13800.   New York State alone has 150 public use airports, and another several hundred private fields. As if that weren’t enough, the Interstate Highway system has plenty of sections of road upon which one could easily land a 767 or B52 if one had to.

Which brings me to sailplanes, the Space Shuttle, and the Gimli Glider.  

Speaking of which, why do you suppose they’re called sailplanes?

What’s a sail? Could a sail have anything to do with airplanes? 

To answer the last question first, yep.

As to what constitutes a sail, that’s a little more complicated.  That is, while we can see that even thin stuff like air can uproot trees and knock people over when it becomes strong wind, and thus understand how a sailboat can be pushed along by the wind behind it, understanding how one can sail into the wind is what complicates the issue.

If you were to hold a square of toilet tissue along  the top edge of your bottom lip so it hung straight down like a beard, and then blew air straight out forcefully, you’d discover the tissue would rise to become almost parallel to the ground.

This high-velocity air is what Bernoulli's Theorem is all about, with the air pressure in the high-velocity jet being lower than ambient atmospheric pressure, and the pressure differential between the two sucking the tissue upward.  Similarly, a wing sucks an airplane into the air and a sail can suck a boat into the wind.

So, when an airplane relies solely on natural wind rather than using an engine to pull the wing through the air, and thus cause the relative wind you feel when you stick your hand out of a car window at 60 mph on an otherwise still, hot, day, it shouldn’t be surprising that it’s called a sailplane.

Sailplane pilots look for thermals, columns of air, which because they’re warmer than the surrounding air, constitute vertical convection currents which a sailplane can dive into to get forward momentum for some lift, while actually climbing with relation to the ground. A light enough airplane with a big enough wing in a strong enough thermal can do the same thing.

‘Light enough’ and ‘big enough’ are expressed in a term referred to as wing loading, how many pounds of airplane each square foot of wing has to lift.  A 2400 pound airplane in level flight has its wing developing 2400 pounds of lift to keep it there.  If the wing develops excess lift, the airplane climbs, less lift and it descends.

An aircraft engine driving an airplane through the air is resisted by the friction of the air flowing around the airplane as well resistance generated in the production of lift, with this resistance being termed drag.

Thus a 2400 pound airplane in level flight which has a lift drag ratio of about 9 to 1 has its engine developing no less than 267 pounds of forward thrust to overcome drag.

Lift is proportional to a combination of the curvature of the wing and, up to a point, its angle of attack into the relative wind, as well as the airspeed over the wing.


This combination has resulted in the engineer’s aphorism that given enough power, one could fly a barn door.

In the absence of power, pointing an aircraft’s nose just a few degrees below its level flight attitude will result in a “best glide” speed where L/D is at its maximum, and where the craft can “coast” down to a normal landing.

So what’s the Gimli Glider?

On July 23 1983, an improbable sequence of events caused flight 143, an Air Canada Boeing 767, to run out of gas during a flight from Montreal to Edmonton.  While the maximum takeoff weight of a fully loaded 767 can go as high as 340,000 pounds, by the time this particular aircraft became a glider it was probably down around a svelte 270,000 pounds.

The event occurred in stages, starting with dwindling fuel pressure in the left engine’s fuel pump at 41,000 feet, causing the crew to decide to divert to Winnipeg 120 miles away.  Twenty-one minutes later, they were down at 28,500 feet and 65 miles from Winnipeg when both engines flamed out within seconds of each other.

Ten minutes after first experiencing the quiet peaceful joy of the glider pilot, the aircrew determined they wouldn’t make Winnipeg, and diverted again, this time to Gimli, just 12 miles away, where the copilot had been stationed when in the RCAF.

In the interim, Gimli had been turned into a public airport, and one of its two parallel  6,800 foot runways (the one, as it happens, which flight 143 was approaching) had been turned into multiple race courses which, on that day, was packed with cars, campers, and people for “Family Day” of the Winnipeg Sports Car Club.

As it turned out the only injures were to passengers exiting via the rear emergency slide when the plane was left tail high after the nosewheel collapsed during the landing.  For more , there’s a book by Wade H. Nelson, Freefall: From 41,000 Feet to Zero – a True Story, as well as a somewhat less faithful TV movie which starred William Devane.

So what’s the glide ratio of a 767?   About 19:1.  A B-52 is a bit better at 20:1, as is an Albatross soaring over the ocean, or a 1960’s vintage sailplane (actually about 22:1 for the latter). Modern sailplanes are 40:1 with some as high as 60:1.

The Space Shuttle is 1:1, a 45 degree approach angle, until in ground effect over the runway, pulling contrails and flattening out.  Training, by the way, consisted of virtual deadsticking of a modified (“dirtied-up”) Gulfstream II with gear and flaps down.

A brick? 0:1.

Next Issue