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.
