Page 189 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 176

176 CHAPTER SEVEN

Gliders have glide ratios of 25 to 60:1. A glide ratio of 25:1 (read “25
to 1”) means that for every 1000 ft an airplane descends it travels
25,000 ft horizontally. That is about 5 miles! A typical airliner has a
glide ratio on the order of 16:1, while small propeller-driven airplanes
have a glide ratio from 10 to 15:1. The glide ratio of the Space Shuttle
is only 4:1. It has been said that the Space Shuttle glides like a bathtub.
A pilot is trained to know the speed to be flown when the airplane
loses power. Part of transitioning to a new airplane is memorizing the
new critical speeds associated with that airplane. But we have seen
that power and drag are functions of altitude. Does the pilot need to
know critical speeds for every altitude? As you will soon see in the
section on Indicated Airspeed, nature has made life a little easier for
the very busy pilot.

OUT OF FUEL
On July 23, 1984, in Ontario, Canada, a Boeing 767 ran out of
fuel. An error was made converting from the English system of
measure to the metric system.The airplane did not have
enough fuel to complete the trip from Montreal to Edmonton.
The Air Canada pilot, Robert Pearson, was a glider pilot and
able to bring the 767 down on an abandoned airfield many
miles off the flight path.
Let us assume that the Boeing 767 has a glide ratio of 16:1
and was cruising at 32,000 ft (6 miles) when it ran out of fuel.
It would be able to glide almost 100 miles before landing and
would have almost 30,000 square miles to land.

Indicated Airspeed

We now come to the concept of indicated airspeed. In critical
maneuvers such as best climb, longest glide, greatest endurance,
etc., the pilot must fly at specific airspeeds. The pilot has committed
these airspeeds to memory before flying the airplane. But these
speeds change with altitude and air density. So how does a pilot
make airspeed corrections for different altitudes and densities? As