Associated Topics || Dr. Math Home || Search Dr. Math

### Effect of Gravity on a Stalled Jet

```
Date: 09/05/97 at 13:11:34
From: Travis George
Subject: Impact of gravity on jet's fwd momentum

If a 747 jet is flying at 30,000 feet and the engines stop working, as
gravity pulls the plane closer to the ground, will the wings provided
forward momentum, in essence allowing the plane to glide to a
reasonably safe landing? Or will the plane fall straight down?
```

```
Date: 09/15/97 at 11:31:10
From: Doctor Pipe
Subject: Re: Impact of gravity on jet's fwd momentum

Travis,

First off, the plane will not fall straight down! So if you e-mailed
this from 30,000 feet, just relax.

It's been many years since my pilot training, but here is a brief
discussion of how a plane flies:

A plane's engine, be it propeller or jet, provides the force, called
thrust, that accelerates the plane forward; this enables the plane to
roll along the runway. As the plane moves down the runway, air moves
over the wings and the movement of the air over the wings creates the
force, called lift, that lifts the plane off the ground and into the
air.

The speed at which an airplane's wings must move through the air to
provide sufficient lift for flight is specific for each aircraft and
is determined by both aircraft and weather conditions at the time of
takeoff. Some of the aircraft conditions are the shape and size of the
wing and the weight of the aircraft at takeoff; some of the weather
conditions are the temperature, humidity, and barometric pressure.
Cold dry air allows for takeoff at a slower speed than in hot humid
air. As long as the plane can maintain a speed faster than this
minimum speed for flight, called the stall speed, the airplane will
fly. When we talk about an airplane "stalling" we mean that the flow
of air over the wings (that produces lift) has been disrupted; the
wings are not producing lift and the plane is no longer flying, but
has stalled.

An aircraft that has stalled IS FALLING!  When learning to fly, pilots
ascend to a safe altitude and then deliberately stall the plane in
order to familiarize themselves with the characteristics of the
aircraft immediately before a stall and to practice the procedure for
recovering from a stall. I have done this in a single engine propeller
plane called the Grumman Tiger. I'm not sure if 747 pilots do the same
thing with a 747 - that would be quite an experience!

Getting back to your question, if a 747 loses power at 30,000 feet
then it has lost its thrust but not its lift. The loss of thrust means
that the plane has lost the force that moves the plane through the air
enabling the wings to produce lift. The momentum imparted to the plane
when the engines were on is what causes the plane to continue
traveling forward (relative to the ground). The pilot will allow the
plane to descend and give up altitude in order to maintain airspeed.
Here, gravity (rather than thrust) is the force accelerating the
aircraft.

In the scenario of your question, the pilot's job, along with the
copilot, would be to locate an airfield close enough and with a long
enough runway to land his 747. I'm not sure how far a 747 can fly
without power from 30,000 feet, but if there is an airport within that
distance the pilot can land the plane. It would be a landing similar
to those made by Space Shuttle pilots (the Shuttle falls out of earth
orbit and flies without power to a landing in Florida or California) -
it can be done, but you only get one chance to do it right. (Pilots
have many witty sayings like that. Another one is: Any landing you can
walk away from is a good landing.)

If you are interested in learning more about aircraft and how they
fly, visit your local library.  It is a subject that has fascinated me
since I was six years old.

-Doctor Pipe,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```
Associated Topics:
High School Physics/Chemistry

Search the Dr. Math Library:

 Find items containing (put spaces between keywords):   Click only once for faster results: [ Choose "whole words" when searching for a word like age.] all keywords, in any order at least one, that exact phrase parts of words whole words

Submit your own question to Dr. Math
Math Forum Home || Math Library || Quick Reference || Math Forum Search