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/