Copyright (C) 1995 by Andrew C Boyd. All rights reserved.


Which Controls What?
--------------------

When you get a group of pilots together, sooner or later an argument
is sure to arise as to which of the following is the best technique
to use during an approach to landing:

use Power to control Altitude, and Pitch to control Airspeed
or
use Pitch to control Altitude, and Power to control Airspeed

Unfortunately, both are oversimplifications, so sometimes one may
work, and sometimes it won't depending on the circumstances.

For that reason, I really don't think either of them are very useful.

The DOT likes to say:

Attitude + Power = Performance

which is probably more correct, but unfortunately, it doesn't tell
you what to do when things go wrong on final approach!

Most pilots just instinctively do what is required (power or pitch,
or both) in the particular circumstance. They usually don't have
time to reason it out.

For a fledgling pilot, it can be confusing. I find it can help to
think a bit about the basic physics involved.

The aircraft has energy due it's height, and due to it's forward motion.
Forgetting about the engine for a moment, we can easily convert from
one form of energy to another.

For example, if you are high and flying slowly, if you shove the nose
down you will be low and going fast. Conversely, if you are low and
fast and you pull the nose up, you will go high and slow.

Glider and aerobatic pilots are _very_ familiar with the above.

Admittedly, I'm not worrying much about the effects of drag, but
approaches are usually flown close to the endurance speed, the
"bottom" of the power curve (remember last month's article? :-)
where the drag is at a minimum.

Check out the following energy chart, which has airspeed along the top,
and altitude (relative to the desired glidepath) on the side:
 

The Right Amount Of Energy
--------------------------

If the airspeed and altitude are perfect (center), we don't need to do
much, except that we should be sure we've trimmed to get rid of pressure
on the control column.

If we are above the glidepath and our airspeed is too slow (bottom left)
all we need to do is to shove the nose down to intercept the glidepath
at the correct speed. Just convert some altitude to airspeed.

If we are below the glidepath, and too fast (top right), all we need
to do is to pull the nose up & intercept the glidepath at the correct
speed. Convert some airspeed to altitude.

All three of the above cases are on the diagonal line, where the aircraft
has the right amount of energy, but the energy may be in the wrong form.
Pretty easy to fix, though.


Too Much Energy
---------------

Let's consider the three cases to the right and below the diagonal
line, where the aircraft has too much energy.

If the aircraft is at the right altitude, but the airspeed is too high
(right center), we cannot just pull the nose up, as that will convert
our excess airspeed into altitude, and we do not wish to be above the
glidepath.

We must _reduce_ our energy, by one of the following techniques:

(1) increase flap (eg: a maximum of 20 degrees until on final)
(2) reduce power (eg: use 250 RPM +/- increments)
(3) sideslip (forward slips are fun!)

If the aircraft is at the correct speed, but above the glidepath
(bottom center), if we all we do is shove the nose down, we will
have excess airspeed when we intercept the glidepath, which is the
case above. Again, we must reduce the energy of the aircraft, using
the techniques above.

If the aircraft is too fast and too high (right bottom), you've got to
get rid of even more energy using the same techniques. Keep in mind,
though, if the approach is that screwed up, maybe it's best to just go
around and try again.

However, in the event that you _cannot_ go around (eg a forced approach)
it sure is handy to know to do all of (1) (2) and (3) above simultaneously.

Cessna pilots may mumble about slips with flaps at this point, a topic
I fervently try to avoid. All I will say on that topic is that the placard
on the dashboard of your C-172 says "AVOID SLIPS WITH FLAPS EXTENDED".
It does not say they are _prohibited_. Read your aircraft flight manual!

Back to our topic ...


Not Enough Energy
-----------------

Let's consider the three cases to the left and above the diagonal line
where the aircraft has too little energy.

If the airspeed is correct, but the aircraft is below the glidepath,
(top center) we do not have enough energy, so we must add power. This
will likely cause the aircraft to climb at the same airspeed, which is
exactly what we want to do to get back up to the glidepath.

If the aircraft is on the glidepath, but the airspeed is too low,
(left center), we cannot just shove the nose down if we are to make
it to the runway. We need more energy, which means we increase power.
Typically, the aircraft will wish to climb with the addition of power,
which we don't want, so we will simultaneously push the nose down to
stay on the glidepath.

If we are low and slow (left top), we are seriously low on energy, and
a smooth application of full power is recommended. We are on the "back
side" of the power curve, and we have no altitude to give up to recover
airspeed.


Summary
-------

We can think of altitude and airspeed as just two different forms
of energy. We can easily convert one to the other by using pitch.

If we add up the two different forms of energy, we can think of the
total energy of the aircraft, and we had better have approximately
the right amount of total energy at the right place during an approach.

If at any point on the approach we have too much total energy, we
must reduce the engine's output, or increase our drag somehow, or
some combination of the two.

If at any point on the approach we have too little total energy,
we must increase the engine's output (or maybe even decrease our
drag somehow, though most pilots frown on raising the flaps or
_gear_ on final).

Flying a slow, light aircraft with lots of drag (eg fixed gear
trainer) we really don't have to worry very much about too much
energy. We just have to make sure that we don't have too little
energy, especially very close to the ground.

However, if you fly a faster, heavier, slippery aircraft you will
find that it needs to be flown "by the numbers" for a successful
approach and now you know why!