Student - Private Pilot Ground School
—by Rod Machado
First, a Little Theory
In our class on slow flight, I showed you how, in order to maintain
sufficient lift for flight, the wing's angle of attack increased as the
airspeed decreased. Perhaps you've wondered if there is a limit to how much
the angle of attack could increase. After all, common sense suggests there are
limits to all things. The ancient Egyptians had common-sense limits,
especially regarding the size of pyramids they could build (I think this is
known as Tutankh-common sense). Wings have limits, too.
A pilot's job is to work the four forces, maintain lift, and avoid the
burbling air condition that results in a stall. As I mentioned in a previous
lesson, this kind of stalling has nothing to do with the engine stopping.
Air begins to burble over the top of the wing when the wing reaches a large
angle of attack (about 18 degrees for most airplanes). This burbling disrupts
the flow of air over the wing, interfering with lift and causing a stall. The
angle at which the air begins burbling followed by the wings stalling is known
as the critical angle of attack.
Okay, here comes an idea that's like the biggest fish you ever
caught—it's a real keeper. Since wings always stall when they exceed the
critical angle of attack, you can recover from the stall by decreasing the
angle of attack to less than the critical value. Everybody got that? Repeat it
to yourselves 10 times, fast.
Stall, Angle of Attack, and How the Nose Knows
To get a handle on how a stall happens, think of air molecules as little
race cars moving over the wing (Figure 1-1).
Figure 1-1 Angle of Attack.
Each car (and air molecule) has one objective: To follow the curve over the
wing's upper cambered surface. Of course, if the wing is at a low angle of
attack, the curve is not sharp, and it's a pretty easy trip (Figure 1-1).
But look at the curve made by these cars and air molecules when the wing is
attacking the wind at a large angle. As the angle of attack exceeds
approximately 18 degrees (known as the critical angle of attack for reasons you
will soon see), these speed-racer air molecules can't negotiate the turn (Figure
When this happens, they spin off, or burble, into the free air, no longer
providing a uniform, high-velocity, laminar airflow over the wing (Figure 1-2).
The wing stalls.
Figure 1-2 A Wing that is Stalled..
Remember, according to Jacob Bernoulli, lower-velocity airflow over the wing
produces less lift. There is still impact lift provided by air molecules
striking the underside of the wing, but we've already learned this doesn't
provide nearly enough lift to sustain the airplane. When there's less lift than
weight, bad things happen to good airplanes. The wing goes on strike and stalls.
Abandoned by Bernoulli, gravity summons the airplane to earth on its own terms.
All wings have a critical angle of attack (the angle varies slightly among
airplanes). Beyond this angle, the wing and the wind don't work and play well
together. All the whispered theory in your heart won't overcome the laws of
physics and aerodynamics. The wing police are always watching. Exceed the
critical angle of attack, and the air molecules won't give you a lift. Sounds
serious—and it can be. Fortunately, there's a readily available solution, and
it is not screaming, "Here, you take it!" to the instructor. At this
point, I'd like you to put your finger in your ear. Why? Because I'm about to
say something really important and I don't want it to go in one ear and out the
other. Here comes the important stuff again: You can unstall a wing by reducing
the angle of attack. You do this by gently lowering the nose of the airplane
using the elevator control (Figures 1-3A and 1-3B).
Figure 1-3 Stalling and exceeding the critical angle of
Easy does it here, Tiger. Once the angle of attack is less than its critical
angle, the air molecules flow smoothly again over the top of the wing and
production of lift resumes. It's as simple as that. Now the airplane can resume
flying and doing what airplanes are supposed to do (Figures 1-3C and 1-3D).
Please don't ever forget this. Okay, you can take your finger out of your ear
Why am I making such a big deal out of this? Because in a moment of stress
(having the wing stop flying creates stress for many pilots), you will be
inclined to do exactly the opposite of what will help. Pilots have a natural
inclination to pull or push on the elevator control to change the airplane's
pitch attitude. During a stall, as the airplane pitches downward, your untrained
instinct is to pull back on the elevator control. You could yank that critter
back into your lap, and the result will not be good. The wing will remain
stalled, and you, my friend, will have the look of a just-gelded bull.
If the wing stalls, you need to do one very important thing: Reduce the angle
of attack to less than its critical value. Only then does the wing begin flying
again. Adding full power also helps in the recovery process by accelerating the
airplane. The increase in forward speed provided by power also helps reduce the
angle of attack.
Don't just sit there with stalled wings. There's a reason why you are called
the pilot in command. Do something. But do the right thing.
Stall at Any Attitude or Airspeed
You should realize that airplanes can be stalled at any attitude or at any
airspeed. Put your finger back in your ear. It makes no difference whether the
nose is pointed up or down or whether you are traveling at 60 knots or 160
knots. Whether an airplane exceeds its critical angle of attack is independent
of attitude or airspeed. Figure 1-4A shows one instance of how this might
Figure 1-4 Stall recovery when exceeding the critical
angle of attack.
Airplanes have inertia, meaning they want to keep on moving in the direction
they are traveling. Airplane A is pointed nose down, diving at 150 knots (don't
try this at home!). The pilot pulled back too aggressively, forcing the wings to
exceed their critical angle of attack, and the airplane stalled. Wow! Imagine
that. It stalls nose-down at 150 knots! Figure 1-4B shows an instance of an
airplane stalling at 100 knots in level flight after the pilot pulled too
abruptly on the elevator control.
What must the pilot do to recover? The first step is to decrease the angle of
attack by moving the elevator control forward or by releasing back pressure on
the joystick (remember, pulling back on the elevator control was probably
responsible for the large angle of attack that induced the stall in the first
place.) This re-establishes the smooth, high-velocity flow of air over the
wings. The airplane is once again flying.
The second step (if necessary) requires applying all available power to
accelerate the airplane and help reduce the angle of attack.
Once the airplane is no longer stalled, it should be put back in the desired
attitude while making sure you don't stall again. Stalling after you've just
recovered from a previous stall is known as a secondary stall. Unlike secondary
school, it is not considered a step up, especially by the participating flight
instructor. (You'll know your instructor is unhappy when you hear her make
subtle statements like, "Hmm, come to think of it, childbirth wasn't all
Stalling an airplane intentionally, at a safe altitude, is actually fun, or
at least educational. Stalls are relatively gentle maneuvers in most airplanes.
Stalling an airplane close to the ground, however, is serious business because
it is usually not an intentional act. During flight training, you'll have ample
practice in stall recovery.
Managing a stalled airplane is one thing; managing your natural instincts,
however, is another. For example, a typical stall trap you could (literally)
fall into involves a high sink rate (that is, a high rate of descent) during
landing. While on approach, you might apply back pressure on the elevator
attempting to shallow the descent. If you exceed the critical angle of attack,
the airplane will stall. The runway now expands in your windshield like a
low-orbit view of a supernova.
If you follow your untrained instincts and continue to pull backwards on the
elevator, the stall deepens. Trained pilots know better. They are aware of the
possibility of stalling and apply the appropriate combination of elevator back
pressure and power during landing to change the airplane's glide path without
exceeding the critical angle of attack. (Your instructor will show you the
appropriate use of elevator and power during landing). How do pilots know the
proper amount of rearward movement to apply to the elevator? How do they know
they won't stall the airplane?
If there was an angle-of-attack indicator in your airplane, stall recognition
would be easy. You'd simply keep the angle of attack less than what's critical
for that wing. Angle-of-attack indicators, although valuable, are rare in small
airplanes. In Flight Simulator, the main clue you have to the onset of a stall
is the stall horn, which will activate when you're a few knots above stall
speed. You'll also have the luxury of seeing the word STALL appear on your
screen. You won't have this in an actual airplane, of course. You may, however,
have a red stall warning light activate, which is almost the same thing.
Now that you have a good foundation in stall aerodynamics, let's examine the
details of stall recovery.
Stop Flying; Start Stalling
Pulling way back on the joystick causes the wings to exceed their critical
angle of attack and stall. During the stall, airflow burbles instead of flowing
smoothly over the top of the wing. This results in insufficient lift for flight,
causing the airplane to pitch forward (provided that the baggage, passengers,
and fuel are loaded properly in the airplane). This automatic nose-down pitch is
somewhat like doing the Heimlich maneuver on yourself; the airplane reduces its
own angle of attack to less than the critical value and regains its ability to
If airplanes are built to recover from stalls themselves, why do you need to
learn any of this? The problem is that pilots often do things that prevent stall
recovery. You need to know what these things are. Also, an accidental stall
close to the ground requires that you know how to quickly recover in order to
minimize your altitude loss. Let's try another stall, but this time, let's see
what happens if you prevent the airplane from pitching forward on its own.
Doing the Wrong Thing in a Stall
What happens if we stall and prevent the airplane from recovering from the
The answer is that the airplane will remain stalled with the joystick held
full aft (that's all the way back). It will not climb no matter how hard you
pull on that joystick. Think about this carefully: You could remain stalled all
the way to the ground while the joystick is pulled full aft, which doesn't bring
you much joy, right? Holding the joystick full aft keeps the wing's angle of
attack at or beyond its critical value. Unfortunately, this is what some pilots
do after stalling an airplane.
Doing the Right Thing in a Stall
That's why we learned that you must release any back pressure on that
joystick and move it forward until the wings are at less than their critical
angle of attack. The proper attitude for recovery is subject to many variables,
so in the Interactive Lessons, we'll use a 5- to 10-degree nose-down pitch for
simulator stall recoveries. You don't want an excessively steep nose-down
attitude since it results in excessive altitude loss and airspeed increase.
How do you know if you've decreased the angle of attack sufficiently? In a
simulator, you should experience these things: the stall horn stops blaring, the
word STALL disappears from the screen, the airplane begins to fly again, the
airspeed begins to increase, and the flight controls become more responsive. If
your instructor were on board, his or her voice would also reduce in pitch, and
whales would no longer be inclined to beach themselves.
With a few exceptions, this is the way pilots have always recognized stalls
and recovered from them. You'll also want to add full power immediately after
reducing the angle of attack. This helps accelerate the stall recovery process.
Be careful not to let the nose pitch up as you add power. This might, once
again, increase the angle of attack sufficiently to induce another stall. When
the airplane is no longer stalled (that is, the stall horn stops blaring), raise
the nose to climb attitude, and establish climb airspeed.
What happens if you stall with full power already applied? Let's say that
you've just lifted off from an airport and are climbing with full power (as you
normally do in this airplane). Suddenly, you find a big bumblebee in the
cockpit. You're distracted and forget to fly the airplane as you swat the
critter with both hands. Of course, all your flailing in the air makes the
cockpit look like the set of a kung fu movie as the airplane stalls. What do you
Well, Grasshopper, all the kung fu in the world won't help you now unless you
do one thing: Reduce the wing's angle of attack to less than its critical value.
Once the airplane is no longer stalled, you can recover back to climbing
attitude. Don't worry about touching the throttle, since full power is already
There you have it: your first introduction to the aerial theme park known as
Stall World. The only problem, however, is that you didn't visit one corner of
the park called Reality Land. Here's what you missed.
It's easy to remember that airplanes stall because they exceed their critical
angle of attack. But don't forget that this can happen in any attitude, at any
airspeed, and at any power setting. Time now for more truth.
In reality, if the airplane was pointed straight down and you pulled back
hard enough on the controls, the airplane would stall. Of course, we wouldn't do
this in the actual airplane (even if it was a rental). Remember, this is a
simulator. We can do things you'd never dream of doing in a real airplane. It's
like visiting Fantasy Land, in that we're not exposed to great risk in the
demonstration. So we can take advantage of our technology and see what others
only talk about and never actually do.
Now it's time for you practice stalls. Click the Fly This Lesson Now
link to practice what you just learned. Have fun!
THIS LESSON IS AVAILABLE IN THE ACTIVE
FLIGHT SIMULATOR PROGRAM