The information found in this article is found in Tom Knauff’s popular flight
training manuals. Teaching stalls is covered in detail in Tom’s Glider CFI
Manual. Tom can be reached at his web site: www.eglider.org
Preventing Stall Accidents
WHAT ARE WE SUPPOSED TO TEACH?
FAA Flight Training Handbook (AC 61-21A)
Page 144 “Fundamentals of Stall Recovery”
“During the practice of intentional stalls, the real objective is not to learn
how to stall an airplane, but to learn how to recognize an incipient stall and
take prompt corrective action.”
I underlined the word, “not.”
Stall recognition training is extremely important, thus our emphasis on the six
signs of a stall during flight training.
1. Excessive back stick pressure
2. Nose high attitude
3. Low airspeed
4. Quietness
5. Mushy controls
6. Buffet
The effectiveness of the elevator is intentionally limited in normally
type-certificated aircraft. The elevator must have enough effectiveness to
perform a normal, tail low, tail wheel/main wheel simultaneous landing. There is
no other maneuver that requires a more effective elevator in a normally type
certificated glider.
Thus, in a wings level attitude, gently bringing the stick back to the full stop
will finally produce a gentle, nose above the horizon, stall.
But suppose we fly with the nose below the horizon at a normal, constant
airspeed? How does a pilot stall the glider?
Some might respond, “ with a violent back stick motion.”
But that violates the premise of keeping the nose below the horizon in a normal
flying attitude.
While it is possible for the wing to stall in violent turbulence, gliders rarely
fly in these severe conditions, and pilots are trained to fly reasonably faster
when turbulence is encountered.
So, to prevent wings level forward stalls, (such as landing stalls) we only need
to develop the discipline of keeping the nose below the horizon in a normal
gliding attitude. Especially when low to the ground as in landings.
The first sign, excessive back stick pressure, is important in stall prevention.
Pilots should be taught to set the trim in a neutral position when entering the
landing pattern. Thus, they may notice they are using “excessive back stick
pressure” when performing a landing.
The problem with the first sign of a stall, excessive back stick pressure, is in
the stress of the moment; the pilot is not likely to notice.
In a typical scenario, a pilot enters the landing pattern too low. The law of
primacy causes the pilot to revert to what we call “stinkin’ thinkin’, and pulls
back on the stick in a futile effort to hold the glider up, or to prevent a
further loss of altitude.
Flight instructors will notice everyone will show the tendency to use excessive
back stick pressure under some stressful situations.
Thus, the second sign of a stall becomes important. That is, “Nose high
attitude.”
Simply developing the habit of keeping the nose of the glider below the horizon
in a normal flying attitude will prevent accidental stalls.
The possible exception is a violent application of the controls, however, pilots
under stress of a low altitude landing pattern do not apply violent application
of controls.
Nose high attitude has a problem in undulating/mountainous terrain due to the
false impressions of the terrain. Pilots are made aware of the problem and
generally understand the horizon means the flat-earth horizon. Regardless, a
sloping horizon can cause a pilot to fly too slowly – higher angle of attack,
when distracted.
The third sign – Low Airspeed, is the indication of the airspeed indicator. When
flying low to the ground such as a landing pattern or rope break emergency,
pilots are trained to frequently monitor the airspeed indicator.
The combination of watching the horizon reference and airspeed indicator, serves
the pilot well in preventing stall accidents, but many pilots have not developed
this discipline when close to the ground.
Airspeed indicators fail (bug in the pitot) and it is interesting to watch a
pilot with an erroneous airspeed indicator reading while disregarding all the
other signs to control airspeed.
The fourth sign is perhaps the most important of all.
Quietness. Pilots will use this important signal and immediately respond with
forward stick pressure. When a pilot flies a new glider – especially the
ultra-quiet new gliders, it is important for them to open the air vent when
entering the landing pattern for this important audio clue.
The fifth sign of a stall is mushy controls. At very slow speeds, move the
controls aggressively and notice how ineffective they are compared with higher
airspeeds. As a pilot approaches stall speed, it is unlikely they will move the
controls aggressively, so this sign is not usually noticed other than in the
training environment.
Finally, the last sign; The buffet. Ironically, when asked to name the signs of
a stall, this is the one most often mentioned first.
The FAA expects instructors to place special emphasis on how to recognize stalls
in an effort to prevent stalls from happening.
Every pilot can finish this quote:
“An aircraft can be stalled at any _______ and any ________.”
This has been drummed into every pilot and every pilot has memorized this
mantra.
Most flight training manuals including the FAA, repeat the quote.
“An aircraft can be stalled at any airspeed and any attitude.”
It is not true for aircraft certificated in the normal category.
Try it. Fly at a steady airspeed – say 60 knots – with the nose below the
horizon.
According to the legend, you should be able to stall the aircraft, “at this
airspeed and this attitude.”
The instant you pull back on the angle of attack control – the stick - the nose
comes up; violating the premise. The attitude has changed. So has the airspeed.
In fact, if pilots were simply trained to keep the nose of the glider below the
horizon in a normal flying attitude, we would have done much to prevent stalling
accidents.
Stall training is supposed to be about stall recognition and stall avoidance.
Many flight instructors spend the majority of stall training teaching how to
perform stalls rather than stall prevention.
Here is another interesting exercise:
Roll into a modest bank angle of 30 degrees, and then pull back on the stick to
bring the nose of the glider above the horizon. Watch carefully what happens.
If one wing stalls, the glider will roll as it stalls. If both wings stall, the
glider will pitch nose down.
(You really must try this.)
With few exceptions, what you will observe is the glider neither rolls nor
pitches, but yaws.
What happens is as the glider loses airspeed, the lift produced is not adequate
to hold the glider up and it begins to fall. Since the glider is tilted, the
falling glider causes the airflow to strike the side of the fuselage, causing
the tail of the glider to be pushed up, causing the yawing motion.
There is no rolling. No pitching.
The wing is not stalled.
Because the wing is not stalled, the ailerons and rudder work properly.
The steeper the angle of bank, the more stall-proof the normally certificated
glider becomes.
In a very steep angle of bank, (60 degrees) at a stable airspeed, the control
stick may be fully back. At this attitude, how would you stall the glider? Since
the angle of attack control is used up (fully back) it is not possible to
increase the angle of attack and thus, the glider becomes stall-proof.
Thus, from the standpoint of stalling, steeper turns are safer than shallow
turns.
Just as important: If you are performing a steeper angle of bank turn, and the
glider seems to be falling/stalling, your instinctive reaction of applying
opposite aileron will be OK. Since the wing is not stalled, opposite aileron
works fine to level the wings.
The same instinctive reaction in a shallow angle of bank turn can create a
stall/spin.
This is so contrary to what pilots are taught, that I expect lots of mail from
you all.
Before you send me mail, please go try it yourself. Let me know what you
experience.
Stall prevention includes:
* Know the signs of a stall in the order they occur.
* Keep the nose of the glider below the horizon.
* Fly an appropriate, stable airspeed.
* Keep the yaw string straight.
* Use steeper bank angles. (30 – 45 degrees)
All of the above is especially important during landings and all low altitude
turns (rope breaks above 200 feet, low altitude thermalling.)
During training, we teach the student pilots to, “Watch the airspeed indicator.”
What we mean is verify you are flying the correct airspeed by looking at the
airspeed indicator every few seconds when landing or during rope break
practice..
We demand the yaw string stay straight during low altitude turns.
The principle reference for airspeed control is not the airspeed indicator, but
the nose below the horizon.
As an aside, if you instruct a student pilot to use a 45 degree angle of bank,
they rarely will bank steeper than 30 degrees. It seems steeper than it really
is.