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Aircraft
Stall and How to Prevent It
BY SARINA HOUSTON
An airplane stall is
an aerodynamic condition in which an aircraft exceeds its given critical angle
of attack and is no longer able to produce the required lift for normal flight.
This type of stall
should not be confused with an engine stall, familiar to anyone who has driven
an automobile.
When flying an
airplane, a stall has nothing to do with the engine or another mechanical part.
In piloting, a stall
is only defined as the aerodynamic loss of lift that occurs when an airfoil
(i.e., the wing of the airplane) exceeds its critical angle of attack.
Angle of Attack
The angle of attack
on an airfoil is measured by the angle between the chord line (i.e., the
imaginary line from the leading edge to the trailing edge of the wing) and the
relative wind.
It is dependent on
the shape of the airfoil, including its platform and aspect ratio. At a high
angle of attack, the airflow over the wing is disrupted.
At the critical angle
of attack, the airflow over the wing is disrupted enough to inhibit lift,
resulting in the nose of the aircraft to fall.
The critical angle of
attack for an airfoil never changes. However, factors such as weight,
configuration (e.g., flaps and gear changes, or other conditions, like airframe
icing), and load factor, can change the airspeed at which an airplane will
stall.
Stall Characteristics
Characteristics of a
stall include a distinctive decrease in lift, which is usually noted by a
sudden (if sometimes gradual) pitch down of the nose of the aircraft.
While this can feel
like the plane is falling and has no lift, in reality, it's only a decrease in
lift and a change in the plane's level.
Additionally, a stall
may be accompanied by a roll or yaw to one side if the aircraft is
uncoordinated. If this happens and recovery procedures are not initiated right
away, an aircraft may enter a spin.
Stability
In a stable airplane,
the drop in the nose at the beginning of a stall often is enough to regain the
proper amount of lift for the airfoil.
If this happens, the
airplane is easily recoverable just by lowering its pitch attitude and
increasing airspeed.
However, in an
unstable airplane, a stall that is not corrected can further develop into a
spin, which can be difficult or impossible to recover from.
Airspeed
Stalls commonly occur
at slow airspeeds. For this reason, slow-speed flight, such as during approach
and departure, are critical phases of flight, and pilots must be particularly
cognizant at these times to prevent stalling the aircraft.
A stall at cruise
altitude offers the pilot enough space to recover. A stall during landing with
limited space doesn't offer the same envelope of security from which to
recover.
While stalling may be
most common at slow speeds, a stall can happen at any airspeed, regardless of
the attitude.
Therefore, a pilot
should not rule out the possibility of a stall based on airspeed or attitude.
For example, when pulling
out of a dive, the airspeed is high, but the angle of attack can be higher than
you think because the plane is still dropping in altitude even though its nose
is raised. If the angle of attack exceeds about 17 percent, the plane can
stall.
Tailplane Stalls
Tailplane stalls
often indicate that something is happening to the aircraft wings, but the
plane's horizontal stabilizer can also stall. While this tailplane stall is
also dangerous, it is a much less common aerodynamic condition.
Practicing Stalls and
Recovery
Stall recovery
procedures are different for each aircraft, but in general, a pilot can
initiate a stall recovery by increasing airflow over the wing.
This is usually
accomplished by lowering the pitch attitude, leveling the wings, and increasing
power or thrust.
When a wing has
stalled, it is usually best to use the rudder to raise the wing, rather than
the ailerons.
Pilots practice
stalls and recovery as part of their training, and they must perform a stall
and recovery to earn a private or commercial certificate.
However, routine
flight reviews often do not involve stalls, and as a result, pilots may forget
how to recognize the indications that an airplane is going into a stall.
Practicing stalls and
recovery at slow speeds — and at sufficient altitude to recover, of course — helps
pilots recognize the early signs of a stall condition so that they can make the
proper corrections.
Sarina Houston
Commercial Pilot and
Flight Instructor with Single and Multi-Engine Instrument ratings
Worked for
Embry-Riddle Aeronautical University as an administrative director
Founding President of
a chapter of Women in Aviation, International
Member of NAFI
(National Association of Flight Instructors)
Experience
Sarina Houston is a
former writer for The Balance Careers covering aviation and aerospace. Houston
is an FAA-certified Commercial Pilot and Flight Instructor with Single and
Multi-Engine Instrument ratings. She has been a flight instructor since 2005.
In addition to flying,
Houston has experience in administrative and nonprofit management. She has
worked for Embry-Riddle Aeronautical University as an administrative director, and
was the founding President of a chapter of Women in Aviation, International—a
nonprofit organization that provides support for women and men who choose to
enter the challenging world of aviation.
She maintains
professional memberships with AOPA (Aircraft Owners and Pilots Association),
WAI (Women in Aviation, International), and NAFI (National Association of
Flight Instructors).
Houston currently
works as an independent flight instructor and a freelance aviation writer.
Education
B.S. in Aeronautical
Science, Embry-Riddle Aeronautical University
M.S. in Aeronautical
Science, ERAU-Worldwide, specializing in Aviation Safety and Operations
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