AOPA Pilot Magazine - August 2021 - 90

RUDDER & WRENCH
FLYING SMART /
Push-pull
Anatomy of a pilot-induced oscillation
BY CATHERINE CAVAGNARO
A pilot-induced oscillation can happen when the pilot approaches with excessive airspeed. When he pulls back on the yoke in the flare, the airplane
increases in altitude. Seeing only sky, the pilot pushes forward on the yoke and points the aircraft toward the ground. The cycle starts again when the pilot
then pulls back on the yoke. This pilot is out of phase with the aircraft because he is exacerbating the oscillations instead of helping them settle down.
THE TYPICAL pilot-induced oscillation
scenario goes something like this: The
winds at the airport are especially gusty
so, to avoid an inadvertent stall, the pilot
maintains a higher airspeed than usual. If
adding five knots seems like a good idea,
then perhaps 10 knots is even better. At
the flare, pulling back on the yoke results
in a ballooning effect because of the extra
airspeed.
Now with a high pitch attitude and
decreasing airspeed, the pilot pushes
forward on the yoke only to be staring at
the ground nearby. Touching down flat
or nose first, the airplane bounces back
into the air. The pushing and pulling continue
while the oscillations worsen, until
the nosewheel contacts the ground first
and collapses.
While these accidents don't typically
involve fatalities, they can prove devastating
to aircraft and bruise pilot egos. Let's
dissect an aircraft longitudinal oscillation
and discuss a remedy not easily found in
90
AOPA PILOT / August 2021
training manuals. Better yet, we'll see
how to avoid them from the start.
If an aircraft in flight is disturbed
by a gust of wind or by a misuse of the
controls, it will typically experience a
superposition of two pitching motions
called phugoid and short period oscillations.
These two oscillations are
essentially independent, meaning that
they can be analyzed separately.
The phugoid oscillation features a
constant angle of attack and changing
airspeed and altitude. After trimming
the aircraft for cruise configuration in
my Cessna 152, I calculated the period-
the time it takes the aircraft to pitch up
through equilibrium position again after
completing one cycle-to be roughly 24
seconds. For this reason, it's also called
the long period oscillation. FAA Part 23
certification requirements don't require
that this oscillation be stable, but any
instability should not be distracting or
go beyond the abilities of a normal pilot
to assist the airplane setting back to its
equilibrium. With a well-defined horizon,
a pilot can probably do this without
much trouble. In instrument conditions,
though, that would be a tall order so
many general aviation aircraft feature a
stable phugoid although the damping can
be light. Think of an aircraft with a stable
phugoid oscillation as one that features
airspeed stability.
The short period oscillation involves
constant airspeed and changing angle of
attack and it must be stable for aircraft
certified under Part 23. In my Cessna the
period of this oscillation appears to be just
a fraction of a second with damping forces
so great that it's easy to miss the oscillations
altogether. Flying an aircraft with an
unstable short period oscillation would be
akin to standing on a basketball, and accruing
ice in flight can result in an unstable
short period oscillation. Consider short
period stability as angle of attack stability
(see " Angle of Attack Stability, " p. 91).
CHARLES FLOYD

AOPA Pilot Magazine - August 2021

Table of Contents for the Digital Edition of AOPA Pilot Magazine - August 2021

Contents
AOPA Pilot Magazine - August 2021 - Intro
AOPA Pilot Magazine - August 2021 - BB1
AOPA Pilot Magazine - August 2021 - BB2
AOPA Pilot Magazine - August 2021 - Cover1
AOPA Pilot Magazine - August 2021 - Cover2
AOPA Pilot Magazine - August 2021 - Contents
AOPA Pilot Magazine - August 2021 - 2
AOPA Pilot Magazine - August 2021 - 3
AOPA Pilot Magazine - August 2021 - 4
AOPA Pilot Magazine - August 2021 - 5
AOPA Pilot Magazine - August 2021 - 6
AOPA Pilot Magazine - August 2021 - 7
AOPA Pilot Magazine - August 2021 - 8
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AOPA Pilot Magazine - August 2021 - Cover3
AOPA Pilot Magazine - August 2021 - Cover4
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