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Ground Reference Maneuvers:
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Constant Radius During Turning Flight
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In a no-wind
condition, the pilot can perform a ground-based
constant radius turn by accurately maintaining
a constant bank angle throughout the turn; however,
with any wind the complexities of maintaining
a ground-based constant radius turn increase.
When wind is present, during ground reference
maneuvers involving turns, the pilot must correct
for wind drift. Throughout the turn, the wind
is acting on the airplane from a constantly
changing angle—increasing or decreasing
the groundspeed in a manner similar to straight
flight. To follow a circular, constant radius
ground track, the bank angle must vary to compensate
for wind drift throughout the turn. The airplane’s
ground-based turn radius is affected by the
airplane’s groundspeed: the faster the
groundspeed, the steeper the airplane must be
banked to maintain a ground-based constant radius
turn. The converse is also true: the slower
the groundspeed, the shallower the airplane
needs to be banked to maintain a ground-based
constant radius turn.
For a given true airspeed, the radius of turn
in the air varies proportionally with the bank
angle. To maintain the constant radius over
the ground, the bank angle is proportional to
ground speed. For example, an airplane is in
the downwind position at 100 knots groundspeed.
In this example, the wind is 10 knots, meaning
that the airplane is at an airspeed of 90 knots
(for this discussion, we ignore true, calibrated,
and indicate airspeed and assume that they are
all the same). If the pilot starts a downwind
turn with a 45° “steepest” bank
angle, the turn radius is approximately 890
feet. Let’s assume the airplane is now
upwind with a groundspeed of 80 knots. In order
to maintain the 890-foot radius, the pilot must
reduce the bank angle to a shallowest bank of
approximately 33°. In another example, if
the downwind is flown at an airspeed of 90 knots
in a 10 knot tailwind with a desired turn radius
of 2,000 feet, the “steepest” bank
angle needs to be at approximately 24° and
the upwind “shallowest” bank angle
at approximately 16°.
To demonstrate the effect that wind has on turns,
the pilot should select a straight-line ground
reference, such as a road or railroad track.
Choosing a straight-line ground reference that
is parallel to the wind, the airplane would
be flown into the wind and directly over the
selectedstraight-line ground reference. Once
a straight-line ground reference is established,
the pilot makes a 360° constant medium banked
turn. As the airplane completes the 360°
turn, it should return directly over the straight-line
ground reference but downwind from the starting
point. Choosing a straight-line ground reference
that has a crosswind, and using the same 360°
constant medium-banked turn, demonstrates how
the airplane drifts away from the reference
even as the pilot holds a constant bank angle.
In both examples, the path over the ground is
an elongated circle, although in reference to
the air, the airplane flew a perfect continuous
radius.
In order to compensate for the elongated, somewhat
circular path over the ground, the pilot must
adjust the bank angle as the groundspeed changes
throughout the turn. Where groundspeed is the
fastest, such as when the airplane is headed
downwind, the turn bank angle must be steepest;
where groundspeed is the slowest, such as when
the airplane is headed upwind, the turn bank
angle must be shallow. It is necessary to increase
or decrease the angle of bank, which increases
or decreases the rate of turn, to achieve the
desired constant radius track over the ground.
Ground reference maneuvers should always be
entered from a downwind position. This allows
the pilot to establish the steepest bank angle
required to maintain a constant radius ground
track. If the bank is too steep, the pilot should
immediately exit the maneuver and re-establish
a lateral position that is further from the
ground reference. The pilot should avoid bank
angles in excess of 45°due to the increased
stalling speed.
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Correcting Drift
During Straight-and-Level Flight
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When flying straight
and level and following a selected straight-line
direct ground track, the preferred method of
correcting for wind drift is to angle the airplane
sufficiently into the wind to cancel the effect
of the sideways drift caused by the wind. The
wind’s speed, the angle between the wind
direction and the airplane’s longitudinal
axis, and the airspeed of the airplane determines
the required wind correction angle. For example,
an airplane with an airspeed of 100 knots, a
20 knot wind at 90° to the airplane’s
longitudinal axis, and a 12° angle into
the wind is required to cancel the airplane’s
drift. If the wind in the above example is only
10 knots, the wind correction angle required
to cancel the drift is six degrees. When the
drift has been neutralized by heading the airplane
into the wind, the airplane will fly the direct
straight ground track.
To further illustrate this point, if a boat
is crossing a river and the river’s current
is completely still, the boat could head directly
to a point on the opposite shore on a straight
course to that opposite point without any drift;
however, rivers tend to have a downstream current
that must be considered if the captain wants
the boat to arrive at the opposite shore using
a direct straight path. Any downstream current
pushes the boat sideways and downstream at the
speed of the current. To counteract this downstream
movement, the boat must move upstream at the
same speed as the river is moving the boat downstream.
This is accomplished by angling the boat upstream
sufficiently to counteract the downstream flow.
If this is done, the boat follows a direct straight
track across the river to the intended destination
point. The amount of angle required is dependent
on the forward speed of the boat and the speed
of the current. The slower the forward speed
of the boat and/or the faster speed of the current,
the greater the angle must be to counteract
the drift. The converse is also true.
As soon as an airplane lifts off the surface
and levels the wings, if there is any crosswind,
the airplane will begin tracking sideways with
the wind. Any wind not directly on the nose
or tail of the airplane will drift the airplane
sideways at a speed up to the speed of the wind.
A wind that is directly to the right or the
left (at a 90° angle) drifts the airplane
sideways at the speed of the wind; when the
wind is halfway between the side and the nose
of the airplane (at a 45° angle), it drifts
the airplane sideways at just over 70 percent
of the speed of the wind. It should be understood
that pilots do not calculate the required drift
correction angles for ground reference maneuvers;
they merely use the references and adjust the
airplane’s relationship to those references
to cancel any drift. The groundspeed of the
airplane is also affected by the wind. As the
wind direction becomes parallel to the airplane’s
longitudinal axis, the magnitude of the wind’s
effect on the groundspeed is greater; as the
wind becomes perpendicular to the longitudinal
axis, the magnitude of the wind’s effect
on the groundspeed is less. In general, When
the wind is blowing straight into the nose of
the airplane, the groundspeed will be less than
the airspeed. When the wind is blowing from
directly behind the airplane, the groundspeed
will be faster than the airspeed. In other words,
when the airplane is headed upwind, the groundspeed
is decreased; when headed downwind, the groundspeed
is increased.
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Tracking Over
and Parallel to a Straight Line
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The pilot should
first be introduced to ground reference maneuvers
by correcting for the effects of a crosswind
over a straight-line ground reference, such
as road or railroad tracks. If a straight road
or railroad track is unavailable, the pilot
will choose multiple references (three minimum)
which, when an imaginary visual reference line
is extended, represents a straight line. The
reference should be suitably long so the pilot
has sufficient time to understand the concepts
of wind correction and practice the maneuver.
Initially, the maneuver should be flown directly
over the ground reference with the pilot angling
the airplane’s longitudinal axis into the
wind sufficiently such as to cancel the effect
of drift. The pilot should scan between far
ahead and close to the airplane to practice
tracking multiple references.
When proficiency has been demonstrated by flying
directly over the ground reference line, the
pilot should then practice flying a straight
parallel path that is offset from the ground
reference. The offset parallel path should not
be more than three-fourths of a mile from the
reference line. The maneuver should be flown
offset from the ground references with the pilot
angling the airplane’s longitudinal axis
into the wind sufficiently to cancel the effect
of drift while maintaining a parallel track.
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Maneuvering
by Reference to Ground Objects
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The purpose of
ground reference maneuvers is to train pilots
to accurately place the airplane in relationship
to specific references and maintain a desired
ground track. Such precision requires that a
pilot simultaneously evaluate the airplane’s
attitude, reference points along the desired
path, and the natural horizon. Vision is the
most utilized sense in maneuvering in orientation
to ground-based references; however, all senses
are actively involved at different levels. For
example, touch provides tactile feedback as
to the required flight control pressures to
overcome flight control surface forces that
indirectly indicate the airplane’s airspeed
and aerodynamic load.
It is a common error for beginning pilots to
fixate on a specific reference, such as a single
location on the ground or the natural horizon.
To be effective, the pilot must scan between
several visual references to determine relative
motion and to determine if the airplane is maintaining,
or drifting to or from, the desired ground track.
A pilot fixating on any one reference eliminates
the ability to determine rate, which significantly
degrades a pilot’s performance. Visual
scanning across several references allows the
pilot to develop the important skill of determining
the rate of closure to a specific point. Consider
a skilled automobile driver in a simple intersection
turn; the driver does not merely turn the steering
wheel some degree and hope that it will work
out. The skilled driver picks out several references,
such as an island to their side, a painted lane
line, or the opposing curb, and they use those
references to make almost imperceptible adjustments
to the amount of deflection on the steering
wheel, as well as the pressure on the accelerator
pedal to smoothly join the lane into which they
are turning. In the same manner, multiple references
are required to precisely control the airplane
in reference to the ground.
Not all ground-based references are visually
equal and some understanding of those differences
is important for their selection and use. For
example, larger objects or references may appear
closer than they actually are when compared
to smaller objects or references. Also, prevailing
visibility has a significant effect on the pilot’s
perception of the distance to a reference. Excellent
visibilities with clear skies tend to make an
object or reference appear closer than when
compared to a hazy day with poor visibility.
Another example is that rain can alter the visual
image in a manner that an illusion of being
at a higher altitude may be perceived, and brighter
objects or references may appear closer than
dimmer objects. Being aware of typical visual
illusions helps a pilot select the best references
for ground reference maneuvers. It is best,
however sometimes impracticable, to find ground-based
references that are similar in size and proportion.
Ground-based references can be numerous. Excellent
examples are breakwaters, canals, fence lines,
field boundaries, highways, railroad tracks,
roads, pipe lines, power lines, water-tanks,
and others; however, choices can be limited
by geography, population density, infrastructure,
or structures. Selecting a ground-based reference
requires prior consideration, such as the type
of maneuver being performed, altitude at which
the maneuver will be performed, emergency landing
requirements, density of structures, wind direction,
visibility, and the type of airspace.
Division of attention is an important skill
that a pilot must develop. A pilot must be able
to fly the airplane affecting the flight controls
in a manner they will place the airplane in
the needed attitude while tracking a specific
path over the ground. In addition, the pilot
must be able to scan for hazards such as other
aircraft, be immediately prepared for an emergency
landing should the need arise, and scan the
flight and engine instruments at regular intervals
to ensure that a pending situation, such as
decreasing oil pressure, does not turn into
an unexpected incident.
Safety is paramount in all aspects of flying.
Awareness and practice of safety-enhancing procedures
must be constantly exercised. Ground reference
maneuvers place the airplane in an environment
where heightened awareness is needed. Pilots
should be looking for other aircraft, including
helicopters, radio towers, and assessing locations
for emergency landings. Pilots should always
clear the area with two 90° clearing turns
looking to the left and the right, as well as
above and below the airplane. The maneuver area
should not cause disturbances and be well away
from groups of people, livestock, or communities.
Before performing any maneuver, the pilot should
complete the required checklist items, make
any radio announcements (such as on a practice
area frequency), and safety clearing turns.
As a general note, a ground reference maneuver
should not exceed a bank angle of 45° or
an airspeed greater than maneuvering speed.
As part of preflight planning, the pilot should
determine the predicted (POH/AFM) stall speed
at 50° or the highest bank angle planned
plus some margin for error in maneuvering.
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