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Ground reference maneuvers should be flown at an altitude
of approximately 600 to 1,000 feet AGL. The
actual altitude will depend on the speed and type of airplane
to a large extent, and the following factors should
be considered.
• The speed with relation to the ground should not
be so apparent that events happen too rapidly.
• The radius of the turn and the path of the airplane
over the ground should be easily noted and
changes planned and effected as circumstances
require.
• Drift should be easily discernable, but not tax the
student too much in making corrections.
• Objects on the ground should appear in their proportion
and size.
• The altitude should be low enough to render any
gain or loss apparent to the student, but in no case
lower than 500 feet above the highest obstruction.
During these maneuvers, both the instructor and the
student should be alert for available forced-landing
fields. The area chosen should be away from communities,
livestock, or groups of people to prevent possible
annoyance or hazards to others. Due to the altitudes at
which these maneuvers are performed, there is little
time available to search for a suitable field for landing
in the event the need arises.
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6-2
DRIFT AND GROUND
TRACK CONTROL
Whenever any object is free from the ground, it is
affected by the medium with which it is surrounded.
This means that a free object will move in whatever
direction and speed that the medium moves.
For example, if a powerboat is crossing a river and
the river is still, the boat could head directly to a point
on the opposite shore and travel on a straight course
to that point without drifting. However, if the river
were flowing swiftly, the water current would have to
be considered. That is, as the boat progresses forward
with its own power, it must also move upstream at the
same rate the river is moving it downstream. This is
accomplished by angling the boat upstream sufficiently
to counteract the downstream flow. If this is
done, the boat will follow the desired track across
the river from the departure point directly to the
intended destination point. Should the boat not be
headed sufficiently upstream, it would drift with the
current and run aground at some point downstream
on the opposite bank. [Figure 6-1]
As soon as an airplane becomes airborne, it is free of
ground friction. Its path is then affected by the air mass
in which it is flying; therefore, the airplane (like the
boat) will not always track along the ground in the
exact direction that it is headed. When flying with the
longitudinal axis of the airplane aligned with a road, it
may be noted that the airplane gets closer to or farther
from the road without any turn having been made. This
would indicate that the air mass is moving sideward in
relation to the airplane. Since the airplane is flying
within this moving body of air (wind), it moves or
drifts with the air in the same direction and speed, just
like the boat moved with the river current. [Figure 6-1]
When flying straight and level and following a
selected ground track, the preferred method of correcting
for wind drift is to head the airplane (wind
correction angle) sufficiently into the wind to cause
the airplane to move forward into the wind at the
same rate the wind is moving it sideways.
Depending on the wind velocity, this may require a
large wind correction angle or one of only a few
degrees. When the drift has been neutralized, the
airplane will follow the desired ground track.
To understand the need for drift correction during
flight, consider a flight with a wind velocity of 30
knots from the left and 90° to the direction the airplane
is headed. After 1 hour, the body of air in which the
airplane is flying will have moved 30 nautical miles
(NM) to the right. Since the airplane is moving with
this body of air, it too will have drifted 30 NM to the
right. In relation to the air, the airplane moved forward,
but in relation to the ground, it moved forward
as well as 30 NM to the right.
There are times when the pilot needs to correct for drift
while in a turn. [Figure 6-2] Throughout the turn the
wind will be acting on the airplane from constantly
changing angles. The relative wind angle and speed
CURRENT CURRENT
No Current - No Drift With a current the boat drifts
downstream unless corrected.
With proper correction, boat
stays on intended course.
No Wind - No Drift With any wind, the airplane drifts
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AIRPLANE FLYING HANDBOOK 飞机飞行手册上(63)
