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Flying
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Flight Maneuvers > Descents And Descending Turns
When an airplane enters a descent, it changes
its flightpath from level to an inclined plane. It is important
that the pilot know the power settings and pitch attitudes that
will produce the following conditions of descent.
PARTIAL POWER DESCENT—The
normal method of losing altitude is to descend with partial
power. This is often termed “cruise” or “enroute”
descent. The airspeed and power setting recommended by the airplane
manufacturer for prolonged descent should be used. The target
descent rate should be 400 – 500 f.p.m. The airspeed may
vary from cruise airspeed to that used on the downwind leg of
the landing pattern. But the wide range of possible airspeeds
should not be interpreted to permit erratic pitch changes. The
desired airspeed, pitch attitude, and power combination should
be preselected and kept constant.
DESCENT AT MINIMUM SAFE AIRSPEED—A
minimum safe airspeed descent is a nose-high, power assisted
descent condition principally used for clearing obstacles during
a landing approach to a short runway. The airspeed used for
this descent condition is recommended by the airplane manufacturer
and normally is no greater than 1.3 VSO. Some characteristics
of the minimum safe airspeed descent are a steeper than normal
descent angle, and the excessive power that may be required
to produce acceleration at low airspeed should “mushing”
and/or an excessive rate of descent be allowed to develop.
GLIDES—A glide is a
basic maneuver in which the airplane loses altitude in a controlled
descent with little or no engine power; forward motion is maintained
by gravity pulling the airplane along an inclined path and the
descent rate is controlled by the pilot balancing the forces
of gravity and lift
Although glides are directly related to the
practice of power-off accuracy landings, they have a specific
operational purpose in normal landing approaches, and forced
landings after engine failure. Therefore, it is necessary that
they be performed more subconsciously than other maneuvers because
most of the time during their execution, the pilot will be giving
full attention to details other than the mechanics of performing
the maneuver. Since glides are usually performed relatively
close to the ground, accuracy of their execution and the formation
of proper technique and habits are of special importance..
Because the application of controls is somewhat
different in glides than in power-on descents, gliding maneuvers
require the perfection of a technique somewhat different from
that required for ordinary power-on maneuvers. This control
difference is caused primarily by two factors—the absence
of the usual propeller slipstream, and the difference in the
relative effectiveness of the various control surfaces at slow
speeds.
The glide ratio of an airplane is the distance
the airplane will, with power off, travel forward in relation
to the altitude it loses. For instance, if an airplane travels
10,000 feet forward while descending 1,000 feet, its glide ratio
is said to be 10 to 1
The glide ratio is affected by all four fundamental
forces that act on an airplane (weight, lift, drag, and thrust).
If all factors affecting the airplane are constant, the glide
ratio will be constant. Although the effect of wind will not
be covered in this section, it is a very prominent force acting
on the gliding distance of the airplane in relationship to its
movement over the ground. With a tailwind, the airplane will
glide farther because of the higher groundspeed. Conversely,
with a headwind the airplane will not glide as far because of
the slower groundspeed..
Variations in weight do not affect the glide
angle provided the pilot uses the correct airspeed. Since it
is the lift over drag (L/D) ratio that determines the distance
the airplane can glide, weight will not affect the distance.
The glide ratio is based only on the relationship of the aerodynamic
forces acting on the airplane. The only effect weight has is
to vary the time the airplane will glide. The heavier the airplane
the higher the airspeed must be to obtain the same glide ratio.
For example, if two airplanes having the same L/D ratio, but
different weights, start a glide from the same altitude, the
heavier airplane gliding at a higher airspeed will arrive at
the same touchdown point in a shorter time. Both airplanes will
cover the same distance, only the lighter airplane will take
a longer time.
Under various flight conditions, the drag factor
may change through the operation of the landing gear and/or
flaps. When the landing gear or the flaps are extended, drag
increases and the airspeed will decrease unless the pitch attitude
is lowered. As the pitch is lowered, the glidepath steepens
and reduces the distance traveled. With the power off, a windmilling
propeller also creates considerable drag, thereby retarding
the airplane’s forward movement.
Although the propeller thrust of the airplane
is normally dependent on the power output of the engine, the
throttle is in the closed position during a glide so the thrust
is constant. Since power is not used during a glide or power-off
approach, the pitch attitude must be adjusted as necessary to
maintain a constant airspeed.
The best speed for the glide is one at which
the airplane will travel the greatest forward distance for a
given loss of altitude in still air. This best glide speed corresponds
to an angle of attack resulting in the least drag on the airplane
and giving the best lift-to-drag ratio (L/DMAX). [figure3-17]
figure3-17. L/DMAX.
Any change in the gliding airspeed will result
in a proportionate change in glide ratio. Any speed, other than
the best glide speed, results in more drag. Therefore, as the
glide airspeed is reduced or increased from the optimum or best
glide speed, the glide ratio is also changed. When descending
at a speed below the best glide speed, induced drag increases.
When descending at a speed above best glide speed, parasite
drag increases. In either case, the rate of descent will increase.
[figure3-18]
This leads to a cardinal rule of airplane flying
that a student pilot must understand and appreciate: The pilot
must never attempt to “stretch” a glide by applying
back-elevator pressure and reducing the airspeed below the airplane’s
recommended best glide speed. Attempts to stretch a glide will
invariably result in an increase in the rate and angle of descent
and may precipitate an inadvertent stall.
lower than that at which it would stabilize.
The pilot must be prepared for this. To keep pitch attitude
constant after a power change, the pilot must counteract the
immediate trim change. If the pitch attitude is allowed to decrease
during glide entry, excess speed will be carried into the glide
and retard the attainment of the correct glide angle and airspeed.
Speed should be allowed to dissipate before the pitch attitude
is decreased. This point is particularly important in so-called
clean airplanes as they are very slow to lose their speed and
any slight deviation of the nose downwards results in an immediate
increase in airspeed. Once the airspeed has dissipated to normal
or best glide speed, the pitch attitude should be allowed to
decrease to maintain that speed. This should be done with reference
to the horizon. When the speed has stabilized, the airplane
should be retrimmed for “hands off” flight
When the approximate gliding pitch attitude
is established, the airspeed indicator should be checked. If
the airspeed is higher than the recommended speed, the pitch
attitude is too low, and if the airspeed is less than recommended,
the pitch attitude is too high; therefore, the pitch attitude
should be readjusted accordingly referencing the horizon. After
the adjustment has been made, the airplane should be retrimmed
so that it will maintain
this attitude without the need to hold pressure on the elevator
control. The principles of attitude flying require that the
proper flight attitude be established using outside visual references
first, then using the flight instruments as a secondary check.
It is a good practice to always retrim the airplane after each
pitch adjustment..
A stabilized power-off descent at the best
glide speed is often referred to as a normal glide. The flight
instructor should demonstrate a normal glide, and direct the
student pilot to memorize the airplane’s angle and speed
by visually checking the airplane’s attitude with reference
to the horizon, and noting the pitch of the sound made by the
air passing over the structure, the pressure on the controls,
and the feel of
figure3-18. Best glide speed provides
the greatest forward distance for a given loss of altitude.
the airplane. Due to lack of experience, the
beginning student may be unable to recognize slight variations
of speed and angle of bank immediately by vision or by the pressure
required on the controls. Hearing will probably be the indicator
that will be the most easily used at first. The instructor should,
therefore, be certain that the student understands that an increase
in the pitch of sound denotes increasing speed, while a decrease
in pitch denotes less speed. When such an indication is received,
the student should consciously apply the other two means of
perception so as to establish the proper relationship. The student
pilot must use all three elements consciously until they become
habits, and must be alert when attention is diverted from the
attitude of the airplane and be responsive to any warning given
by a variation in the feel of the airplane or controls, or by
a change in the pitch of the sound.
After a good comprehension of the normal glide
is attained, the student pilot should be instructed in the differences
in the results of normal and “abnormal” glides.
Abnormal glides being those conducted at speeds other than the
normal best glide speed. Pilots who do not acquire an understanding
and appreciation of these differences will experience difficulties
with accuracy landings, which are comparatively simple if the
fundamentals of the glide are thoroughly understood.
Too fast a glide during the approach for landing
invariably results in floating over the ground for varying distances,
or even overshooting, while too slow a glide causes undershooting,
flat approaches, and hard touchdowns. A pilot without the ability
to recognize a normal glide will not be able to judge where
the airplane will go, or can be made to go, in an emergency.
Whereas, in a normal glide, the flightpath may be sighted to
the spot on the ground on which the airplane will land. This
cannot be done in any abnormal glide.
GLIDING TURNS—The action
of the control system is somewhat different in a glide than
with power, making gliding maneuvers stand in a class by themselves
and require the perfection of a technique different from that
required for ordinary power maneuvers. The control difference
is caused mainly by two factors—the absence of the usual
slipstrea and the difference or relative effectiveness of the
various control surfaces at various speeds and particularly
at reduced speed. The latter factor has its effect exaggerated
by the first, and makes the task of coordination even more difficult
for the inexperienced pilot. These principles should be thoroughly
explained in order that the student may be alert to the necessary
differences in coordination.
After a feel for the airplane and control touch
have been developed, the necessary compensation will be automatic;
but while any mechanical tendency exists, the student will have
difficulty executing gliding turns, particularly when making
a practical application of them in attempting accuracy landings.
Three elements in gliding turns which tend
to force the nose down and increase glide speed are:
• Decrease in effective lift due to the
direction of the lifting force being at an angle to the pull
of gravity.
• The use of the rudder acting as it does in the entry
to a power turn.
• The normal stability and inherent characteristics of
the airplane to nose down with the power off.
These three factors make it necessary to use
more back pressure on the elevator than is required for a straight
glide or a power turn and, therefore, have a greater effect
on the relationship of control coordination.
When recovery is being made from a gliding
turn, the force on the elevator control which was applied during
the turn must be decreased or the nose will come up too high
and considerable speed will be lost. This error will require
considerable attention and conscious control adjustment before
the normal glide can again be resumed.
In order to maintain the most efficient or
normal glide in a turn, more altitude must be sacrificed than
in a straight glide since this is the only way speed can be
maintained without power. Turning in a glide decreases the performance
of the airplane to an even greater extent than a normal turn
with power.
Still another factor is the difference in rudder
action in turns with and without power. In power turns it is
required that the desired recovery point be anticipated in the
use of controls and that considerably more pressure than usual
be exerted on the rudder. In the recovery from a gliding turn,
the same rudder action takes place but without as much pressure
being necessary. The actual displacement of the rudder is approximately
the same, but it seems to be less in a glide because the resistance
to pressure is so much less due to the absence of the propeller
slipstream. This often results in a much greater application
of rudder through a greater range than is realized, resulting
in an abrupt stoppage of the turn when the rudder is applied
for recovery. This factor is particularly important during landing
practice since the student almost invariably recovers from the
last turn too soon and may enter a cross-control condition trying
to correct the landing with the rudder alone. This results in
landing from a skid that is too easily mistaken for drift.
There is another danger in excessive rudder
use during gliding turns. As the airplane skids, the bank will
increase. This often alarms the beginning pilot when it occurs
close to the ground, and the pilot may respond by applying aileron
pressure toward the outside of the turn to stop the bank. At
the same time, the rudder forces the nose down and the pilot
may apply back-elevator pressure to hold it up. If allowed to
progress, this situation may result in a fully developed cross
control condition. A stall in this situation will almost certainly
result in a spin.
The level-off from a glide must be started
before reaching the desired altitude because of the airplane’s
downward inertia. The amount of lead depends on the rate of
descent and the pilot’s control technique. With too little
lead, there will be a tendency to descend below the selected
altitude. For example, assuming a 500-foot per minute rate of
descent, the altitude must be led by 100 – 150 feet to
level off at an airspeed higher than the glide speed. At the
lead point, power should be increased to the appropriate level
flight cruise setting so the desired airspeed will be attained
at the desired altitude. The nose tends to rise as both airspeed
and downwash on the tail section increase. The pilot must be
prepared for this and smoothly control the pitch attitude to
attain level flight attitude so that the level-off is completed
at the desired altitude.
Particular attention should be paid to the
action of the airplane’s nose when recovering (and entering)
gliding turns. The nose must not be allowed to describe an arc
with relation to the horizon, and particularly it must not be
allowed to come up during recovery from turns, which require
a constant variation of the relative pressureson the different
controls.
Common errors in the performance of descents
and descending turns are:
• Failure to adequately clear the area.
• Inadequate back-elevator control during glide entry
resulting in too steep a glide.
• Failure to slow the airplane to approximate glide speed
prior to lowering pitch attitude.
• Attempting to establish/maintain a normal glide solely
by reference to flight instruments.
• Inability to sense changes in airspeed through sound
and feel.
• Inability to stabilize the glide (chasing the airspeed
indicator).
• Attempting to “stretch”
the glide by applying back-elevator pressure.
• Skidding or slipping during gliding turns due to inadequate
appreciation of the difference in rudder action as opposed to
turns with power.
• Failure to lower pitch attitude during gliding turn
entry resulting in a decrease in airspeed.
• Excessive rudder pressure during recovery from gliding
turns.
• Inadequate pitch control during recovery from straight
glides.
• “Ground shyness”—resulting in cross-controlling
during gliding turns near the ground.
• Failure to maintain constant bank angle during gliding
turns.
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