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than the International Standard Atmosphere (ISA) prediction
an adjustment must be made to performance predictions and
various instrument indications.
Pressure Altitude
There are two measurements of the atmosphere that affect
performance and instrument calibrations: pressure altitude
and density altitude. Pressure altitude is the height above the
standard datum pressure (SDP) (29.92" Hg, sea level under
ISA) and is used for standardizing altitudes for flight levels
(FL). Generally, flight levels are at or above 18,000 feet
(FL 180), providing the pressure is at or above 29.92"Hg.
For calculations involving aircraft performance when the
altimeter is set for 29.92" Hg, the altitude indicated is the
pressure altitude.
Density Altitude
Density altitude is pressure altitude corrected for nonstandard
temperatures, and is used for determining aerodynamic
performance in the nonstandard atmosphere. Density altitude
increases as the density decreases. Since density varies
directly with pressure, and inversely with temperature, a
wide range of temperatures may exist with a given pressure
altitude, which allows the density to vary. However, a
known density occurs for any one temperature and pressure
altitude combination. The density of the air has a significant
effect on aircraft and engine performance. Regardless of the
2-6
Figure 2-7. Relationship of Lift to Angle of Attack.
actual altitude above sea level an aircraft is operating at, its
performance will be as though it were operating at an altitude
equal to the existing density altitude.
If a chart is not available the density altitude can be estimated
by adding 120 feet for every degree Celsius above the ISA. For
example, at 3,000 feet pressure altitude (PA), the ISA prediction
is 9° C (15° C - [lapse rate of 2° C per 1,000 feet x 3 = 6° C]).
However, if the actual temperature is 20° C (11° C more than
that predicted by ISA) then the difference of 11° C is multiplied
by 120 feet equaling 1,320. Adding this figure to the original
3,000 feet provides a density altitude of 4,320 feet (3,000 feet
+ 1,320 feet).
Lift
Lift always acts in a direction perpendicular to the relative
wind and to the lateral axis of the aircraft. The fact that lift is
referenced to the wing, not to the Earth’s surface, is the source
of many errors in learning flight control. Lift is not always
“up.” Its direction relative to the Earth’s surface changes as
the pilot maneuvers the aircraft.
The magnitude of the force of lift is directly proportional to
the density of the air, the area of the wings, and the airspeed. It
also depends upon the type of wing and the angle of attack. Lift
increases with an increase in angle of attack up to the stalling
angle, at which point it decreases with any further increase
in angle of attack. In conventional aircraft, lift is therefore
controlled by varying the angle of attack and speed.
Pitch/Power Relationship
An examination of Figure 2-7 illustrates the relationship
between pitch and power while controlling flight path and
airspeed. In order to maintain a constant lift, as airspeed is
reduced, pitch must be increased. The pilot controls pitch
through the elevators, which control the angle of attack.
When back pressure is applied on the elevator control, the tail
lowers and the nose rises, thus increasing the wing’s angle of
attack and lift. Under most conditions the elevator is placing
downward pressure on the tail. This pressure requires energy
that is taken from aircraft performance (speed). Therefore,
when the CG is closer to the aft portion of the aircraft the
elevator downward forces are less. This results in less energy
used for downward forces, in turn resulting in more energy
applied to aircraft performance.
Thrust is controlled by using the throttle to establish or
maintain desired airspeeds. The most precise method
of controlling flight path is to use pitch control while
simultaneously using power (thrust) to control airspeed. In
order to maintain a constant lift, a change in pitch requires a
change in power, and vice versa.
If the pilot wants the aircraft to accelerate while maintaining
altitude, thrust must be increased to overcome drag. As
the aircraft speeds up, lift is increased. To prevent gaining
altitude, the pitch angle must be lowered to reduce the
angle of attack and maintain altitude. To decelerate while
maintaining altitude, thrust must be decreased to less than the
value of drag. As the aircraft slows down, lift is reduced. To
　
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