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is required, an additional correction of not more than one-half
horizon bar wide normally counteracts any deviation from
normal flight.
4-6
Figure 4-8. Vertical Speed Indicator.
fluctuations should be considered as the correct reading.
Reference to the altimeter helps in turbulent air because it is
not as sensitive as the VSI.
Vertical speed is represented in feet per minute (fpm).
[Figure 4-8] The face of the instrument is graduated with
numbers such as 1, 2, 3, etc. These represent thousands of
feet up or down in a minute. For instance, if the pointer is
aligned with .5 (1/2 of a thousand, or 500 fpm) the aircraft
will climb 500 feet in one minute. The instrument is
divided into two regions, one for climbing (up) and one for
descending (down).
During turbulence, it is not uncommon to see large
fluctuations on the VSI. It is important to remember that small
corrections should be employed to avoid further exacerbating
a potentially divergent situation.
Overcorrecting causes the aircraft to overshoot the desired
altitude; however, corrections should not be so small that
the return to altitude is unnecessarily prolonged. As a guide,
the pitch attitude should produce a rate of change on the VSI
about twice the size of the altitude deviation. For example,
if the aircraft is 100 feet off the desired altitude, a 200 fpm
rate of correction would be used.
During climbs or descents, the VSI is used to change the
altitude at a desired rate. Pitch attitude and power adjustments
are made to maintain the desired rate of climb or descent on
the VSI.
When pressure is applied to the controls and the VSI shows
an excess of 200 fpm from that desired, overcontrolling is
indicated. For example, if attempting to regain lost altitude
at the rate of 500 fpm, a reading of more than 700 fpm
would indicate overcontrolling. Initial movement of the
needle indicates the trend of vertical movement. The time
for the VSI to reach its maximum point of deflection after a
correction is called lag. The lag is proportional to speed and
magnitude of pitch change. In an airplane, overcontrolling
may be reduced by relaxing pressure on the controls, allowing
the pitch attitude to neutralize. In some helicopters with
servo-assisted controls, no control pressures are apparent.
In this case, overcontrolling can be reduced by reference to
the attitude indicator.
Some aircraft are equipped with an instantaneous vertical
speed indicator (IVSI). The letters “IVSI” appear on the face
of the indicator. This instrument assists in interpretation by
instantaneously indicating the rate of climb or descent at a
given moment with little or no lag as displayed in a VSI.
Occasionally, the VSI is slightly out of calibration and
indicates a gradual climb or descent when the aircraft is in
level flight. If readjustments cannot be accomplished, the error
in the indicator should be considered when the instrument is
used for pitch control. For example, an improperly set VSI
may indicate a descent of 100 fpm when the aircraft is in
level flight. Any deviation from this reading would indicate
a change in pitch attitude.
Airspeed Indicator
The airspeed indicator gives an indirect reading of the pitch
attitude. With a constant power setting and a constant altitude,
the aircraft is in level flight and airspeed remains constant. If the
airspeed increases, the pitch attitude has lowered and should be
raised. [Figure 4-9] If the airspeed decreases, the pitch attitude
has moved higher and should be lowered. [Figure 4-10] A
rapid change in airspeed indicates a large change in pitch;
a slow change in airspeed indicates a small change in pitch.
Although the airspeed indicator is used as a pitch instrument,
it may be used in level flight for power control. Changes in
pitch are reflected immediately by a change in airspeed. There
is very little lag in the airspeed indicator.
Pitch Attitude Instrument Cross-Check
The altimeter is an important instrument for indicating pitch
attitude in level flight except when used in conditions of
exceptionally strong vertical currents, such as thunderstorms.
With proper power settings, any of the pitch attitude
instruments can be used to hold reasonably level flight
attitude. However, only the altimeter gives the exact altitude
information. Regardless of which pitch attitude control
instrument indicates a need for a pitch attitude adjustment,
the attitude indicator, if available, should be used to make the
4-7
Figure 4-9. Pitch attitude has lowered. Figure 4-10. Pitch attitude has moved higher.
　
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