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deviation. The result is tension on the controls, erratic control
response, and increasingly extreme control movements. The
correct technique, which is slower and smoother, will return
the airplane to the desired attitude more quickly, with positive
control and no confusion.
When a pitch error is detected, corrective action should be
taken promptly, but with light control pressures and two
distinct changes of attitude: (1) a change of attitude to stop
the needle movement and (2) a change of attitude to return
to the desired altitude.
When the altimeter indicates an altitude deviation, apply
just enough elevator pressure to decrease the rate of needle
movement. If it slows down abruptly, ease off some of the
pressure until the needle continues to move, but ease off
slowly. Slow needle movement means the airplane attitude
is close to level flight. Add slightly more corrective pressure
to stop the direction of needle movement. At this point level
flight is achieved; a reversal of needle movement means
the aircraft has passed through it. Relax control pressures
carefully, continuing to cross-check since changing airspeed
will cause changes in the effectiveness of a given control
pressure. Next, adjust the pitch attitude with elevator pressure
for the rate of change of altimeter needle movement that is
correlated with normal pitch corrections, and return to the
desired altitude.
As a rule of thumb, for errors of less than 100 feet, use a half
bar width correction. [Figures 5-9 and 5-10] For errors in
excess of 100 feet, use an initial full bar width correction.
[Figures 5-11 and 5-12] Practice predetermined altitude
changes using the altimeter alone, then in combination with
the attitude indicator.
Vertical Speed Indicator (VSI)
The VSI, like the altimeter, gives an indirect indication of
pitch attitude and is both a trend and a rate instrument. As
a trend instrument, it shows immediately the initial vertical
movement of the airplane, which disregarding turbulence
can be considered a reflection of pitch change. To maintain
level flight, use the VSI in conjunction with the altimeter and
attitude indicator. Note any positive or negative trend of the
needle from zero and apply a very light corrective elevator
5-5
Figure 5-9. Altitude Error, Less Than 100 Feet.
Figure 5-10. Pitch Correction, Less Than 100 Feet—One-Half Bar
Low to Correct Altitude Error.
Figure 5-11. Altitude Error, Greater Than 100 Feet.
Figure 5-12. Pitch Correction, Greater Than 100 Feet—One Bar
Correction Initially.
pressure. As the needle returns to zero, relax the corrective
pressure. If control pressures have been smooth and light, the
needle reacts immediately and slowly, and the altimeter shows
little or no change of altitude. As a rate instrument, the VSI
requires consideration of lag characteristics.
Lag refers to the delay involved before the needle attains a
stable indication following a pitch change. Lag is directly
proportional to the speed and magnitude of a pitch change.
If a slow, smooth pitch change is initiated, the needle moves
with minimum lag to a point of deflection corresponding
to the extent of the pitch change, and then stabilizes as the
aerodynamic forces are balanced in the climb or descent.
A large and abrupt pitch change produces erratic needle
movement, a reverse indication, and introduces greater time
delay (lag) before the needle stabilizes. Pilots are cautioned
not to chase the needle when flight through turbulent
conditions produces erratic needle movements. The apparent
lag in airspeed indications with pitch changes varies greatly
among different airplanes and is due to the time required for
the airplane to accelerate or decelerate when the pitch attitude
is changed. There is no appreciable lag due to the construction
or operation of the instrument. Small pitch changes, smoothly
executed, result in an immediate change of airspeed.
When using the VSI as a rate instrument and combining it
with the altimeter and attitude indicator to maintain level
flight, a pilot should know that the amount the altimeter
needle moves from the desired altitude governs the rate which
should be used to return to that altitude. A rule of thumb is
to make an attitude change that will result in a vertical-speed
rate approximately double the error in altitude. For example,
if altitude is off by 100 feet, the rate of return to the desired
altitude should be approximately 200 feet per minute (fpm).
If it is off by more than 100 feet, the correction should
be correspondingly greater, but should never exceed the
　
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