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instrumentation. That is why partial panel flying training
is important. If the pilot understands how to utilize each
instrument independently, no significant change is encountered
in carrying out the flight when other instruments fail.
VSI Tape
The VSI tape provides for an indirect indication of pitch
attitude and gives the pilot a more immediate indication of a
pending altitude deviation. In addition to trend information,
the vertical speed also gives a rate indication. By using the
VSI tape in conjunction with the altitude trend tape, a pilot
will have a better understanding of how much of a correction
needs to be made. With practice, the pilot will learn the
performance of a particular aircraft and know how much
pitch change is required in order to correct for a specific
rate indication.
Unlike older analog VSIs, new glass panel displays have
instantaneous VSIs. Older units had a lag designed into the
system that was utilized to indicate rate information. The
new glass panel displays utilize a digital air data computer
that does not indicate a lag. Altitude changes are shown
immediately and can be corrected for quickly.
The VSI tape should be used to assist in determining what
pitch changes are necessary to return to the desired altitude.
A good rule of thumb is to use a vertical speed rate of change
that is double the altitude deviation. However, at no time
should the rate of change be more than the optimum rate of
climb or descent for the specific aircraft being flown. For
example, if the altitude is off by 200 feet from the desired
altitude, then a 400 feet per minute (fpm) rate of change
5-37
would be sufficient to get the aircraft back to the original
altitude. If the altitude has changed by 700 feet, then doubling
that would necessitate a 1,400 fpm change. Most aircraft
are not capable of that, so restrict changes to no more than
optimum climb and descent. An optimum rate of change
would vary between 500 and 1,000 fpm.
One error the instrument pilot encounters is overcontrolling.
Overcontrolling occurs when a deviation of more than
200 fpm is indicated over the optimum rate of change. For
example, an altitude deviation of 200 feet is indicated on the
altimeter, a vertical speed rate of 400 feet should be indicated
on the gauge. If the vertical speed rate showed 600 fpm (200
more than optimum), the pilot would be overcontrolling the
aircraft.
When returning to altitude, the primary pitch instrument
is the VSI tape. If any deviation from the desired vertical
speed is indicated, make the appropriate pitch change using
the attitude indicator.
As the aircraft approaches the target altitude, the vertical
speed rate can be slowed in order to capture the altitude in a
more stabilized fashion. Normally within 10 percent of the
rate of climb or descent from the target altitude, begin to
slow the vertical speed rate in order to level off at the target
altitude. This will allow the pilot to level at the desired altitude
without rapid control inputs or experiencing discomfort due
to G-load.
Airspeed Indicator (ASI)
The ASI presents an indirect indication of the pitch attitude.
At a constant power setting and pitch attitude, airspeed
remains constant. As the pitch attitude lowers, airspeed
increases, and the nose should be raised.
As the pitch attitude is increased, the nose of the aircraft will
raise, which will result in an increase in the angle of attack
as well as an increase in induced drag. The increased drag
will begin to slow the momentum of the aircraft which will
be indicated on the ASI. The airspeed trend indicator will
show a trend as to where the airspeed will be in 6 seconds.
Conversely, if the nose of the aircraft should begin to fall, the
angle of attack as well as induced drag will decrease.
There is a lag associated with the ASI when using it as a pitch
instrument. It is not a lag associated with the construction
of the ASI, but a lag associated with momentum change.
Depending on the rate of momentum change, the ASI may not
indicate a pitch change in a timely fashion. If the ASI is being
used as the sole reference for pitch change, it may not allow
for a prompt correction. However, if smooth pitch changes
are executed, modern glass panel displays are capable of
indicating 1 knot changes in airspeed and also capable of
projecting airspeed trends.
When flying by reference to flight instruments alone, it
is imperative that all of the flight instruments be crosschecked
for pitch control. By cross-checking all pitch related
instruments, the pilot can better visualize the aircraft attitude
　
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