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Normal instrument flight relies in part on three gyroscope instruments: an attitude
indicator (artificial horizon), a heading indicator (directional gyro, or "DG")
and a turn and slip indicator ("needle and ball," or "turn and bank," or
"turn coordinator").
These gyroscopic instruments may be powered by pneumatic (vacuum or pressure) or by
airplane electrical systems. Which power source is used for which instruments may
vary in the same make and model of airplane, depending on use intended at time of
manufacture or modifications made after manufacture.
The most common arrangement for single engine airplanes without back-up instrumentation,
or systems, is a single vacuum system which powers the directional and attitude gyroscope
instruments. The other gyro instrument, the "turn and bank" or "turn
coordinator" is usually electrically driven.
The gage on the instrument panel may be marked as either a "suction gage,"
a "vacuum gage," or a "pressure gage," and indicates in inches of
mercury. The correct operating range (around 4.5" to 5.5" HG.) is given in the
Airplane Flight Manual (AFM) or Pilot's Operating Handbook (POH) for each airplane.
Some airplanes also have warning lights when the vacuum or pressure is out of tolerance.
Pneumatic systems, like other mechanical systems, can malfunction suddenly or slowly.
A slow decrease in gage indication may indicate a dirty filter, dirty screens, sticking
regulator, worn out air pump or leak in the system. Zero pressure could indicate a
sheared pump drive, pump failure, a collapsed line, or a malfunctioning gage.
Any operation out of the normal range requires immediate attention by a mechanic.
A complete pneumatic loss is noticeable immediately on the gage or within minutes
by incorrect gyro readings. A slow deterioration may lead to sluggish or incorrect
readings which may trap a pilot who is not constantly cross-checking all instruments -
including the vacuum or pressure gage.
An additional factor involves an initial lack of recognition of the cause of the
conflicting instrument indication which develops when one instrument, usually the
attitude indicator, malfunctions. Although possibly proficient in flying
"partial panel," many pilots are not trained or skilled in deciding to revert
to a partial panel scan unless an instructor or safety pilot has forced the scan
by covering the attitude indicator. It is important for pilots to scan all
instruments whenever conflicting information develops, and not attempt to make
control inputs on the basis of the attitude indicator alone.
Once the all-important first step of recognition of the need for partial panel scan is
accepted, it is also helpful to remove the malfunctioning instrument from the scan,
usually by covering it with a disk or piece of paper.
The possibility of pneumatic system or gyroscope instrument failure is the reason
every instrument instructor drills students on partial panel flying without reference
to gyroscope heading and attitude instruments. It is very rare that the failure itself
results in a fatal accident, but it can set the stage for one if the pilot is not proficient
in partial panel flying and the failure occurs during instrument flight conditions.
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