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Aviation Learning Center Document Silent Emergency: Pneumatic System Failure - P-8740-52
Author: Federal Aviation Administration Date: unknown
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Gyroscopic Instrument Power
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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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