6.6 Instrument Systems: Pitot-Static, Gyros, and Displays

The Pitot-Static System

The pitot-static system supplies air-pressure information to three primary flight instruments. The pitot tube faces the airstream and senses ram (impact) plus static pressure; the static port(s), mounted flush where airflow is undisturbed, sense ambient static pressure only.

Because these instruments drive the pilot's airspeed, altitude, and trend information, line integrity and leak checks are safety-critical. The static system is checked for leaks per the manufacturer and, for IFR aircraft, the altimeter/static system test required by 14 CFR 91.411 every 24 calendar months (and the transponder per 91.413). The pitot heat anti-ice element is checked because a blocked pitot in icing makes the ASI useless. Never blow into a pitot/static line toward an instrument — you can destroy it.

The altimeter is an aneroid (sealed bellows) instrument that expands as static pressure falls; the pilot sets local barometric pressure in the Kollsman window. The airspeed indicator measures the difference between pitot ram pressure and static pressure with a diaphragm. The VSI senses how fast static pressure changes by comparing case pressure (delayed through a calibrated leak) with the instantaneous static line, which is why the VSI lags and reads zero when the static port is blocked.

Apply suction/pressure for leak checks only with a calibrated tester, slowly, and always within published rates — abrupt application bends the mechanism.

Blockage Errors and Instrument Markings

Knowing blockage behavior is a guaranteed exam topic:

• Blocked pitot ram inlet (drain open): ASI drops toward zero.
• Blocked pitot inlet AND drain (trapped pressure): the ASI acts like an altimeter — it reads higher as the aircraft climbs and lower as it descends.
• Blocked static port: the altimeter freezes at the blockage altitude, the VSI stops (reads zero), and the ASI reads low above the blockage altitude and high below it.
• Alternate static source (vents to the cabin, where pressure is slightly lower): the altimeter and ASI read slightly high.

Airspeed color markings (memorize):

Other markings appear on engine and pressure gauges (green = normal, yellow = caution, red = limit/do-not-exceed). A white radial line across the glass and case (a slippage mark) lets the mechanic confirm the instrument face has not rotated in its case. These markings are applied per approved data and are part of the airworthiness inspection — a missing or wrong arc is a discrepancy.

When an instrument reads wrong, separate an instrument fault from a system fault: a single bad airspeed reading with a normal altimeter points at the pitot side; all three pitot-static instruments misbehaving points at a shared static line or port; a wandering heading points at the gyro or its power, not the compass. This source-isolation habit is exactly what the practical examiner wants to see, because replacing the instrument when the line is at fault wastes parts and leaves the squawk.

Magnetic, Gyroscopic, and Electronic Instruments

The magnetic compass is the only self-contained direction instrument; it has known errors — variation (chart vs. true), deviation (aircraft metal/electrics, recorded on a compass card after a swing), and acceleration/turning errors. It is serviced with compass fluid, kept bubble-free, and swung after major electrical changes.

Gyroscopic instruments rely on two properties:

• Rigidity in space — a spinning gyro resists tilting and stays fixed. The attitude indicator (artificial horizon) and heading indicator (directional gyro) use rigidity; the aircraft moves around the rigid gyro.
• Precession — a force applied to a spinning gyro is felt 90 degrees later in the direction of rotation. The turn coordinator / turn-and-slip indicator uses precession to show turn rate.

Gyros are powered by vacuum/pressure (attitude, heading) or electricity (turn coordinator) — split sources so one failure does not blank all gyros. Handle mechanical gyros gently; shock can damage bearings/rotors, and a dropped gyro is removed even if it looks fine. Modern aircraft use electronic displays (EFIS/PFD/MFD, glass cockpit) fed by an air-data computer and an AHRS (attitude-heading reference system using solid-state/laser gyros). Avionics work demands electrostatic-discharge (ESD) precautions — a discharge you cannot feel can destroy a circuit card.

Engine and pressure gauge markings mirror the airspeed logic: a green arc is the normal operating range, a yellow arc is a caution range (time-limited or transient), and a red line/radial is a maximum or minimum limit not to be exceeded. These appear on oil-pressure, oil-temperature, cylinder-head-temperature, EGT, fuel-pressure, tachometer, and manifold-pressure gauges. Markings are applied per the Type Certificate Data Sheet (TCDS) and approved data, not painted by guess.

When a candidate is asked what a yellow arc means on any gauge, the answer is caution range — operate there only briefly and with attention. Treat any intermittent warning/annunciator as a real discrepancy: check wiring, sensors, the annunciator, and system logic before clearing it.