The Pitot-Static Instruments

The pitot-static system drives three instruments using ram air pressure (pitot) and ambient static pressure (static ports).

A sealed aneroid measures static pressure to display altitude. Set the Kollsman window to the local altimeter setting. Remember "High to low, look out below": flying from high pressure/temperature into low pressure or cold air makes the altimeter read higher than you actually are. In very cold conditions, apply the ICAO cold-temperature correction to published altitudes.

The ASI compares ram (pitot) pressure to static pressure. Key speeds:

• IAS — what you read.
• CAS — corrected for installation/position error.
• TAS — corrected for nonstandard pressure and temperature (rises ~2% per 1,000 ft).
• Groundspeed — TAS corrected for wind.

The VSI measures the rate of change of static pressure. It lags several seconds — use trend information, not the instantaneous needle, as a performance reference.

| Blockage | Symptom |

|---|---|

| Pitot tube blocked, drain open | Airspeed drops toward zero |

| Pitot and drain blocked (acts as altimeter) | Airspeed reads like an altimeter: increases in climb, decreases in descent |

| Static port blocked | Altimeter freezes; airspeed inaccurate; VSI reads zero |

If the static port ices over, select the alternate static source (often cabin air). Because cabin pressure is slightly lower than outside static, the altimeter reads slightly high, airspeed reads slightly high, and the VSI momentarily jumps. Apply the POH correction.

Pitot heat prevents ice from blocking the ram-air inlet. Turn it on whenever you fly in visible moisture near freezing or anytime in IMC. It is a required item on your IFR equipment check.

Bottom line: know what each instrument senses, and you can predict exactly how it fails.