The Pitot-Static System

The two sources

• Pitot tube — captures ram (dynamic + static) air pressure from forward motion. Located somewhere on the leading edge or front of the aircraft.
• Static port — captures ambient atmospheric (static) pressure. Usually flush-mounted on the side of the fuselage where airflow is undisturbed.

Removing pitot covers and inspecting drains is a preflight item for a reason — moisture and ice in this system manifest as the most disorienting failures in IMC.

Altimeter — sensitive aneroid barometer

An aneroid barometer displays altitude by measuring the difference between current static pressure and a reference set in the Kollsman window.

• Inside is a sealed aneroid capsule. As pressure decreases (climbing), the capsule expands and mechanical linkages drive the needle clockwise. Pressure increase (descent) compresses the capsule, needle goes counter-clockwise.
• Kollsman window adjustment: 1 inHg = 1,000 ft change in indication.
• Setting 29.92 inHg displays pressure altitude (used above 18,000 ft / FL180 in the US).
• Setting the local altimeter setting displays indicated altitude (height above sea level corrected for non-standard pressure).
• ISA standard: 29.92 inHg / 15°C / 2°C lapse per 1,000 ft.

Errors: instrument (manufacturing), position (static port location), pressure (non-standard day), temperature (non-standard day).

Cardinal rule: "When flying from hot to cold or from high to low, look out below." Going from a high-pressure area to a low-pressure area, the altimeter over-reads — true altitude is lower than indicated. Same effect when flying from warm air into colder air.

Vertical Speed Indicator (VSI)

Senses the rate of change of static pressure and displays it in feet per minute.

• An aneroid capsule receives static pressure directly. The case around it receives static pressure through a calibrated leak — typically delaying case pressure by 4-5 seconds.
• The pressure difference between capsule (current) and case (5 seconds ago) drives the needle. That's why the VSI inherently lags actual rate changes.
• More sensitive than the altimeter for trend detection — you'll see a 50 fpm climb on the VSI before the altimeter has moved noticeably.
• The VSI is not required for IFR flight — if blocked, the AIM permits breaking the VSI glass to provide an alternate static source for the altimeter and ASI.

Airspeed Indicator (ASI)

A differential pressure gauge measuring dynamic pressure — the pressure of air pressing into the aircraft due to forward motion.

• A diaphragm receives total pressure from the pitot tube. The case around it receives static pressure from the static port.
• Total = static + dynamic. Subtracting static from total leaves dynamic — that's what the ASI displays.
• Increased airspeed → diaphragm expands → needle clockwise.

Errors: Instrument (manufacturing), position (static port location, worse at high pitch attitudes), density (uncompensated for non-standard density), compressibility (above ~180 kt — not a helicopter problem).

Types of Airspeed (IAS → CAS → EAS → TAS → GS)

• IAS — Indicated: what the ASI reads. Uncorrected.
• CAS — Calibrated: IAS corrected for instrument and position errors. POH airspeed correction table. Helicopters introduce position error as airspeed increases (pitch-down attitude angles the pitot away from relative wind).
• EAS — Equivalent: CAS corrected for compressibility. Below 180 kt, EAS = CAS.
• TAS — True: EAS corrected for non-standard density (altitude and temperature). Actual speed through the air mass. TAS ≈ CAS at sea level on an ISA day. Increases ~2% per 1,000 ft.
• GS — Ground Speed: TAS corrected for wind. Speed over the ground.

Pitot-Static Blockages — the diagnostic table

Diagnose by remembering which instrument is fed by which source.

Blocked pitot tube only (drain still open):

• ASI loses its dynamic pressure source — drops slowly toward zero.
• Altimeter and VSI: unaffected.

Blocked pitot tube AND drain (full pitot block):

• The trapped pressure inside the diaphragm becomes a fixed reference. The ASI now behaves like an altimeter:
  • Climb → indicated airspeed increases
  • Descend → indicated airspeed decreases
• Altimeter and VSI: unaffected.
• Mnemonic: "blocked pitot acts like an altimeter."

Blocked static port:

• Altimeter freezes at the altitude where the blockage occurred.
• VSI indicates zero (no rate change since both case and capsule are sealed).
• ASI behaves opposite to a pitot block:
  • Climb → indicated airspeed decreases
  • Descend → indicated airspeed increases
• Use the alternate static source (typically inside the cabin — reads slightly low because cabin pressure is lower than ambient). If unavailable, breaking the VSI glass works as a last resort.