3.pitot-static-instruments-detail. Pitot-Static Instruments
The pitot-static system supplies the three primary flight instruments that derive their indications from atmospheric pressure: the airspeed indicator (ASI), the altimeter, and the vertical speed indicator (VSI). Understanding how each instrument senses ram air pressure, static pressure, or both is essential for diagnosing failures and interpreting unusual indications in instrument flight.
System Components
- Pitot tube: Mounted facing the relative wind, it captures ram (impact) air pressure and routes it to the airspeed indicator only. It is heated electrically (pitot heat) to prevent icing.
- Static port(s): Flush-mounted on the side of the fuselage in undisturbed air, they sense ambient (static) pressure and feed all three instruments.
- Alternate static source: A cockpit-selectable port (often venting into the cabin) used when the primary static port is blocked. Because cabin pressure in an unpressurized aircraft is slightly lower than outside ambient, selecting alternate static typically causes the altimeter to read higher, the VSI to momentarily show a climb, and airspeed to read higher than actual. The POH provides correction tables.
- Drains and lines: Low points include drains to remove moisture that could otherwise freeze and block the system.
Altimeter
The altimeter is an aneroid barometer calibrated in feet. A stack of sealed aneroid wafers expands as static pressure decreases (climb) and compresses as pressure increases (descent). A mechanical linkage drives the pointers. The Kollsman window lets the pilot set local altimeter setting (29.92 "Hg standard) so the instrument indicates altitude above mean sea level (MSL).
Key altitudes:
- Indicated altitude — what the altimeter shows with the current setting.
- Pressure altitude — height above the standard datum plane (29.92 "Hg).
- Density altitude — pressure altitude corrected for nonstandard temperature.
- True altitude — actual height above MSL.
- Absolute altitude — height above ground level (AGL).
A useful rule: an altimeter setting change of 0.01 "Hg ≈ 10 feet. Failing to update the setting when flying from high to low pressure (or warm to cold air) causes the altimeter to read higher than actual — "from high to low, look out below."
Airspeed Indicator
The ASI is a differential pressure gauge that compares ram pressure (pitot) to static pressure. The difference is dynamic pressure (q = ½ρV²), which the instrument converts to a calibrated airspeed reading.
Airspeed types:
- Indicated airspeed (IAS) — direct instrument reading.
- Calibrated airspeed (CAS) — IAS corrected for installation and instrument error (POH table).
- Equivalent airspeed (EAS) — CAS corrected for compressibility (significant above ~200 KCAS / 10,000 ft).
- True airspeed (TAS) — EAS corrected for nonstandard density. TAS increases roughly 2% per 1,000 ft above sea level for a given IAS.
Color-coded markings: white arc (flap operating range, V_S0 to V_FE), green arc (normal range, V_S1 to V_NO), yellow arc (caution, smooth air only), red line (V_NE), and a red radial for V_MC on multiengine aircraft.
Vertical Speed Indicator
The VSI measures the rate of change of static pressure. Inside the case, a diaphragm vents to the static line directly while the surrounding case vents through a calibrated leak. When altitude changes, a pressure differential develops across the diaphragm proportional to climb or descent rate, displayed in feet per minute. Because of the calibrated leak, the VSI has a 6–9 second lag before stabilizing. An instantaneous VSI (IVSI) uses accelerometer-driven pistons to eliminate most of the lag.
Blockages and Failures
Correct diagnosis is a frequent checkride topic.
- Pitot tube blocked, drain hole open: Ram pressure bleeds off; airspeed drops to zero. Altimeter and VSI unaffected.
- Pitot tube and drain both blocked (typical icing): Trapped pressure makes the ASI act like an altimeter — airspeed increases in a climb and decreases in a descent, regardless of actual speed.
- Static port blocked: Altimeter freezes at the altitude where blockage occurred, VSI reads zero, and airspeed reads low when climbing above and high when descending below the blockage altitude. Use alternate static; if unavailable, breaking the glass on the VSI in an unpressurized aircraft provides a cabin static reference (the VSI will then read in reverse).
Preflight and Operating Practice
- Visually inspect pitot tube, static ports, and drains; remove the pitot cover.
- Verify pitot heat operation during runup (per POH).
- During takeoff roll, confirm airspeed is alive.
- Set the altimeter to current local setting; tolerance per 14 CFR 91.411 for IFR is ±75 feet from field elevation. Within the preceding 24 calendar months, the static system, altimeter, and transponder must be tested and inspected for IFR flight.