7.altimeter-airspeed-vsi. Pitot-Static Instruments: Altimeter, Airspeed Indicator, and VSI
The pitot-static system supplies the three primary flight instruments that operate on the principles of differential and absolute pressure: the altimeter, the airspeed indicator (ASI), and the vertical speed indicator (VSI). The pitot tube faces the relative wind and senses ram (impact) pressure, while one or more static ports sense the undisturbed ambient (static) atmospheric pressure. The altimeter and VSI are connected only to the static line; the ASI is connected to both pitot and static.
Altimeter
The sensitive altimeter is an aneroid barometer that measures absolute static pressure and displays it as altitude in feet above a selected pressure datum. A stack of sealed aneroid wafers expands as static pressure decreases (climbing) and contracts as pressure increases (descending). The motion is geared to three pointers (10,000 ft, 1,000 ft, and 100 ft).
The Kollsman window allows the pilot to set a barometric reference between 28.00 and 31.00 inHg. With the local altimeter setting entered, the instrument indicates altitude above mean sea level (MSL).
Key altimetry rules of thumb:
- A change of 1 inHg ≈ 1,000 ft of altitude.
- ISA standard: 29.92 inHg, 15 °C at sea level; lapse rate −2 °C per 1,000 ft and −1 inHg per 1,000 ft in the lower atmosphere.
- "From a high to a low (pressure or temperature), look out below" — the altimeter reads higher than true altitude.
Types of altitude:
- Indicated — what the altimeter reads with the current setting.
- Pressure — height above the standard datum plane (set 29.92).
- Density — pressure altitude corrected for nonstandard temperature.
- True — actual height above MSL.
- Absolute — height above ground level (AGL).
Per 14 CFR 91.121, when operating below 18,000 ft MSL, set the altimeter to a station within 100 NM; at and above FL180, set 29.92.
Airspeed Indicator
The ASI is a differential pressure gauge. Ram air from the pitot tube is fed into a sealed diaphragm; static pressure surrounds it. The difference (dynamic pressure, q = ½ρV²) deflects the diaphragm and drives a pointer calibrated in knots.
Standard color-coded markings:
- White arc — flap operating range; from V_S0 (lower) to V_FE (upper).
- Green arc — normal operating range; V_S1 to V_NO.
- Yellow arc — caution range; smooth air only.
- Red line — V_NE, never-exceed speed.
- Red/white blue radial — V_MC and V_YSE on multiengine aircraft.
Airspeed types:
- Indicated (IAS) — direct reading.
- Calibrated (CAS) — IAS corrected for installation/position error (see POH).
- Equivalent (EAS) — CAS corrected for compressibility (significant above ~200 KCAS or high altitude).
- True (TAS) — EAS corrected for nonstandard density. A practical rule: TAS increases about 2% per 1,000 ft above sea level for a given CAS.
- Groundspeed (GS) — TAS corrected for wind.
Vertical Speed Indicator
The VSI displays rate of climb or descent in feet per minute (fpm). Inside the case is a diaphragm vented directly to the static line and a surrounding case vented to static through a calibrated leak (restricted diffuser). In level flight, pressures equalize and the needle reads zero. During a climb, static pressure in the diaphragm decreases faster than in the case, deflecting the pointer to indicate a climb; descent reverses the indication.
Because of the calibrated leak, the VSI has lag of 6–9 seconds before stabilizing, so it is a trend instrument first and a rate instrument second. An instantaneous VSI (IVSI) uses accelerometer-actuated pistons (dashpots) to eliminate most of this lag.
System Errors and Blockages
- Blocked pitot ram inlet only (drain open): ASI reads zero.
- Blocked pitot inlet AND drain: ASI behaves like an altimeter — reads high in a climb, low in a descent.
- Blocked static port (alternate static unavailable): altimeter freezes, VSI reads zero, ASI is inaccurate (reads low in a climb, high in a descent).
- Alternate static source in unpressurized aircraft typically draws from the cockpit, where pressure is slightly lower than ambient — altimeter reads slightly high, ASI slightly high, VSI momentarily indicates a brief climb. Consult the POH for corrections.
Preflight Checks
During preflight, verify pitot tube and static ports are clear and the pitot cover is removed. Before takeoff, set the current altimeter setting and confirm indicated elevation is within ±75 ft of the known field elevation (an IFR airworthiness check under 14 CFR 91.411 requires accuracy within established tolerances). On the takeoff roll, the ASI should come alive by rotation; if it does not, abort.