Airspeed Indicator
The airspeed indicator (ASI) is a flight instrument that measures and displays the speed of an aircraft relative to the surrounding air mass. It works by comparing total pressure from the pitot tube to static pressure from the static port — the difference is dynamic pressure, which is directly related to airspeed.
Airspeed Types
- Indicated Airspeed (IAS): Raw ASI reading; used for all normal operations and aircraft limitations
- Calibrated Airspeed (CAS): IAS corrected for instrument and position error
- Equivalent Airspeed (EAS): CAS corrected for compressibility at high altitude
- True Airspeed (TAS): Actual speed through the air mass; used for flight planning and navigation
ASI Colour Coding
- White arc: Flap operating range (VS0 to VFE)
- Green arc: Normal operating range (VS1 to VNO)
- Yellow arc: Caution range — smooth air only (VNO to VNE)
- Red line: Never exceed speed (VNE)
- Blue radial (multi-engine): Best single-engine rate of climb speed (VYSE)
Pitot-Static Failures
A blocked pitot tube causes the ASI to over-read in a climb and under-read in a descent. A blocked static port causes erroneous readings on the ASI, altimeter, and VSI. Pitot heat should be selected ON whenever visible moisture or icing conditions are present.
On the TC Written Exam
The ASI colour coding is heavily tested. Know all arcs and lines, what each represents, and the corresponding V-speeds. Also know pitot-static failure modes.
Related Terms
See also: IAS, CAS, True Airspeed, VNE.