Airspeed Indicator Errors

Types of Airspeed Indicator Errors

• Instrument Error: Caused by imperfections in the mechanical or electronic components of the airspeed indicator. Older mechanical systems are more prone to friction, calibration drift, and wear, while modern digital systems reduce but do not eliminate these errors.

• Position Error: Results from the placement of pitot and static ports on the aircraft. Disturbed airflow around these ports—due to aircraft configuration, attitude, or speed—can cause the indicator to over-read or under-read. Position error is often systematic and may be corrected via calibration tables or air data computers.

• Pitot/Static System Errors: Blockages or leaks in the pitot tube or static lines can cause significant errors. A blocked pitot tube with an open static port will cause the indicator to behave like an altimeter; a blocked static port causes the airspeed to freeze or react incorrectly to altitude changes. Leaks, especially into a pressurized cabin, can make the instrument respond to cabin pressure changes rather than ambient air.

• Compressibility Error: At high speeds and altitudes, air becomes compressible, causing the airspeed indicator to over-read. This error becomes significant as aircraft approach transonic speeds and is corrected in calibrated airspeed (CAS) and true airspeed (TAS) calculations.

• Density Error: The indicator assumes standard sea-level density. At higher altitudes or non-standard temperatures, the instrument under-reads true airspeed for a given dynamic pressure. Pilots must correct for this when calculating TAS.

Recognizing Unreliable Airspeed

Unreliable airspeed can be detected by:

• Discrepancies between multiple airspeed indicators
• Aircraft behavior inconsistent with expected pitch and power settings
• System warnings or flags on electronic displays
• Unexpected buffeting or control feedback

Procedures for Unreliable Airspeed

If airspeed becomes unreliable, pilots should:

• Fly known pitch attitudes and power settings for the phase of flight
• Listen for ambient wind noise as a secondary cue
• Use GPS groundspeed as a reference, but note its limitations (wind is not accounted for)

Airspeed Indicator Displays

Airspeed can be shown via:

• Analog pointer dials
• Vertical tape displays
• Digital readouts, including HUDs

Correction Tables

Position error correction tables are used to adjust indicated airspeed to calibrated airspeed, especially in aircraft where position error is predictable and repeatable.