Instrument Error Types

Main Instrument Error Types Explained

Instrument Error

Instrument error is caused by imperfections in the instrument itself—such as manufacturing tolerances, mechanical wear, friction, or calibration drift. Even with modern technology, some degree of instrument error is always present, though digital systems have greatly reduced its impact compared to older mechanical designs.

Position Error

Position error, sometimes called installation or static system error, results from the placement of sensors (like static ports or pitot tubes) on the aircraft. Disturbed airflow around these sensors—due to aircraft geometry, configuration changes, or specific manoeuvres—can cause readings to deviate from true values. This error has both systematic (configuration) and unpredictable (manoeuvre-induced) components.

Barometric and Temperature Errors

Barometric error occurs when the altimeter is set incorrectly or when atmospheric pressure differs from standard assumptions. Temperature error affects instruments like the altimeter because the air column may not match International Standard Atmosphere (ISA) conditions, leading to incorrect altitude indications.

Lag

Lag is the delay in instrument response to a change in the measured parameter. For example, traditional vertical speed indicators (VSIs) exhibit lag due to their mechanical design, causing a delay before a climb or descent is indicated.

Random and Systematic Errors

Systematic errors are predictable and repeatable, often correctable through calibration or compensation (e.g., configuration error). Random errors are unpredictable, arising from transient factors like turbulence or rapid manoeuvres.

Other Notable Errors

• Compressibility error (notable at high speeds for airspeed indicators)
• Density error (affecting true airspeed calculations)
• Instrument drift (gradual change in instrument accuracy over time)

Minimising and Correcting Errors

• Regular calibration and maintenance reduce instrument error.
• Modern air data computers can compensate for systematic position errors.
• Correct barometric setting and temperature awareness help mitigate related errors.
• Understanding instrument lag helps pilots anticipate and interpret indications correctly.

Typical Locations and Error Considerations

• Static ports: susceptible to position and manoeuvre errors
• Pitot tubes: affected by position and compressibility errors
• Temperature probes: installation affects position and instrument errors

Recognising and compensating for these error types is essential for accurate instrument interpretation and safe flight.