What are the errors of the magnetic compass in aviation?

The magnetic compass has several errors that pilots must understand: Variation (magnetic declination) — the angular difference between true north and magnetic north, which varies by location. Deviation — the error caused by the aircraft's own magnetic fields, corrected by a deviation card. Turning error — in the northern hemisphere, when turning through north the compass lags; when turning through south it leads. Acceleration error — on easterly/westerly headings, acceleration causes an apparent turn north, deceleration causes an apparent turn south (ANDS: Accelerate North, Decelerate South). Magnetic dip — the tendency of the compass needle to tilt toward the earth's magnetic field, worst at high latitudes.

How does a pitot-static system work and what are its errors?

The pitot-static system uses pitot (ram air) pressure and static (ambient atmospheric) pressure to drive the airspeed indicator, altimeter and vertical speed indicator. The airspeed indicator measures the difference between pitot and static pressure. The altimeter measures absolute static pressure against a reference. Errors include: position error (static port location causes inaccurate static readings, especially at high AOA), instrument error (manufacturing tolerances), blocked pitot tube (ASI freezes or reads incorrectly), blocked static port (altimeter and VSI freeze, ASI shows reverse errors if pitot is also blocked). Icing is the primary cause of pitot-static blockages.

What is the difference between indicated, calibrated, equivalent and true airspeed?

Indicated Airspeed (IAS) is the raw reading from the airspeed indicator, uncorrected. Calibrated Airspeed (CAS) is IAS corrected for instrument and position error — used for most performance calculations. Equivalent Airspeed (EAS) is CAS corrected for compressibility error at higher speeds — significant above 200 knots or FL250. True Airspeed (TAS) is EAS corrected for air density (altitude and temperature) — actual speed through the airmass. The relationship: IAS → CAS → EAS → TAS, each correction adding speed at altitude. TAS = EAS × √(sea level density / actual density).

What are the gyroscopic properties used in flight instruments?

Two gyroscopic properties are used in flight instruments: Rigidity in space (gyroscopic inertia) — a spinning gyroscope maintains its orientation in space regardless of external forces. This property is used in the attitude indicator and heading indicator to provide a stable reference. Precession — when a force is applied to a spinning gyroscope, the resultant motion occurs 90° later in the direction of rotation. This property is used in the turn coordinator and turn-and-slip indicator, where aerodynamic or mechanical forces are converted into rotation about a different axis to indicate rate of turn.

What does TCAS Resolution Advisory (RA) require the crew to do?

When TCAS II issues a Resolution Advisory (RA), the crew must immediately follow the RA manoeuvre instructions, which appear on the VSI/IVSI as green (fly-to) and red (avoid) arcs. The crew must: disconnect autopilot if necessary to comply with the RA, follow the vertical manoeuvre indicated (climb, descend, maintain/increase vertical speed, level off), not manoeuvre in the opposite direction to the RA, notify ATC of the RA after the conflict is resolved. ATC instructions must be disregarded if they conflict with an RA — the RA takes priority. After the conflict, ATC must be informed and aircraft returned to cleared altitude.

ATPL Theory — Instrumentation

Master Aircraft Instruments for your ATPL theory exam

Free practice questions covering pitot-static systems, gyroscopic instruments, compass errors, EFIS, TCAS, GPWS, FMS and autopilot — with detailed explanations for every answer.

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10 Topic areas

75% EASA pass mark

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Questions

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Syllabus Coverage

Topics covered in this question bank

All major Instrumentation topic areas from the EASA ATPL syllabus, from basic pitot-static principles to integrated glass cockpit systems.

⊙Pitot-Static Systems

⊙Gyroscopic Instruments

⊙Magnetic Compass & Errors

⊙EFIS / PFD / ND / MFD

⊙Engine & Navigation Instruments

⊙TCAS II & Resolution Advisories

⊙GPWS & EGPWS

⊙Flight Management Systems

⊙Autopilot Modes & Operations

⊙Instrument Errors & Limitations

Key Concept — Airspeed Definitions

IAS, CAS, EAS and TAS — what each means and when it matters

One of the most tested areas in ATPL Instrumentation is understanding the four airspeed types, the corrections between them, and which is used for which purpose. The progression is always: IAS → CAS → EAS → TAS.

Airspeed Type	What it corrects for	Primary use
IAS (Indicated)	Nothing — raw instrument reading	Cockpit display, stall/VMO limits
CAS (Calibrated)	Instrument error + position error	Performance calculations, takeoff/landing
EAS (Equivalent)	CAS + compressibility (above ~200 kt / FL250)	Structural load calculations
TAS (True)	EAS + air density (altitude + temperature)	Navigation, flight planning, wind correction

EASA Exam Format

What to expect on the real exam

Instrumentation is a mix of conceptual understanding and precise knowledge of error types, instrument principles and system procedures. TCAS RA procedures, gyroscopic properties, pitot-static blockage effects and compass errors are all regularly tested with scenario-based questions.

~60

Questions in exam

75%

Pass mark required

90 min

Exam duration

About This Subject

Why Instrumentation underpins safe flight

Modern aircraft are entirely dependent on accurate instrumentation for safe operation, especially in IMC. Understanding how each instrument derives its information, what can go wrong, and how to interpret conflicting indications is a core professional pilot skill. The ATPL instrumentation paper tests both theoretical principles and practical application.

Questions are aligned with the EASA ATPL syllabus and cover the full instrument chain from basic pitot-static physics through to integrated glass cockpit displays, traffic avoidance systems and flight management computers.

Sample Question

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Instrumentation — Sample Question

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