Instrumentation - Complete ATPL Subject Guide

Instrumentation, officially designated as ATPL Subject 022, covers the instruments and avionics systems that provide pilots with essential information about aircraft state, position, and performance. From traditional analog gauges to modern glass cockpits with integrated displays, understanding aircraft instrumentation is fundamental to safe and efficient flight operations.

For professional pilots, instrumentation knowledge enables effective monitoring, cross-checking, identifying instrument failures, and managing automated systems in both normal and abnormal situations.

What is Instrumentation?

Instrumentation encompasses the study of:

• Pitot-static instruments (ASI, altimeter, VSI)
• Gyroscopic instruments (attitude indicator, heading indicator, turn coordinator)
• Magnetic compass and compass errors
• Engine instruments (EPR, N1, N2, EGT, fuel flow, etc.)
• Electronic Flight Instrument System (EFIS)
• Engine Indicating and Crew Alerting System (EICAS) / Electronic Centralized Aircraft Monitor (ECAM)
• Flight Management System (FMS)
• Warning and alerting systems
• Standby instruments and redundancy

Subject 022 Exam Details

Number of Questions: 32 questions Exam Duration: 1 hour Pass Mark: 75% (24 correct answers) Difficulty Level: Medium Recommended Study Hours: 40-50 hours Prerequisites: Basic understanding of Principles of Flight and Aircraft General Knowledge helpful

Instrumentation is a moderate-length exam with emphasis on understanding instrument operating principles, error sources, and modern avionics systems.

Pitot-Static Instruments

Pitot-static instruments derive information from air pressure measurements. They are fundamental to flight operations and understanding their operating principles and error sources is critical.

The Pitot-Static System

Components:

• Pitot tube: Measures total pressure (static + dynamic pressure)
• Static port(s): Measure static (ambient atmospheric) pressure
• Pitot-static instruments: ASI, altimeter, VSI
• Alternate static source: Backup static pressure source (usually inside cockpit)

• Total Pressure (P_T): Static pressure + dynamic pressure
• Static Pressure (P_S): Ambient atmospheric pressure
• Dynamic Pressure (q): Pressure due to aircraft motion = ½ρV²
• PT = PS + q

• Electric heating prevents ice blockage
• Critical: Must be ON when flying in visible moisture and temperatures ≤10°C
• Blockage leads to unreliable airspeed indications

Airspeed Indicator (ASI)

Operating Principle:

• Measures difference between total pressure (pitot) and static pressure (static port)
• This differential pressure is proportional to dynamic pressure (q = ½ρV²)
• Calibrated to indicate airspeed under standard conditions

• Rule of thumb: TAS = CAS + (2% × CAS × altitude in thousands of feet)
• Example: CAS = 250 kts at FL350 → TAS ≈ 250 + (0.02 × 250 × 35) = 425 kts

• White arc: Flap operating range (VS0 to VFE)
• Green arc: Normal operating range (VS1 to VNO)
• Yellow arc: Caution range (VNO to VNE)
• Red line: Never-exceed speed (V_NE)
• Barber pole: Maximum operating speed (varies with altitude, Mach number)

Altimeter

Operating Principle:

• Aneroid barometer measures static pressure
• As altitude increases, static pressure decreases
• Altimeter converts pressure to altitude indication

• QNH: Pressure at sea level → Altimeter reads altitude above mean sea level (AMSL)
• QFE: Pressure at airfield elevation → Altimeter reads height above airfield (used in some countries)
• Standard setting (1013.25 hPa / 29.92 inHg): Used at and above transition altitude → Reads flight level

• Transition Altitude: Altitude below which altitude (QNH) is used
• Transition Level: Flight level above which flight levels (1013) are used
• Transition Layer: Airspace between transition altitude and transition level
• Example: Transition altitude 10,000 ft; if QNH = 1003 hPa, transition level might be FL110

• "High to Low, Look Out Below" (pressure or temperature)
• "From Hot to Cold, Look Out Below"
• Flying from high pressure to low pressure (or hot to cold) without updating altimeter → indicated altitude higher than actual altitude → risk of terrain impact

• True altitude affected by non-standard temperature
• Approximation: True altitude error ≈ 4 ft per 1°C deviation per 1,000 ft altitude
• Example: At 10,000 ft with temperature -20°C (ISA is -5°C, so 15°C colder) → altimeter over-reads by ~600 ft → true altitude is ~9,400 ft

Vertical Speed Indicator (VSI)

Operating Principle:

• Measures rate of change of static pressure
• Diaphragm: Connected directly to static port, responds instantly to pressure changes
• Case: Connected to static port via calibrated leak (restrictor), responds slowly
• Pressure difference between diaphragm and case indicates rate of pressure change

• Graduated in feet per minute (fpm) or meters per second (m/s)
• Typical range: ±2,000 fpm or ±4,000 fpm
• Zero indication in level flight (stabilized altitude)

• 6-9 second lag before indication stabilizes after pitch change
• Instantaneous Vertical Speed Indicator (IVSI) reduces lag to ~1 second using accelerometers

• Instrument error: Usually minimal
• Position error: Negligible
• Lag: Inherent design limitation (except IVSI)

Alternate Static Source

Purpose:

• Provides backup static pressure if primary static port(s) blocked (e.g., by ice, insects)

• Usually inside cockpit (unpressurized aircraft) or in a different location on fuselage

• Cabin pressure slightly lower than outside static pressure (due to venturi effect)
• ASI: Reads slightly high
• Altimeter: Reads slightly high
• VSI: Momentarily shows a climb, then stabilizes

Gyroscopic Instruments

Gyroscopic instruments use the properties of spinning gyroscopes to provide attitude, heading, and rate-of-turn information.

Gyroscopic Principles

Rigidity in Space:

• A spinning gyroscope maintains its orientation in space
• The faster the spin, the greater the rigidity
• Used in attitude indicator and heading indicator

• Applying force to a spinning gyroscope causes it to move 90° in direction of rotation
• Used in turn coordinator

Attitude Indicator (AI) / Artificial Horizon

Operating Principle:

• Gyroscope maintains reference plane (typically horizontal)
• Aircraft symbol moves relative to fixed horizon line
• Displays pitch (nose up/down) and bank (roll left/right)

• Vacuum/pressure system: Traditional (engine-driven vacuum pump)
• Electric: Backup or primary in modern aircraft
• Typical gyro speed: 12,000-15,000 RPM

• Vertical gyro: Spin axis vertical, maintains horizontal reference
• Erection system: Slowly corrects gyro tilt due to precession, uses gravity reference (pendulous vanes or electrolytic level switch)

• Exceeding gimbal limits causes gyro to "tumble" (become unreliable)
• Modern instruments have wider limits or are "non-tumbling"

Heading Indicator (HI) / Directional Gyroscope (DG)

Operating Principle:

• Gyroscope with horizontal spin axis maintains directional reference
• Aircraft rotates around gyro, which maintains fixed direction
• Displays magnetic heading

• No turning errors, acceleration errors, or oscillation
• Easier to read and interpret

• Real wander: Earth's rotation causes apparent drift (15°/hour × sine latitude)
• Maximum at poles (15°/hour), zero at equator
• Apparent wander: Gyro maintains orientation in space while aircraft moves over curved Earth
• 15°/hour × cos latitude × sin heading
• Mechanical drift: Friction, imperfections (typically 1-3°/hour)
• Total drift: Combination of above factors

• Must be periodically realigned with magnetic compass (typically every 10-15 minutes)
• Align in straight and level flight at constant speed (to minimize compass errors)

Turn Coordinator (TC) / Turn and Slip Indicator

Turn Coordinator:

• Displays rate of turn and, in some designs, roll rate
• Gyroscope tilted ~30° allows sensing both yaw and roll
• Standard Rate Turn: 3° per second (360° in 2 minutes)
• Indication: Aircraft symbol wings align with index marks
• Powered by: Electric motor

• Inclinometer: Curved glass tube with ball inside, damped with fluid
• Coordinated turn: Ball centered (no slip or skid)
• Slip: Ball falls to inside of turn (insufficient bank for rate of turn, or insufficient rudder)
• Skid: Ball moves to outside of turn (too much bank for rate of turn, or too much rudder)
• Memory aid: "Step on the ball" (apply rudder in direction ball has moved)

• Similar to turn coordinator but senses only yaw rate (no roll rate)
• Gyroscope vertical, senses yaw only

Magnetic Compass

The magnetic compass is the primary self-contained heading reference, requiring no external power.

Operating Principle

• Magnets aligned with Earth's magnetic field
• Compass card attached to magnets, floats in liquid (damping fluid)
• Lubber line: Reference line on compass bowl, aligned with aircraft longitudinal axis

Magnetic Variation and Deviation

Magnetic Variation (Declination):

• Angle between True North and Magnetic North
• Easterly variation: Magnetic North east of True North (subtract from True to get Magnetic)
• Westerly variation: Magnetic North west of True North (add to Magnetic to get True)
• Memory aid: "Variation East, Magnetic Least; Variation West, Magnetic Best"

• Error caused by aircraft magnetic fields (electrical systems, metal structures)
• Compass swing: Procedure to measure and minimize deviation
• Deviation card: Posted near compass, shows deviation on various headings

• True → Magnetic → Compass: Apply variation, then apply deviation
• Compass → Magnetic → True: Remove deviation, then remove variation

Magnetic Compass Errors

Variation and Deviation:

• Systematic errors, corrected using charts and deviation card

• Cause: Magnetic dip (vertical component of Earth's magnetic field) + pendulous mounting
• Northern Hemisphere:
• Acceleration: Compass indicates turn toward North (ANDS: Accelerate North, Decelerate South)
• Deceleration: Compass indicates turn toward South
• Southern Hemisphere: Opposite
• Maximum error: On East-West headings
• Zero error: On North-South headings
• Memory aid: "ANDS" (Accelerate North, Decelerate South) in Northern Hemisphere

• Cause: Magnetic dip + compass card tilt during turn
• Northern Hemisphere:
• Turn to North: Compass lags behind actual heading (undershoot North)
• Turn to South: Compass leads actual heading (overshoot South)
• Southern Hemisphere: Opposite
• Maximum error: On North-South headings
• Zero error: On East-West headings
• Memory aid: "UNOS" (Undershoot North, Overshoot South) in Northern Hemisphere

• Turn to North: Lead rollout by ~(latitude / 2)° (e.g., 30° at 60°N latitude)
• Turn to South: Lag rollout by ~(latitude / 2)°
• Turn to East/West: Rollout on heading

• Compass swings back and forth during turbulence, turns, or acceleration
• Damping fluid reduces oscillation but doesn't eliminate it
• Allow compass to settle before reading heading

• Vertical component of Earth's magnetic field
• Zero at magnetic equator: Compass works best
• Maximum at magnetic poles: Compass nearly useless (vertical magnetic field)

Engine Instruments

Engine instruments monitor powerplant performance and health. Understanding these instruments allows pilots to optimize performance, identify malfunctions, and prevent engine damage.

Turbine Engine Instruments

Primary Power Indication (Thrust):

• Modern turbofans: N1 primary power indicator (more directly related to thrust)

• N2 (%): High-pressure spool RPM
• Indicates engine core health and operability
• Typical: 50-60% idle, 100% maximum

• EGT (Exhaust Gas Temperature): Temperature at turbine exit
• Also called TGT (Turbine Gas Temperature), ITT (Interstage Turbine Temperature), TOT (Turbine Outlet Temperature)
• Most critical parameter for engine health
• Typical range: 400-950°C depending on engine model and phase of flight
• Limits: Red line typically 900-1,000°C for max 5-10 minutes

• Indicates fuel consumption rate (kg/hour or lbs/hour)
• Useful for performance monitoring and fuel management

• Oil pressure: Typically 40-60 PSI, varies by engine
• Oil temperature: Typically 50-100°C operating range
• Low oil pressure or high oil temperature → potential engine damage

• Measured in quarts or liters
• Monitored for leaks or consumption

• Measures engine vibration levels
• High vibration indicates imbalance, bearing wear, blade damage, or FOD

Piston Engine Instruments

Tachometer:

• Indicates engine RPM
• Typically 2,000-2,700 RPM for cruise in most piston aircraft

• Measures pressure in intake manifold (induction system)
• Used with tachometer to set power (e.g., "25 inches, 2,500 RPM")
• Without engine running, reads ambient pressure (~30 inHg at sea level)

• Similar to turbine engines, critical for engine health

• Measures temperature of hottest cylinder
• Indicates cooling effectiveness, mixture setting
• Typical: 300-450°F

• Measures temperature of exhaust gases
• Used for mixture leaning (peak EGT indicates stoichiometric mixture)

• Fuel pressure ensures proper fuel flow
• Fuel quantity indicates remaining fuel

• Shows alternator output or electrical load
• Positive reading indicates charging, negative indicates battery discharge

Electronic Flight Instrument System (EFIS)

EFIS replaces traditional analog instruments with electronic displays, providing flexibility, reliability, and integration.

EFIS Components

Primary Flight Display (PFD):

• Displays primary flight information:
• Attitude (artificial horizon)
• Airspeed (with speed bugs, V-speeds)
• Altitude (with altitude alerting)
• Vertical speed
• Heading
• Flight director (if engaged)
• Autopilot / autothrottle modes
• Approach mode information (ILS deviation, glideslope, localizer)

• Displays navigation and situational information:
• Map mode: Moving map with flight plan, waypoints, navaids
• Plan mode: Top-down view of flight plan
• VOR mode: Traditional VOR display
• ILS mode: ILS approach information
• Weather radar
• TCAS (Traffic Collision Avoidance System)
• EGPWS / TAWS (Enhanced Ground Proximity Warning System / Terrain Awareness Warning System)

EFIS Advantages

• Reduced pilot workload: Integrated information reduces scanning
• Improved situational awareness: Graphical displays easier to interpret
• Flexibility: Display modes configurable
• Reliability: Solid-state electronics, no moving parts
• Redundancy: Multiple displays, reversionary modes

Reversionary Modes

If one display fails, critical information can be shown on remaining display:

• PFD fails → Essential flight information shown on ND or standby display
• ND fails → Essential navigation information shown on PFD

Standby Instruments

Even in glass cockpits, standby instruments provide backup:

• Standby airspeed indicator
• Standby altimeter
• Standby attitude indicator (often electric, independent power source)
• Standby magnetic compass (required by regulations)

Engine Indicating and Crew Alerting System (EICAS) / Electronic Centralized Aircraft Monitor (ECAM)

EICAS (Boeing) and ECAM (Airbus) are systems that monitor aircraft systems and provide alerts to the crew.

EICAS (Boeing)

EICAS Displays:

• Upper EICAS: Primary engine parameters (N1, EGT, N2, fuel flow, etc.)
• Lower EICAS: Secondary engine parameters, system synoptics, alerts

• Master Warning (red): Illuminates with red alerts, aural warning
• Master Caution (amber): Illuminates with amber alerts, aural attention
• Push to cancel aural alert and extinguish master warning/caution lights

ECAM (Airbus)

ECAM Displays:

• Engine/Warning Display (E/WD): Upper display, engine parameters and warnings/cautions
• System Display (SD): Lower display, detailed system synoptics

• Automatic display of relevant system pages when malfunction detected
• Checklists presented on display (electronic checklist)
• Crew actions:
• Immediate action items (red): Perform from memory
• Checklist items (blue/white): Follow ECAM prompts

• Automatically changes displayed information based on flight phase (takeoff, climb, cruise, descent, approach, landing)
• Suppresses non-critical alerts during critical phases (e.g., takeoff, landing)

• ECAM more automated, presents checklists
• EICAS requires crew to reference paper QRH for most checklists
• Both effective, differences reflect manufacturer philosophy

Flight Management System (FMS)

The Flight Management System is the heart of modern airliner avionics, integrating navigation, flight planning, performance, and guidance.

FMS Functions

Flight Planning:

• Route entry: Enter departure, arrival, airways, waypoints
• SID/STAR: Standard Instrument Departures / Standard Terminal Arrival Routes
• Performance initialization: Enter weights, cost index, cruise altitude, cruise speed

• Position determination: Combines GPS, IRS, VOR/DME, and other sources
• RNP (Required Navigation Performance): Specifies navigation accuracy required
• CDI (Course Deviation Indicator): Displays lateral deviation from planned track

• V-speeds calculation: Calculates V1, VR, V2 based on weight, temperature, runway
• Optimum altitude: Recommends most efficient cruise altitude
• Fuel predictions: Estimates fuel at waypoints, destination, alternate

• LNAV (Lateral Navigation): Autopilot follows horizontal flight plan
• VNAV (Vertical Navigation): Autopilot follows vertical profile (climbs, descents, level-offs)

FMS Components

Control Display Unit (CDU) / Multifunction Control Display Unit (MCDU):

• Interface for crew interaction with FMS
• Keypad and display for data entry and retrieval
• Common pages:
• INIT (Initialization): Aircraft data, route, performance init
• RTE (Route): Flight plan entry
• PERF (Performance): Takeoff, climb, cruise, descent performance data
• PROG (Progress): Current position, estimates, fuel predictions
• NAV (Navigation): Navigation accuracy, position updating

• Processes navigation data, computes flight plan, guidance commands
• Dual or triple redundancy typical

• GPS (Global Positioning System): Primary position source, highly accurate
• IRS (Inertial Reference System): Inertial navigation, no external references required
• VOR/DME: Radio navigation, cross-check for GPS/IRS
• Updating: FMC compares sources, selects most accurate, alerts crew if discrepancy

FMS Navigation Accuracy

RNP (Required Navigation Performance):

• Specifies navigation accuracy required for specific airspace or procedure
• Example: RNP 1.0 = aircraft must be within 1 NM of planned path 95% of the time
• FMS Navigation Specifications:
• RNP 10: Oceanic (10 NM)
• RNP 4: Remote continental
• RNP 1: Terminal area, some SIDs/STARs
• RNP 0.3: Precision approach (RNP AR APCH)

• RAIM (Receiver Autonomous Integrity Monitoring): GPS receiver checks satellite signal integrity
• RAIM prediction: FMS predicts GPS availability along route
• Required for GPS-based navigation in IFR

FMS Vertical Navigation (VNAV)

VNAV Modes:

• VNAV CLB: Climbs at optimum speed/Mach, economy climb
• VNAV CRZ: Maintains cruise altitude, can execute step climbs
• VNAV DES: Descends at optimum profile, idle thrust when possible, arrives at altitude constraints

• Geometric path: Fixed vertical angle (e.g., 3° glidepath)
• Vertical speed: Fixed rate of descent (e.g., 800 fpm)

• AT: Cross waypoint at specified altitude
• AT OR ABOVE: Cross at specified altitude or higher
• AT OR BELOW: Cross at specified altitude or lower
• BETWEEN: Cross between two altitudes

• Entered in flight plan (e.g., 250 kts below 10,000 ft)
• VNAV adjusts vertical profile to meet speed and altitude constraints

Warning and Alerting Systems

Master Warning and Master Caution

Master Warning (Red):

• Critical situations requiring immediate action
• Examples: Fire, engine failure, hydraulic failure, pressurization loss
• Aural warning (continuous horn, siren, voice alert)
• Silenced by pressing master warning button/switch

• Abnormal situations requiring awareness and possible action
• Examples: Low fuel pressure, generator off-line, anti-ice required
• Aural attention-getter (single chime, beep)
• Silenced by pressing master caution button/switch

Ground Proximity Warning System (GPWS) / Enhanced GPWS (EGPWS)

GPWS (Older System):

• Uses radar altimeter to detect proximity to terrain
• Modes:
• Mode 1: Excessive descent rate ("Sink Rate, Pull Up")
• Mode 2: Excessive terrain closure rate ("Terrain, Pull Up")
• Mode 3: Altitude loss after takeoff ("Don't Sink")
• Mode 4: Unsafe terrain clearance ("Too Low Terrain", "Too Low Gear", "Too Low Flaps")
• Mode 5: Excessive deviation below glideslope ("Glideslope")

• Uses GPS position and terrain database (in addition to radar altimeter)
• Look-ahead capability: Predicts terrain conflicts ahead of aircraft
• Visual alerts: Display terrain on ND (red = warning, yellow = caution)
• Reduced nuisance alerts: Better discrimination of actual threats

• Some modes inhibited during approach (e.g., Mode 4 "Too Low Gear" inhibited when gear intentionally up for go-around)

Traffic Collision Avoidance System (TCAS)

TCAS II (Standard for Transport Aircraft):

• Provides traffic advisories (TA) and resolution advisories (RA)
• TA (Traffic Advisory): "Traffic, Traffic" + visual display → Increases awareness, no maneuver
• RA (Resolution Advisory): "Climb, Climb" or "Descend, Descend" → Maneuver required
• TCAS operates independently of ATC: Pilots must follow RA even if contrary to ATC instruction (inform ATC afterward)

• Integrated on ND
• Intruder symbols:
• Other traffic (white): No threat
• Proximate traffic (cyan/white): Within 6 NM and ±1,200 ft
• TA (amber): Traffic advisory
• RA (red): Resolution advisory
• Vertical speed indicator: Shows RA guidance (green arc = fly here, red arc = avoid)

• TCAS II systems coordinate RAs between aircraft
• One aircraft instructed to climb, the other to descend (or one to maintain current VS, the other to maneuver)

Stall Warning System

Angle of Attack (AOA) Sensors:

• Measure angle between chord line and relative wind
• Primary input for stall warning

• Stick shaker: Mechanically vibrates control yoke/stick, activated at ~5-10 kts above stall speed
• Stick pusher: Automatically pushes yoke/stick forward if stall imminent (some aircraft)
• Aural warning: Voice alert "Stall, Stall"

• Automatic thrust increase when AOA approaches critical level
• Prevents stall by maintaining adequate speed/AOA

Overspeed Warning

VMO / MMO Overspeed:

• Activated when airspeed exceeds maximum operating speed
• Aural warning: Clacker, horn, or voice "Overspeed"
• Visual warning: Barber pole on ASI/PFD

EASA Learning Objectives - Subject 022

According to the EASA ATPL syllabus, candidates must demonstrate knowledge of:

Pitot-Static Instruments

• ASI, altimeter, VSI: Operating principles, components, errors (instrument, position, density, compressibility, temperature, pressure)
• IAS, CAS, TAS, groundspeed, Mach number: Definitions, relationships, calculations
• Altimeter settings: QNH, QFE, standard (1013.25 hPa), transition altitude/level
• Blockages: Effects of pitot and static blockages on each instrument
• Alternate static source: Purpose, effects on instruments

Gyroscopic Instruments

• Gyroscopic principles: Rigidity, precession
• Attitude indicator: Operating principle, power sources, errors (real/apparent wander, acceleration, turning), gimbal limits
• Heading indicator: Operating principle, drift (real wander, apparent wander, mechanical), resetting procedure
• Turn coordinator: Operating principle, standard rate turn, slip/skid indicator

Magnetic Compass

• Operating principle: Magnetic field, compass card, damping
• Variation and deviation: Definitions, corrections, compass swing
• Compass errors: Acceleration error, turning error, oscillation, magnetic dip
• Memory aids: ANDS, UNOS

Engine Instruments

• Turbine engines: EPR, N1, N2, EGT, fuel flow, oil pressure/temperature/quantity, vibration
• Piston engines: Tachometer, manifold pressure, oil pressure/temperature, CHT, EGT, fuel pressure/quantity

EFIS

• PFD and ND: Components, displays, modes
• Advantages and disadvantages
• Reversionary modes
• Standby instruments

EICAS / ECAM

• Purpose and components
• Alert levels: Warning (red), caution (amber), advisory, memo
• Master warning / caution
• ECAM automatic checklists (Airbus)

FMS

• Functions: Flight planning, navigation, performance, guidance
• Components: CDU/MCDU, FMC, position sensors
• Navigation accuracy: RNP, GPS, RAIM
• LNAV and VNAV: Lateral and vertical navigation
• Flight plan entry and modification

Warning and Alerting Systems

• Master warning and caution
• GPWS / EGPWS: Modes, alerts, look-ahead capability
• TCAS: TA, RA, display symbology, crew response
• Stall warning: Stick shaker, stick pusher, AOA sensors
• Overspeed warning

Exam Tips & Common Questions

Memory Aids

Pitot-Static Blockages:

• "Pitot blocked, drain blocked → ASI acts as altimeter"
• "Static blocked → ASI, ALT, VSI all unreliable"

• "High to Low, Look Out Below" (pressure or temperature)
• "Hot to Cold, Look Out Below"

• "ANDS" (Accelerate North, Decelerate South) - Northern Hemisphere acceleration error
• "UNOS" (Undershoot North, Overshoot South) - Northern Hemisphere turning error

• "TAS = CAS + 2% per 1,000 ft"

• "3° per second = 360° in 2 minutes"
• Bank angle for standard rate ≈ (TAS / 10) + 7°
• Example: At 150 kts TAS → (150/10) + 7 = 22° bank

High-Yield Topics

Based on historical exam analysis, these topics appear frequently:

1. Pitot-static system and blockages (15-20% of questions)

• Effects of pitot blocked (drain open/closed)
• Effects of static blocked
• Alternate static source effects

1. Altimeter errors (10-15% of questions)

• Temperature error (hot/cold, high/low)
• Pressure error (QNH setting)
• True altitude calculations

1. Magnetic compass errors (10-15% of questions)

• Acceleration error (ANDS)
• Turning error (UNOS)
• Deviation and variation

1. Gyroscopic instrument errors (8-12% of questions)

• Attitude indicator errors during acceleration, turning
• Heading indicator drift

1. EFIS components and displays (8-12% of questions)

• PFD and ND information
• Reversionary modes

1. FMS navigation and guidance (8-12% of questions)

• LNAV and VNAV modes
• RNP values
• GPS/RAIM

1. GPWS/EGPWS and TCAS (8-12% of questions)

• GPWS modes and alerts
• TCAS TA vs. RA
• Crew responses

1. EICAS/ECAM alert levels (5-10% of questions)

• Red (warning), amber (caution), white (memo)
• Master warning/caution

Common Mistakes to Avoid

• Confusing IAS, CAS, and TAS: Understand the differences and when each is used
• Forgetting to apply temperature correction to altimeter: Cold temperatures are especially critical
• Reversing compass error memory aids: ANDS and UNOS are for Northern Hemisphere only
• Mixing up N1 and N2: N1 is fan/LP spool, N2 is core/HP spool
• Not understanding TCAS priorities: RA must be followed immediately, takes priority over ATC instructions
• Ignoring standby instruments: Even in glass cockpits, standby instruments are critical backups

Tricky Question Types

Blockage Scenarios:

• Questions often present specific blockage scenarios and ask about instrument behavior
• Example: "If the pitot tube is blocked but the drain hole remains open, the ASI will..."
• Answer: Read zero (no pressure differential)

• Questions may ask for true altitude given indicated altitude and temperature deviation
• Example: "Flying at 10,000 ft indicated, OAT -20°C (ISA -5°C). True altitude?"
• Temperature 15°C colder than ISA → ~4 ft/°C/1,000 ft error → 600 ft over-read → ~9,400 ft true altitude

• Questions often ask about rollout heading from turns
• Example: "Turning from 270° to 360° (North) at 60°N latitude. When should you begin rollout?"
• Answer: ~30° early (latitude/2), so rollout at ~330° indicated

Practical Application for Pilots

Instrument Cross-Checking

Basic T Scan Pattern:

• Central focus on attitude indicator
• Cross-check with:
• Airspeed (left)
• Altimeter (right)
• Heading indicator (bottom)
• VSI (right of altimeter)
• Expand scan to engine instruments, navigation displays as needed

• Unreliable airspeed: Compare captain and first officer ASI, cross-check with GPS groundspeed and pitch/power
• Altimeter discrepancy: Compare altimeters, check QNH setting, consider temperature error
• Attitude indicator failure: Use standby attitude indicator, recognize excessive bank/pitch indications

Glass Cockpit Transition

• Scan pattern: Similar to analog but information more integrated
• Mode awareness: Constantly monitor autopilot and FMS modes
• Automation management: Understand what the automation is doing and what it will do next
• Reversion skills: Practice using standby instruments, be ready for display failures

FMS Operation

• Flight plan verification: Always cross-check FMS route against ATC clearance, charts
• Performance data accuracy: Incorrect weights or temperatures lead to incorrect V-speeds and performance predictions
• VNAV monitoring: Don't blindly follow VNAV, ensure altitude and speed constraints will be met
• GPS navigation: Understand GPS limitations (satellite coverage, RAIM, signal interference)

Alerting System Response

GPWS/EGPWS Warnings:

• Immediate response: Disconnect autopilot, max thrust, rotate to initial pitch (15-20°), retract speedbrakes if extended
• "Terrain, Terrain, Pull Up": Escape maneuver, don't delay

• Immediate response: Follow RA guidance on VSI (fly into green, avoid red)
• Inform ATC: "TCAS RA" call once clear of conflict
• Do not maneuver opposite to RA: Even if ATC instructs differently

• Stick shaker activation: Immediate nose-down input, max thrust, minimize bank
• Do not pull: Pulling back on controls worsens stall

Study Strategy for Instrumentation

Recommended Study Sequence

1. Pitot-static system (1 week)

• Master ASI, altimeter, VSI operating principles
• Focus heavily on errors and blockages

1. Gyroscopic instruments (1 week)

• Understand gyroscopic principles first
• Study each instrument's operating principle and errors

1. Magnetic compass (2-3 days)

• Memorize ANDS and UNOS
• Practice compass error calculations

1. Engine instruments (3-4 days)

• Focus on turbine engine instruments (EPR, N1, N2, EGT)
• Understand normal indications and limits

1. EFIS, EICAS/ECAM (1 week)

• Study PFD and ND displays and modes
• Understand alert levels and crew response

1. FMS (1 week)

• Understand FMS functions and components
• Practice flight plan entry and navigation concepts

1. Warning systems (3-4 days)

• GPWS/EGPWS, TCAS, stall warning, overspeed
• Focus on crew responses

Study Resources

• EASA Syllabus: Review the official learning objectives
• Aircraft Flight Manuals: Real-world instrument descriptions (Boeing, Airbus)
• Flight simulator: Practice with EFIS and FMS (even desktop simulators like X-Plane, MSFS helpful)
• Question banks: Aviationexam, Bristol Ground School, ATPL Ground Training
• YouTube: Cockpit videos showing real instrument behavior

Study Tips

• Draw instrument diagrams: Sketch pitot-static system, gyroscopic instruments to solidify understanding
• Use flashcards: Especially for errors, memory aids, definitions
• Practice calculations: TAS from CAS, true altitude from indicated, compass rollout headings
• Watch cockpit videos: See how instruments behave in real flight (takeoff, climb, cruise, descent, approach, landing)
• Flight simulator practice: If available, use simulator to familiarize with EFIS, FMS, alerting systems

Integration with Other Subjects

Instrumentation connects with several other ATPL subjects:

• Aircraft General Knowledge: Systems that power instruments (electrical, pneumatic, hydraulic)
• Principles of Flight: Airspeed definitions, angle of attack
• General Navigation: Position determination, navigation aids
• Radio Navigation ^[?]: VOR, DME, ILS displayed on instruments
• Flight Planning: FMS flight planning and performance
• Human Performance: Instrument scanning, automation management, situational awareness

Conclusion

Instrumentation is a foundational subject for professional pilots, directly relevant to daily flight operations. From basic analog instruments to advanced integrated avionics, understanding how instruments work, their limitations, and how to use them effectively is critical for safe and efficient flight.

The key to success in Instrumentation is understanding operating principles and error sources. Don't just memorize that "cold temperatures cause the altimeter to over-read"—understand why (pressure lapse rate changes with temperature). This deeper understanding will help you answer exam questions and, more importantly, make sound decisions as a pilot.

Focus on high-yield topics (pitot-static blockages, altimeter errors, compass errors, GPWS/TCAS responses) but don't neglect other areas. The exam is comprehensive, and a well-rounded understanding is necessary.

Practice with question banks, but always refer back to source materials when you get questions wrong. Use flight simulators if available to gain hands-on experience with EFIS and FMS. Watch cockpit videos to see instruments in action during real flights.

Instrumentation knowledge will serve you throughout your aviation career, from your first type rating to your last flight. Master the fundamentals, understand the systems, and practice effective instrument cross-checking—these skills will keep you safe and proficient for years to come.

Related Articles:

• Aircraft General Knowledge - ATPL Subject 021
• Principles of Flight - ATPL Subject 081
• General Navigation - ATPL Subject 061
• Radio Navigation - ATPL Subject 062 ^[?]
• Flight Planning - ATPL Subject 033
• Operational Procedures - ATPL Subject 070
• Human Performance - ATPL Subject 040

• Review EASA Learning Objectives thoroughly
• Practice pitot-static blockage scenarios
• Memorize compass error memory aids (ANDS, UNOS)
• Begin practicing with Instrumentation question banks