Aircraft Instruments and Systems: CASA RPL/PPL Study Guide

The pitot-static system relies on measuring air pressure to provide you with altitude, rate of climb, and airspeed information. The system uses two types of pressure: pitot (dynamic) pressure, which is the pressure of the forward-moving air ramming into the pitot tube, and static pressure, which is the ambient atmospheric pressure surrounding the aircraft.

The three primary pitot-static instruments

Each instrument in the pitot-static system uses pressure differently. Understanding which pressures feed which instruments is essential for diagnosing blockage failures in flight.

• Airspeed Indicator (ASI): The only instrument that uses both pitot and static pressure. It measures the difference between the two pressures to display Indicated Airspeed (IAS). Pitot pressure is fed to one side of a flexible diaphragm (or capsule) inside the instrument case, while static pressure fills the case surrounding the diaphragm. The expansion or contraction of the diaphragm drives the ASI needle.
• Altimeter: Uses only static pressure to measure the aircraft's height above a given pressure datum — for example, QNH for height above mean sea level, or QFE for height above a specific aerodrome. An aneroid capsule inside the instrument expands or contracts as static pressure changes with altitude.
• Vertical Speed Indicator (VSI): Uses only static pressure to measure the rate of change in altitude. It compares the current static pressure with a slightly delayed reference pressure inside a sealed case, achieved through a calibrated leak (metering unit). The resulting pressure differential moves the needle to indicate a rate of climb or descent in feet per minute.

System blockages

Instrument failures often occur due to blockages in the pitot tube or static port, commonly caused by ice, insects, or water. Understanding the specific blockage scenario is critical for recognising instrument malfunctions in flight.

Blocked static port

A blocked static port affects all three instruments — the ASI, Altimeter, and VSI — since all three rely on static pressure.

• The Altimeter will freeze at the altitude where the blockage occurred, as it can no longer sense changes in ambient pressure.
• The VSI will indicate zero, as there is no pressure differential across the metering unit.
• The ASI will become inaccurate. Since the static pressure trapped inside the instrument case is fixed, the ASI will overread at altitudes lower than where the blockage occurred (because the trapped static pressure is lower than actual, making the pressure differential appear larger) and underread at altitudes higher than where the blockage occurred (because the trapped static pressure is higher than actual, making the pressure differential appear smaller).

Blocked pitot tube

A blocked pitot tube affects only the ASI. The Altimeter and VSI continue to operate normally. However, the exact behaviour of the ASI depends on whether the pitot drain hole is also blocked.

• Pitot ram air intake blocked and drain hole blocked: The air pressure inside the pitot line is trapped. The ASI effectively behaves like an altimeter — as the aircraft climbs, the static pressure in the instrument case decreases while the trapped pitot pressure remains constant, causing the diaphragm to expand and the ASI to overread (indicate a higher airspeed than actual). Conversely, during a descent, static pressure increases while the trapped pitot pressure remains constant, compressing the diaphragm and causing the ASI to underread (indicate a lower airspeed than actual). At a constant altitude, the ASI may appear to read approximately correctly if the trapped pressure happens to match, but any altitude change will produce erroneous readings. This is an extremely dangerous malfunction because the pilot may not immediately recognise the error.
• Pitot ram air intake blocked but drain hole open: The dynamic pressure trapped in the pitot line will gradually bleed away through the open drain hole. The ASI will progressively drop toward zero as the pitot pressure equalises with the static pressure, since there is no longer any ram air entering the system.

Alternate static source

Many aircraft are equipped with an alternate static source, typically located inside the cockpit or a sheltered area of the airframe. If the primary static port becomes blocked, you can select the alternate static source. Because the alternate source is usually inside the fuselage where pressure is slightly lower than outside ambient pressure (due to aerodynamic suction effects around the airframe), the altimeter will read slightly higher than actual altitude, and the ASI may read slightly higher than actual airspeed when using the alternate static source. You should be aware of these minor inaccuracies.

Pitot heat

To prevent ice from blocking the pitot tube, most aircraft are equipped with a pitot heater. You should activate pitot heat when flying in conditions conducive to icing, including visible moisture at temperatures near or below freezing. Some aircraft also have heated static ports.

Pre-flight checks

During the pre-flight inspection, you should visually check that the pitot tube and static port(s) are clear of obstructions such as insects, dirt, or protective covers. The pitot cover must be removed before flight and the pitot tube opening inspected. On the ground run-up, instruments should be checked for correct indications — the altimeter set to QNH should read within acceptable tolerance of aerodrome elevation, the VSI should read approximately zero, and the ASI should read zero or within a small tolerance.

The altimeter is essentially a sensitive barometer calibrated to indicate height based on the International Standard Atmosphere (ISA) pressure lapse rate. To read correctly, the pilot must input the correct pressure datum into the altimeter's subscale.

Common subscale settings

Two subscale settings are used regularly in Australian operations:

• Local QNH — provides the aircraft's altitude above Mean Sea Level (AMSL).
• Standard Pressure (1,013.2 hPa) — used when flying at Flight Levels.

Setting the subscale incorrectly directly impacts the altitude displayed. As a rule of thumb, increasing the subscale pressure setting winds the altimeter needles up (indicating a higher altitude), while decreasing it winds the needles down. This means that setting a QNH higher than the actual ambient pressure will cause the altimeter to overread the aircraft's true height.

Pressure altitude and density altitude

Understanding the difference between pressure altitude and density altitude is critical for performance calculations. Pressure altitude is the height in the standard atmosphere corresponding to the current atmospheric pressure, and can be read directly from the altimeter by setting 1,013.2 hPa on the subscale.

Gyroscopic instruments rely on two fundamental physical properties of a rapidly spinning rotor: rigidity in space (the tendency of the rotor to remain in a fixed plane) and precession (the reaction of the rotor 90° in the direction of rotation when a force is applied).

The three gyroscopic instruments

There are generally three gyroscopic instruments fitted to a light aircraft, each serving a distinct purpose and powered by a specific source.

• Artificial Horizon (Attitude Indicator): provides pitch and roll information. Usually vacuum (suction) driven.
• Directional Indicator (DI / Heading Indicator): provides stable heading information. Usually vacuum driven. Must be periodically synchronised with the magnetic compass due to apparent wander.
• Turn Coordinator: provides rate of turn and roll information. Usually electrically driven.

Understanding power sources and in-flight failures

Recognising the power source of each gyroscopic instrument is vital for managing in-flight failures. If the suction gauge reads zero, you will lose reliable indications from both the Artificial Horizon and the Directional Indicator. However, an electrically driven Turn Coordinator will remain unaffected, making it a key instrument to cross-check if you suspect a vacuum system failure.

The direct reading magnetic compass is a fundamental, self-contained navigation instrument found in all aircraft. It requires no electrical power or vacuum source, making it an essential backup and primary reference for heading information. It consists of a set of magnetic needles (or a magnetised ring) attached to a compass card, suspended on a pivot within a fluid-filled chamber (bowl).

The compass fluid (typically white spirit or kerosene-based liquid) serves two key purposes:

• Buoyancy: The fluid buoyantly supports the weight of the magnet assembly, significantly reducing the effective weight bearing on the pivot point. This reduces friction on the pivot, allowing the magnets to align freely with the Earth's magnetic field.
• Damping: The fluid damps (resists) oscillations of the compass card, preventing excessive swinging and helping the card settle more quickly on a steady indication.

It is crucial that the chamber does not contain any air bubbles, as bubbles reduce the damping and buoyancy effects and can obscure the compass reading. If a bubble is present, the compass should be reported as unserviceable.

How the compass card is read

Unlike many other instruments, the compass card is read from behind. The card rotates relative to the pilot — the pilot reads the heading on the card at the lubber line (a fixed reference marker on the compass bowl nearest the pilot). Because the magnets remain aligned with the Earth's magnetic field while the aircraft (and lubber line) turns around them, the heading indication changes as the aircraft changes direction.

Magnetic dip and pendulous suspension

The Earth's magnetic lines of flux are only parallel to the surface near the magnetic equator. At all other latitudes, the lines of flux dip towards the magnetic poles. This phenomenon is called magnetic dip. In the Southern Hemisphere, the south-seeking end of the magnetic assembly is pulled downward; in the Northern Hemisphere, the north-seeking end dips.

To prevent the compass card from tilting excessively due to magnetic dip, the magnet assembly is designed with its centre of gravity (CG) slightly below the pivot point, creating a pendulous suspension. This pendulosity keeps the card approximately level but introduces specific compass errors during turns and accelerations. The magnitude of these errors increases with magnetic latitude — that is, they are greater closer to the magnetic poles — because the dip angle increases.

Turning errors

Turning errors are most pronounced when turning through Northerly and Southerly headings. They are caused by the interaction between the pendulous magnet system and the vertical component of the Earth's magnetic field during a banked turn. When the aircraft banks, the compass card tilts and the vertical component of the magnetic field exerts a force on the displaced magnet assembly, causing the card to rotate erroneously.

In the Southern Hemisphere, the following rule applies — ONUS: Overshoot North, Undershoot South:

• When turning through a Northerly heading, the compass will overshoot (lead the turn) — the compass indication will turn faster than the aircraft. You must roll out after the compass passes through your desired heading.
• When turning through a Southerly heading, the compass will undershoot (lag behind the turn) — the compass indication will turn slower than the aircraft. You must roll out before the compass reaches your desired heading.
• When turning through Easterly or Westerly headings, turning errors are minimal and the compass reads approximately correctly.
• The magnitude of turning error increases with magnetic latitude and with the angle of bank.

Note: In the Northern Hemisphere, the rule is reversed — Undershoot North, Overshoot South (UNOS).

Practical tip: When making timed turns using the magnetic compass, it is generally best to roll out of the turn early or late (depending on the error) by a number of degrees that accounts for the turning error. A common technique is to apply a correction of approximately the amount of dip angle for your latitude, adjusting for the bank angle used. At moderate Australian latitudes, a rule of thumb is to allow approximately 15–20° of lead or lag when turning through North or South respectively, though this varies. Practice and experience at your local training area will refine this estimate.

Acceleration and deceleration errors

Acceleration errors occur when the aircraft changes speed while flying on or near Easterly or Westerly headings. The inertia of the pendulous magnet system causes the compass card to tilt when the aircraft accelerates or decelerates. This tilt allows the vertical component of the Earth's magnetic field to act on the magnet assembly, producing a false indication of a turn.

In the Southern Hemisphere, the following rule applies — SAND: South Accelerate, North Decelerate:

• When accelerating on an Easterly or Westerly heading, the compass will falsely indicate a turn towards South.
• When decelerating on an Easterly or Westerly heading, the compass will falsely indicate a turn towards North.
• When flying on Northerly or Southerly headings, acceleration errors are minimal.

The reason for this in the Southern Hemisphere is that the south-seeking end of the magnet is heavier (pulled down by dip). During acceleration, inertia causes the magnet assembly to lag behind, tilting its weighted (southern) end, and the vertical component of the Earth's field rotates the card to indicate a false turn towards South. The reverse occurs during deceleration.

Note: In the Northern Hemisphere, the rule is reversed — the mnemonic is ANDS (Accelerate North, Decelerate South), because the north-seeking end of the magnet is the heavier end due to dip in that hemisphere.

Key exam mnemonic summary for the Southern Hemisphere:

• Turning errors: ONUS — Overshoot North, Undershoot South
• Acceleration errors: SAND — South Accelerate, North Decelerate

Other compass considerations

Variation: The magnetic compass points to Magnetic North, not True North. The angular difference between True North and Magnetic North is called variation (or magnetic declination) and must be applied when converting between true and magnetic headings. In Australia, variation values are shown on aeronautical charts and range from approximately 3°E in Western Australia to approximately 13°E in eastern Australia (these values change slowly over time). The mnemonic for applying variation is: Variation East, Magnetic Least (subtract easterly variation from true heading to get magnetic heading); Variation West, Magnetic Best (add westerly variation).

Deviation: Local magnetic fields within the aircraft (electrical equipment, metal structures, avionics, cargo) can deflect the compass from Magnetic North. This error is called deviation and is recorded on a compass deviation card (compass swing card) located near the compass. Deviation varies with heading and is checked by performing a compass swing. A compass swing must be performed after any maintenance that may have affected the aircraft's magnetic environment (for example, avionics installation, engine work, or structural repairs), after a lightning strike, or if the deviation card is missing or out of date.

Heading calculation: The relationship between True Heading, Magnetic Heading, and Compass Heading is:

True Heading ± Variation = Magnetic Heading ± Deviation = Compass Heading

Compass serviceability: Before flight, check that:

• The compass fluid is clear and free of bubbles.
• The compass card moves freely and is legible.
• The deviation card is current and displayed near the compass.
• The compass reads a sensible heading relative to the aircraft's known orientation (for example, consistent with the runway heading during taxi or pre-takeoff checks).
• No magnetic items (mobile phones, tablets, keys, tools) are placed near the compass, as these will cause additional deviation errors.

Compass accuracy limitations: Even after applying variation and deviation corrections, the magnetic compass is subject to turning and acceleration errors as described above. It is most reliable when the aircraft is in steady, unaccelerated, straight-and-level flight. Pilots should allow the compass to stabilise before reading it after any manoeuvre.

Monitoring engine and electrical instruments is vital for preventing mechanical failures and ensuring safe flight operations. This section covers the key gauges you need to understand for the CASA RPL and PPL exams, including how to interpret abnormal readings and respond appropriately.

Engine instruments

The following engine gauges are commonly tested in the CASA exam. Understanding what each indicates — and what abnormal readings mean — is essential for safe aircraft operation.

• Tachometer (RPM): Indicates engine speed in revolutions per minute. For fixed-pitch propeller aircraft, the tachometer is the primary power indicator.
• Oil pressure gauge: Indicates the pressure of oil circulating through the engine. Oil pressure should rise to the normal (green arc) range within 30 seconds of engine start (or as specified by the manufacturer). A drop in oil pressure demands immediate attention.
• Oil temperature gauge: Indicates the temperature of the engine oil. A rise in oil temperature may indicate insufficient cooling, low oil quantity, or excessive engine load.
• Fuel pressure gauge: Indicates fuel delivery pressure to the engine. A drop in fuel pressure may indicate fuel pump failure or fuel supply issues.
• Cylinder Head Temperature (CHT): Monitors the temperature of the cylinder heads, helping to detect overheating or improper mixture settings.
• Exhaust Gas Temperature (EGT): Used primarily for leaning the fuel–air mixture at cruise power settings.

A combination of low oil pressure and high oil temperature in an aeroplane fitted with a fixed-pitch propeller is the primary indication of low oil quantity. This is a serious condition requiring immediate action — reduce power, monitor the gauges closely, and plan for a precautionary landing if the situation does not improve or worsens.

Electrical system instruments

Electrical system health is monitored via voltmeters, loadmeters, and ammeters. It is critical to distinguish between the two types of ammeter, as each indicates alternator health differently. This is a frequently tested area in the CASA exam.

• Centre-zero ammeter: This instrument shows whether the battery is charging (positive/right deflection) or discharging (negative/left deflection). Shortly after engine start, a positive (charge) reading is expected as the alternator recharges the battery after the energy used during starting. During normal cruise flight with a fully charged battery and a healthy charging system, the reading should settle to at or near zero, indicating the alternator output is meeting the electrical load and the battery is neither charging nor significantly discharging. A continuous negative (discharge) reading during flight indicates an alternator failure, meaning the battery alone is supplying all electrical power and will eventually become depleted.
• Left-zero ammeter (loadmeter): This instrument shows the total electrical load being supplied by the alternator. During normal flight, it displays a positive reading corresponding to the current electrical demand. A zero reading on a left-zero ammeter during flight indicates an alternator failure, as the alternator is producing no output.

Responding to alternator failure

If an alternator failure is suspected — indicated by a continuous discharge reading on a centre-zero ammeter, or a zero reading on a left-zero ammeter — follow these steps:

1. Check the alternator circuit breaker or fuse — reset if tripped (once only).
2. Check the alternator/field switch is ON.
3. If the alternator cannot be restored, shed non-essential electrical load (turn off unnecessary electrical equipment) to conserve battery power.
4. Advise ATC of the situation, as you may eventually lose radio and other electrical services.
5. Plan to land at the nearest suitable aerodrome before battery power is exhausted.
6. Refer to the aircraft's Pilot Operating Handbook (POH) for the specific emergency checklist.

Voltmeter

A voltmeter indicates the electrical system bus voltage. In a typical 14-volt system, normal bus voltage in flight is approximately 13.5–14.5 volts. A declining voltage reading can provide an early indication of alternator problems or excessive electrical load.

Related Resources

• Aircraft Engines
• Navigation
• RPL Instruments Blog