RPL Exam Instruments: Pitot-Static System, Gyroscopic Instruments, and Compass

Cockpit instruments are the pilot's primary source of information about what the aircraft is doing. The RPL exam — administered by CASA via its approved testing providers — tests how each instrument works, what it measures, and critically, what happens when it fails or gives incorrect readings. Understanding the system behind each instrument is what the exam is really about.

The pitot-static system

Three flight instruments are powered by the pitot-static system: the Airspeed Indicator (ASI), the Altimeter, and the Vertical Speed Indicator (VSI). This system uses two sources of air pressure:

• Pitot pressure (ram air): total air pressure from a forward-facing pitot tube — can be blocked by ice, insects, or a pitot cover left on
• Static pressure: ambient atmospheric pressure from static ports on the side of the fuselage. An alternate static source is available in most aircraft if the primary ports become blocked.

Pitot-static failure modes

Understanding what each failure does to each instrument is a core exam topic. The table below summarises the effects of the most commonly tested failure scenarios.

Failure	ASI	Altimeter	VSI
Pitot tube blocked	Reads zero (or acts like altimeter — rises in climb)	Unaffected	Unaffected
Static port blocked	Freezes at blocked-moment value; over/under-reads with altitude change	Freezes at altitude when blocked	Reads zero
Alternate static selected	Slightly high (cabin pressure slightly lower)	Slightly high	Momentary indication then normal

Airspeed Indicator (ASI)

The ASI measures the difference between pitot and static pressure (dynamic pressure), which relates directly to airspeed. It is the instrument most directly affected by a pitot tube blockage, and one of the most frequently examined instruments in the RPL theory exam.

Types of airspeed

• IAS (Indicated Airspeed): what the ASI reads directly
• CAS (Calibrated Airspeed): IAS corrected for instrument and position error
• TAS (True Airspeed): actual speed through the air mass — CAS corrected for air density. At altitude, TAS is higher than IAS because the air is thinner.
• Groundspeed: TAS corrected for wind

ASI colour coding

The coloured arcs and markings on the ASI face define the aircraft's operating speed ranges. These are fixed for the aircraft type and must be understood for both normal operations and the exam.

• White arc: flap operating range — from V_S0 (stall speed in landing configuration) to V_FE (maximum flap extended speed)
• Green arc: normal operating range — from V_S1 (stall speed in clean configuration) to V_NO (maximum structural cruising speed)
• Yellow arc: caution range — smooth air only, from V_NO to V_NE
• Red line: V_NE — Never Exceed Speed

Altimeter

The altimeter measures static pressure and converts it to altitude using the International Standard Atmosphere (ISA) pressure/altitude relationship. The Kollsman window (subscale) sets the pressure reference — either the local QNH or the standard pressure setting of 1013.2 hPa.

Altimeter errors

Two errors are commonly tested in the RPL exam. Both can result in actual altitude being lower than indicated, which makes them safety-critical.

• Temperature error: in cold temperatures the altimeter over-reads — actual altitude is lower than indicated. The memory aid "high to low, look out below" applies here — flying from high pressure to low pressure also means you are lower than the altimeter shows.
• Lag: the altimeter lags slightly during rapid climbs or descents.

Vertical Speed Indicator (VSI)

The VSI measures the rate of change of static pressure, displaying climb or descent in feet per minute. It has a lag of approximately 6–9 seconds, meaning it does not instantly reflect changes in vertical speed. When the static port is blocked, the VSI reads zero. When alternate static is selected, there may be a momentary indication before returning to normal.

Gyroscopic instruments

Three instruments use gyroscopes: the Attitude Indicator (AI), the Heading Indicator (HI), and the Turn Coordinator. Gyroscopes exploit two key properties:

• Rigidity in space: a spinning gyro resists changes to its axis of rotation
• Precession: a force applied to a gyro causes movement 90° ahead in the direction of rotation

Attitude Indicator (AI)

The AI uses a vacuum-driven gyro to maintain a stable reference horizon, displaying both pitch and bank attitude. It is one of the most important instruments for maintaining controlled flight.

Key exam points for the AI:

• Allow 5 minutes warm-up before flight for the gyro to erect fully
• If the aircraft exceeds the operating limits — typically around ±60° pitch or ±110° bank — the gyro tumbles and gives false readings. Wait for re-erection before trusting the instrument.

Heading Indicator (HI)

The HI provides a stable heading reference without the oscillation errors of the magnetic compass. However, it must be aligned with the magnetic compass regularly — every 10–15 minutes is the standard practice. The HI precesses due to Earth's rotation and bearing friction, causing it to drift off the correct heading over time.

Turn Coordinator

The Turn Coordinator shows rate of turn. A standard rate turn is 3°/second, completing a full 360° in 2 minutes. The inclinometer ball within the instrument shows whether the turn is coordinated:

• Ball centred: the turn is coordinated
• Ball displaced: apply rudder in the direction the ball has moved — the memory aid is "step on the ball"

Magnetic compass

The magnetic compass is the primary heading reference — and the backup to the HI — and requires no electrical or vacuum power to operate. Because it needs no power source, it remains functional even during electrical or vacuum system failures. However, it is subject to several errors that pilots must understand.

• Variation: the angle between true north and magnetic north — a geographic phenomenon, not an instrument error
• Deviation: local magnetic interference from the aircraft's own components, recorded on the deviation card in the cockpit
• Northerly turning error: near north headings, the compass leads or lags due to magnetic dip
• Acceleration/Deceleration error (ANDS): on east/west headings — Accelerate North, Decelerate South in the Southern Hemisphere
• Oscillation: the compass swings in turbulence — only read the compass in steady, straight-and-level flight

Vacuum system

The AI and HI are typically driven by an engine-driven vacuum pump. Suction should read between 4.5 and 5.5 inHg. A failed vacuum pump causes subtle AI and HI errors that worsen over time — the magnetic compass and VSI remain functional. This type of failure is particularly insidious because the instruments degrade gradually, especially if the pilot is relying heavily on them.

Common exam traps

The following points represent the most frequently tested distinctions in the RPL instruments topic. Review these carefully before your exam.

• Blocked pitot: only the ASI is affected — the altimeter and VSI continue to work normally
• Blocked static: the altimeter freezes, the VSI reads zero — the ASI under- or over-reads depending on the direction of altitude change
• HI drift: the HI must be aligned to the compass regularly — it drifts over time due to precession
• ANDS: Accelerate North, Decelerate South — applies on east/west headings in the Southern Hemisphere
• Ball correction: ball displaced to the left → apply left rudder; ball displaced to the right → apply right rudder
• Standard pressure: the standard pressure setting used in Australian CASA materials is 1013.2 hPa

Related Resources

• Aircraft Instruments Study Guide
• All RPL Exam Topics