RPL Exam Aircraft General Knowledge: Engines, Fuel & Instruments

Aircraft General Knowledge (AGK) is one of the most technical subjects on the CASA RPL theory exam. It covers how your aircraft actually works — from the piston engine and fuel system to the instruments in the cockpit and the structure holding it together. This guide covers the core AGK topics with the depth needed to answer exam questions confidently.

The piston engine: four strokes

Most RPL training aircraft use a four-stroke, air-cooled, horizontally-opposed piston engine. The four strokes are:

1. Induction — piston moves down, intake valve opens, fuel-air mixture enters the cylinder.
2. Compression — both valves close, piston moves up, mixture is compressed (approximately 8:1 ratio).
3. Power — spark plug fires, mixture ignites, piston is forced down.
4. Exhaust — exhaust valve opens, piston moves up, burnt gases are expelled.

Dual ignition system

Aircraft engines have two completely independent magneto systems, each with its own spark plugs. This provides redundancy and improves combustion efficiency — two plugs burn the mixture more completely than one.

The magneto check before flight involves switching from BOTH to LEFT (note the RPM drop), then back to BOTH, then from BOTH to RIGHT. An excessive drop or an unacceptable difference between magnetos indicates a problem. A zero drop on one magneto is also dangerous — it may indicate the magneto is not grounding (not cutting out), which means it cannot be switched off. Do not fly if this occurs. Always check your aircraft's Pilot Operating Handbook (POH) for the specific maximum allowable RPM drop, as this varies between aircraft types.

Carburettor icing: a critical exam topic

Carburettor ice can form even on warm days. As air passes through the venturi, pressure drops and temperature falls significantly. Combined with the cooling effect of fuel evaporating, ice can form on carburettor walls and eventually block airflow.

Symptoms of carburettor icing include unexplained power loss and rough running. Conditions: carburettor icing can occur at outside air temperatures (OAT) up to 38°C when humidity is high, especially at reduced power settings (such as during descent). It is most likely in moist conditions with moderate temperatures, but the risk extends well beyond what many pilots expect. Cure: apply carb heat. Initial application may cause a brief RPM drop (the heated air enriches the mixture; also, melting ice may temporarily restrict airflow). If ice was present, the rough running will clear and RPM will recover above the previous setting.

Fuel systems

AVGAS 100LL

Standard fuel for piston training aircraft is AVGAS 100LL. Colour: blue. It is 100 octane and low lead. Using the wrong fuel — for example, Jet-A1, which is clear or straw-coloured — can cause immediate engine damage. Never fuel a piston aircraft with anything other than the specified AVGAS grade.

Pre-flight fuel check

Sump (drain) each fuel tank and the fuel strainer before flight. Check for:

• Water — appears as a clear bubble at the bottom of the sample. Water is denser than AVGAS and does not mix with it. Even a small amount can cause engine failure.
• Sediment — visible as particles or cloudiness in the sample.
• Wrong fuel — check colour and smell against the expected grade.

VFR fuel reserves

For day VFR operations in piston-engine or turboprop aircraft at or below 5,700 kg Maximum Take-off Weight (MTOW), CASR Part 91 requires a minimum fuel reserve of 30 minutes at normal cruise consumption over and above the fuel required for the planned flight. Night VFR requires a 45 minutes reserve.

Pitot-static instruments

Three cockpit instruments depend on pitot and/or static pressure. Understanding how each one fails is as important as knowing how each one works — both are frequently tested in the CASA exam.

Airspeed indicator (ASI)

The airspeed indicator (ASI) measures the difference between pitot (dynamic) pressure and static pressure. If the pitot tube is blocked by ice or insects, the ASI will freeze at its current reading or behave erratically. In some configurations it may read like an altimeter if the pitot tube has a drain hole that is also blocked, increasing its reading as the aircraft climbs. If the static port is blocked, all three pitot-static instruments are affected.

Altimeter

The altimeter measures static pressure and converts it to altitude. It must be set to local QNH before flight. Key exam point: flying from a high-pressure area to a low-pressure area without resetting QNH causes the altimeter to over-read — the aircraft is lower than indicated. This is dangerous near terrain.

Memory aid: HALT — High to Low, Altimeter reads Too high (true altitude is lower than indicated).

Vertical speed indicator (VSI)

The vertical speed indicator (VSI) measures the rate of change of static pressure. It has a lag of 6–9 seconds and does not show instantaneous rate of climb or descent. It is useful for confirming an established climb or descent, but not for the initial detection of altitude changes.

Gyroscopic instruments

The two main gyroscopic instruments in a light aircraft are the directional indicator and the attitude indicator. Both are driven by the vacuum system and will fail if the vacuum pump fails.

Directional indicator (DI)

The directional indicator (DI) is driven by a vacuum pump. It shows heading but does not respond to Earth's magnetic field — it simply maintains its orientation and drifts due to gyroscopic precession and Earth's rotation. It must be aligned with the magnetic compass every 10–15 minutes. A DI that drifts rapidly may indicate a vacuum pump problem.

Attitude indicator (AI)

The attitude indicator (AI) shows pitch and bank attitude. It takes approximately 5 minutes to erect fully after startup and may topple in extreme attitudes. If the vacuum pump fails in flight, the AI and DI will slowly spin down and give unreliable readings. Suspect vacuum failure if both instruments behave erratically at the same time.

Magnetic compass errors

The magnetic compass is self-contained (no power required) but has predictable errors that are frequently tested in the CASA exam:

• Variation — the angle between magnetic north and true north. In eastern Australia, variation is approximately 12°E (magnetic north is east of true north).
• Deviation — caused by the aircraft's own metal components and electrical equipment. Recorded on the cockpit deviation card.
• Turning errors — the compass leads and lags on turns through north and south. In the southern hemisphere, the compass leads on turns through south and lags on turns through north.
• Acceleration errors (ANDS) — Accelerate = compass swings towards North; Decelerate = swings towards South. These errors occur on headings near east or west in the southern hemisphere.

Electrical systems

Most light aircraft have a 14V or 28V DC electrical system. Key components are:

• Alternator — engine-driven, charges the battery and powers electrical loads. Works effectively at lower RPM and is lighter than older generator designs.
• Battery — provides startup power and backup if the alternator fails. It has limited capacity, so reducing electrical load extends battery life after alternator failure.
• Ammeter — monitors the electrical system. On a left-zero ammeter (loadmeter), a zero reading while the engine is running indicates the alternator is no longer producing current. On a centre-zero ammeter, a continuous discharge (needle deflected to the negative side) indicates the battery is supplying all electrical power because the alternator has failed. In either case, reduce non-essential electrical load immediately and land as soon as practical.

Load factors and stall speed

Load factor (measured in G) increases in banked turns. Stall speed increases with load factor — this is why steep turns can result in an inadvertent stall. The table below shows how load factor and stall speed change with bank angle:

Bank angle	Load factor	Stall speed increase
0°	1.0 G	0%
30°	1.15 G	7%
45°	1.41 G	19%
60°	2.0 G	41%

At 60° bank, an aircraft that stalls at 55 kt in level flight now stalls at approximately 78 kt. Pilots who enter steep turns at low altitude risk stall or spin entry if they do not maintain sufficient airspeed.

Common AGK exam questions

The following questions reflect the patterns you will see in the CASA RPL AGK exam. This is frequently tested material — make sure you understand the reasoning behind each answer, not just the answer itself.

• "What indicates a magneto not grounding properly?" — A zero RPM drop when switched to one magneto position. The magneto is live and cannot be deactivated — this is a safety defect. Do not fly.
• "What colour is AVGAS 100LL?" — Blue.
• "Why does the altimeter over-read when flying from high to low pressure?" — The ambient pressure decreases but the altimeter still reads as if at the higher pressure datum, making the aircraft appear higher than it actually is.
• "What happens on initial carb heat application if ice is present?" — RPM drops further briefly (enriched mixture plus melting ice restricting airflow) before clearing and recovering to above the original setting.
• "What is the ANDS rule?" — Accelerate North, Decelerate South: the magnetic compass swings north on acceleration and south on deceleration when on east or west headings in the southern hemisphere.
• "What indicates alternator failure on a left-zero ammeter?" — A zero reading while the engine is running. On a centre-zero ammeter, a continuous discharge (negative deflection) indicates alternator failure.
• "At what OAT can carburettor icing occur?" — Carburettor icing can occur at outside air temperatures up to 38°C when humidity is sufficient, particularly at reduced power settings.

Practice AGK questions

AGK questions follow recognisable patterns once you have studied the core concepts. Work through the AGK question set on The Pilot Exam — our 1,000+ question bank covers all subjects with immediate answer explanations, so you understand the reasoning rather than just memorising answers.