RPL Exam Engines: 4-Stroke Cycle, Magnetos, Fuel System, and Carburettor Icing

The engine section tests whether you understand what's happening inside the engine — enough to recognise abnormal indications and make correct power management decisions. This guide covers everything CASA tests on engines in the RPL exam, including the 4-stroke cycle, dual magneto ignition, fuel and oil systems, mixture control, and carburettor icing.

The 4-stroke cycle

The piston engine operates on a repeating 4-stroke cycle. Understanding each stroke helps you answer questions about combustion, valve timing, and power production.

1. Induction: piston moves down, inlet valve opens, and the fuel-air mixture is drawn into the cylinder.
2. Compression: both valves close, piston moves up, and the mixture is compressed.
3. Power: the spark plug fires, the mixture burns, and expanding gases push the piston down — this is the only power-producing stroke.
4. Exhaust: the exhaust valve opens, the piston moves up, and burnt gases are expelled from the cylinder.

A common exam question asks: "In which stroke does combustion occur?" The answer is the 3rd stroke — the power stroke. This is frequently tested in the CASA exam.

Dual magneto ignition

Aircraft engines use two completely independent ignition systems — two magnetos — each firing one spark plug per cylinder. This means two spark plugs fire per power stroke. The dual system provides redundancy: if one magneto fails, the engine continues running on the other, with a slight RPM and power reduction. The two-plug system also improves combustion completeness.

Magneto check during run-up

The magneto check is performed during the pre-flight run-up to confirm both ignition systems are functioning correctly. The expected results are:

• Switch to LEFT only — a small RPM drop is expected (typically 25–75 RPM).
• Switch to RIGHT only — a similar small RPM drop is expected.
• Return to BOTH — RPM recovers to the original value.

Exam trap: no RPM drop on the magneto check is also a problem. If there is no drop when switching to a single magneto, it means that magneto is still live even when selected off — indicating a grounding wire fault. In this condition, the engine can fire even with the ignition key removed, which is a serious safety hazard.

Fuel system

Training aircraft use AVGAS 100LL (low-lead, dyed blue). It is important not to confuse AVGAS with MOGAS (automotive fuel, green or clear) or AVTUR (jet fuel, straw-coloured). Using the wrong fuel type can cause serious engine damage or failure.

Pre-flight fuel check

Always sump drain from each tank and the fuel strainer before flight. You are checking for water contamination — water sinks to the bottom of the fuel and appears as globules or causes the sample to appear blue-tinted or separated. Do not rely on the previous pilot's check; always perform your own.

Fuel selector

Select the correct tank before flight and check fuel balance periodically during flight. On aircraft where it is available, the BOTH position feeds from both tanks simultaneously. Fuel starvation — where the engine is starved of fuel because an empty tank is selected while a full tank remains selected off — is a leading cause of in-flight power loss, and it is entirely preventable.

Mixture control

As altitude increases, air density decreases. Without adjusting the mixture, the fuel-air ratio becomes progressively richer because the same volume of air contains less oxygen. Correct mixture management is essential for efficient engine operation and preventing damage.

• Rich mixture: required for full power operations — take-off, go-around, and landing — and at low altitude.
• Lean mixture: used at cruise altitude to reduce fuel burn. Set to peak Exhaust Gas Temperature (EGT) or per the Pilot's Operating Handbook (POH) procedure.
• Over-leaning: causes engine roughness, high Cylinder Head Temperature (CHT), and potential detonation.

Carburettor icing — critical exam topic

Carburettor icing is one of the most tested engine topics in the CASA RPL exam. It is particularly dangerous because it can form on warm, humid days — conditions where pilots do not expect ice to be a threat.

How carburettor ice forms

Two cooling effects combine inside the carburettor to create conditions for ice formation:

1. Venturi cooling: air accelerates through the venturi throat, causing the temperature to drop significantly below ambient.
2. Fuel evaporation: vaporising fuel absorbs heat from the surrounding air, further cooling the mixture.

Combined, these effects can drop the temperature inside the carburettor well below freezing, even on a warm day. When humidity is high, moisture in the air freezes on the carburettor walls and throttle plate, restricting airflow and reducing engine power.

Conditions most likely to cause carburettor ice

Carburettor icing risk is not limited to cold weather. Per the Visual Flight Rules Guide (VFRG), the risk can be high at outside air temperatures (OAT) ranging from around 0°C up to 38°C. Peak risk typically occurs in the +10°C to +25°C range with high humidity, but you should not assume you are safe above 25°C or 30°C. The conditions most associated with carburettor icing are:

• OAT between approximately 0°C and 38°C — well above freezing (per the VFRG).
• High relative humidity (above 60%).
• Low power settings — such as during descent, circuit flying, or any reduced-throttle operation.

Exam trap: carburettor icing is more common on warm, humid days than on cold days. Cold days typically have low humidity, meaning less moisture is available to freeze. This is frequently tested in the CASA exam — remember that icing risk extends up to 38°C OAT, well above what many pilots instinctively expect.

Symptoms of carburettor icing

Carburettor icing develops gradually and can be easy to miss if you are not actively monitoring engine instruments. Common symptoms include:

• Gradual, unexplained RPM drop on a fixed-pitch propeller aircraft.
• Manifold pressure decrease on a constant-speed propeller aircraft.
• Engine roughness as airflow becomes increasingly restricted.
• In severe cases: complete engine stoppage.

Carburettor heat

Carburettor heat works by applying hot air drawn from around the exhaust manifold to the carburettor throat, melting any existing ice and preventing further formation. Knowing what to expect when you apply carb heat is essential for the exam.

• Expect a temporary RPM drop when carb heat is first applied, as hot air is less dense than cold air.
• If ice was present: RPM will drop further initially, then recover above the original RPM as the ice melts and airflow is restored.
• If no ice was present: RPM drops slightly and remains lower (hot air is less dense, so the engine produces marginally less power).

Do not use carburettor heat at full power — including during take-off and go-around. Applying carb heat at full power reduces available power and can cause detonation in high-compression engines.

Oil system

The oil system lubricates, cools, and protects engine components. Two instruments are used to monitor oil system health, and both should be checked during every pre-flight and monitored continuously in flight.

• Oil pressure is the most critical engine instrument. Low oil pressure indicates potential bearing failure, which can occur within minutes — if oil pressure drops out of the green arc, land as soon as possible.
• Oil temperature must reach the normal operating range before high-power operation is applied. Always check the oil dipstick level during the pre-flight inspection.

Detonation vs pre-ignition

Both detonation and pre-ignition are abnormal combustion events that cause engine roughness and high CHT, but they have different causes and levels of severity.

• Detonation occurs when the fuel-air mixture ignites explosively rather than burning progressively from the spark plug. It causes knocking and high CHT. Common causes include: use of low-octane fuel, an over-lean mixture, or high power combined with low airspeed.
• Pre-ignition occurs when the mixture ignites before the spark plug fires, caused by a hot spot in the cylinder such as carbon deposits. Pre-ignition is generally more destructive than detonation. Both conditions produce engine roughness and elevated CHT readings.

Common exam traps — engines

The following points are frequently tested in the CASA RPL exam and are common sources of errors. Review these carefully before your exam.

• Carburettor icing is most likely on warm, humid days — not cold days — and risk extends up to 38°C OAT per the VFRG.
• No RPM drop on the magneto check indicates a dead grounding wire — the magneto remains live and is a serious safety hazard.
• When applying carb heat with ice present: RPM first drops, then recovers above the original RPM.
• Use rich mixture for take-off and landing; use lean mixture for cruise.
• Oil pressure must be in the green arc before applying high power.

Related Resources

• Aircraft Engines Study Guide
• Aircraft General Knowledge
• All RPL Exam Topics