Module 16: Piston Engine

Module 16: Piston Engine – Comprehensive Study Material

1. Module Overview

Module 16 of the EASA Part-66 syllabus (Appendix I) covers the design, construction, operation, and maintenance of piston engines used in aircraft. For the B3 licence category (light aeroplanes and helicopters), this module provides the foundational knowledge required for certifying staff to perform maintenance, troubleshooting, and airworthiness determinations. The module is structured into sub-modules covering fundamentals, engine performance, lubrication, ignition, fuel metering, induction, cooling, and propellers.

The knowledge levels range from Level 1 (overview) to Level 3 (detailed theory), with the latter requiring a thorough understanding of system operation, failure modes, and maintenance procedures. This study material synthesises the core knowledge areas, with emphasis on the practical diagnostic and maintenance scenarios that certifying staff will encounter.

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2. Key Concepts Explained in Detail

2.1 Engine Fundamentals and Construction (Module 16.1)

Four-Stroke Cycle Operation

The four-stroke (Otto) cycle is the fundamental operating principle of most aircraft piston engines. The complete cycle consists of:

1. Intake (Induction) Stroke: The piston moves from TDC to BDC with the intake valve open, drawing a fuel-air mixture into the cylinder.
2. Compression Stroke: Both valves are closed; the piston moves from BDC to TDC, compressing the mixture to a ratio typically between 6:1 and 10:1.
3. Power (Expansion) Stroke: The spark plug ignites the compressed mixture; expanding gases drive the piston from TDC to BDC.
4. Exhaust Stroke: The exhaust valve opens; the piston moves from BDC to TDC, expelling combustion products.

The valve timing sequence is critical: intake opens slightly before TDC and closes after BDC; exhaust opens before BDC and closes after TDC. This overlap allows for efficient gas exchange at high RPM.

Crankshaft Counterweights

Counterweights are fitted to the crank cheeks to balance the dynamic forces of reciprocating and rotating masses. Their primary purpose is to minimise vibration by counteracting the inertia forces of the pistons and connecting rods. On engines such as the Lycoming O-360 series, counterweights may be retained by bolts with witness marks. These marks are installed to detect rotation; if the marks have not moved between inspections, the bolts have not loosened and no further action is required. If movement is detected, the bolts must be removed, inspected, and re-torqued per the engine maintenance manual (EMM).

Cylinder Baffles and Cooling

Air-cooled engines rely on directed airflow over the cylinder fins. Cylinder baffles route cooling air evenly across all cylinders, particularly the rear cylinders which receive less direct airflow. Damaged or missing baffles disrupt this airflow, leading to inadequate cooling and overheating. This is a significant airworthiness risk and must be rectified before further flight.

Propeller Reduction Gearbox

In engines where the crankshaft speed exceeds the optimal propeller speed, a reduction gearbox is interposed. This allows the engine to operate at its most efficient RPM while the propeller operates at its optimal speed. The gearbox typically uses spur or planetary gears and requires specific lubrication and inspection procedures.

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2.2 Engine Performance and Valve Mechanisms (Module 16.2)

Valve Clearance (Lash) Adjustment

Valve clearance must be set with the lifter on the base circle of the cam, which occurs at TDC on the compression stroke for that cylinder. This ensures correct lash and valve timing. The procedure typically involves:

1. Rotating the crankshaft to TDC on the compression stroke for the cylinder being adjusted.
2. Inserting a feeler gauge of the specified thickness between the valve stem and rocker arm.
3. Adjusting the clearance via the adjusting screw and locknut.
4. Re-checking the clearance after tightening the locknut.

For example, on a Rotax 912 engine, the specified intake clearance is 0.10 mm and exhaust is 0.20 mm. A measurement of 0.15 mm for intake is outside the specified value and must be adjusted; 0.25 mm for exhaust is within the typical tolerance of ±0.05 mm and is acceptable.

Valve Guides

Valve guides are an integral part of the cylinder head in many light aircraft engines (e.g., Lycoming, Continental). When guide clearance reaches the maximum allowable limit, the EMM typically specifies cylinder replacement, as the guides are not replaceable as separate items.

Ignition Timing

Ignition timing is expressed in degrees of crankshaft rotation before top dead centre (BTDC). Over-advanced timing (e.g., 25° BTDC instead of the specified 20° BTDC) causes the fuel-air mixture to ignite earlier in the compression stroke, increasing cylinder pressures and temperatures. This leads to detonation and overheating, which can cause severe engine damage. Timing is set using a timing light or by aligning marks on the crankshaft and magneto drive.

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2.3 Carburettors and Fuel Injection Systems (Module 16.3)

Float-Type Carburettor Operation

The float-type carburettor meters fuel based on the pressure differential created by airflow through the venturi. Key components include:

• Float chamber: Maintains a constant fuel level.
• Venturi: Creates a low-pressure area to draw fuel from the float chamber.
• Main metering jet: Controls fuel flow at cruise and full power.
• Idle jet: Supplies fuel at low throttle settings.
• Accelerator pump: Provides an extra shot of fuel when the throttle is opened rapidly to prevent a temporary lean mixture and hesitation.

Carburettor Heat

Carburettor heat is a critical safety feature, especially in light aircraft. Ice forms in the carburettor due to the temperature drop in the venturi caused by fuel vaporisation and the pressure drop. The temperature can drop by as much as 20–30°C, causing ice to form even in warm ambient conditions when relative humidity is high.

When carburettor heat is selected to HOT, air is routed through a heat exchanger around the exhaust system. This heated air is less dense, which causes a drop in manifold pressure of up to 4 inHg. This is a normal condition and not a fault. The heat is used to:

• Prevent ice formation when conditions are conducive (high humidity, temperatures between -10°C and +20°C).
• Clear existing ice by melting it.
• Provide an alternate air source if the air filter becomes blocked.

Mixture Control and Idle Cut-Off

The mixture control adjusts the fuel-air ratio for altitude and operating conditions. Moving the mixture to 'idle cut-off' during shutdown stops fuel flow to the carburettor, shutting down the engine by fuel starvation. This is a standard procedure to prevent after-firing and ensure the engine stops cleanly.

Pressure Carburettors (Bendix/Stromberg)

Pressure carburettors do not have a float chamber. Instead, they use a metering system that maintains a constant pressure differential across the fuel metering jet. The automatic mixture control (AMC) maintains the correct fuel-air ratio with altitude. A faulty AMC can cause improper leaning and rough idle. Float level is not applicable to pressure carburettors.

Fuel Injection Systems

In a continuous-flow fuel injection system, the key components are:

• Fuel control unit (FCU): Meters fuel based on airflow and other parameters (e.g., throttle position, mixture control, altitude).
• Fuel flow divider (manifold valve): Receives metered fuel from the FCU and distributes it equally to the individual fuel injectors at each cylinder, ensuring balanced fuel delivery.
• Fuel injector nozzles: Atomise the fuel for efficient combustion.

The FCU is responsible for metering the correct amount of fuel; the nozzles only atomise the fuel.

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2.4 Ignition Systems (Module 16.3)

Magneto Systems

Aircraft piston engines typically use dual magnetos for redundancy. Each magneto is self-contained, generating its own electrical power. Key components include:

• Magneto: Generates high voltage through a rotating magnet and coil.
• Distributor: Routes the high voltage to the correct spark plug at the correct time.
• Points (contact breaker): Interrupt the primary circuit to induce a high voltage in the secondary coil.
• Condenser (capacitor): Prevents arcing across the points and sharpens the voltage collapse.
• Impulse coupling: Retards the spark during starting (typically 20° retarded) to prevent kickback and provides a hot spark for reliable starting. The 'snap' occurs at the retarded timing point, which is after the normal advanced firing point.

Magneto Checks

During a magneto check at run-up, the engine is run on one magneto at a time. A significant RPM drop on one magneto indicates a fault in that magneto's ignition circuit, such as a faulty spark plug, lead, or internal magneto fault (e.g., faulty condenser). If the drop exceeds the manufacturer's limit, the aircraft is unserviceable and maintenance action is required.

'P' Lead

The 'P' lead (primary lead) is used to ground the magneto to stop it firing. If disconnected, the magneto remains live and cannot be shut off via the ignition switch, posing a serious safety hazard.

Spark Plug Heat Range

Heat range is a measure of the spark plug's thermal characteristics:

• Hot plugs retain more heat, preventing fouling at low power settings.
• Cold plugs dissipate heat faster, preventing pre-ignition at high power settings.

Correct heat range prevents both pre-ignition and fouling. The heat range is determined by the length of the insulator nose; a longer nose retains more heat.

Points Gap

The points gap is critical for correct ignition timing and performance. For example, a specified gap of 0.35 mm measured at 0.30 mm is outside the typical tolerance and must be adjusted to the specified gap.

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2.5 Lubrication Systems (Module 16.4)

Wet Sump vs. Dry Sump

• Wet sump: Oil is stored in a sump integral with the crankcase. A pressure pump circulates oil through the engine.
• Dry sump: Oil is stored in an external tank. A pressure pump supplies oil to the engine, and a scavenge pump returns oil from the crankcase to the external tank.

In some wet sump systems, a scavenge pump may be used to return oil from specific areas (e.g., rocker boxes) to the sump.

Oil Pressure and Temperature

Oil pressure is affected by:

• Oil viscosity: As oil warms up, viscosity decreases, reducing resistance to flow and consequently lowering indicated oil pressure. This is a normal characteristic, not a fault.
• Relief valve: A relief valve stuck open will bypass oil directly to the sump, causing low oil pressure, especially at idle. A stuck-closed relief valve would cause high pressure.
• Blocked oil cooler: Prevents oil from passing through the cooler, resulting in high oil temperatures. A stuck-open bypass valve would also prevent cooling.

Crankcase Breather

The crankcase breather vents the crankcase to the atmosphere, allowing the release of blow-by gases and internal pressure that would otherwise cause oil leaks and seal damage. It also prevents pressure build-up from piston blow-by.

Oil Dilution System

Oil dilution systems introduce fuel into the oil before shutdown in cold weather, lowering viscosity to aid cranking and oil circulation during start. The fuel evaporates once the engine warms up.

Oil Seepage

Slight oil seepage around rocker box covers or cylinder base nuts is common and may be acceptable. It should be recorded and monitored. If it becomes a leak, maintenance is required. The appropriate action is to clean the area and monitor during subsequent inspections.

Metallic Particles in Oil

Metallic particles in the oil filter indicate abnormal wear or damage within the engine. It is essential to investigate the source, which may include a borescope inspection of the cylinders and examination of the filter for type and quantity of particles. Returning the aircraft to service without investigation is unsafe.

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2.6 Engine Inspection and Testing (Module 16.5)

Compression Testing

Compression testing is a key diagnostic tool. A reading of 60/80 psi is below the typical minimum of 70/80 psi and indicates a potential leak path (piston rings, valves, or head gasket). A differential pressure test will localise the leak, and a borescope inspection can visually confirm the condition.

Leak-Down (Differential Pressure) Testing

Air escaping from the oil filler cap during a compression test indicates that compressed air is leaking past the piston rings into the crankcase. This is a classic sign of worn or broken piston rings. Leaking valves would show air from the intake or exhaust; a blown head gasket would show air from the adjacent cylinder or coolant.

Holding the Propeller During Compression Tests

For radial engines, the propeller must be held to prevent rotation when compressed air is applied. A wooden block placed between a propeller blade and the ground (or a solid structure) is the standard method, as hands or ropes are not safe.

Cylinder Replacement

After torquing cylinder hold-down nuts to the specified value, re-torquing after a run is critical. This ensures proper clamping force, as the nuts can settle after initial installation and thermal cycling.

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2.7 Supercharging and Turbocharging (Module 16.7)

Turbocharger Operation

A turbocharger uses exhaust gas energy to drive a turbine, which drives a compressor to increase the density of the intake air. This allows the engine to maintain sea-level power at altitude.

Wastegate

The wastegate is a valve that diverts a portion of the exhaust gases away from the turbine, thereby controlling the speed of the turbocharger and limiting the maximum boost pressure delivered to the engine. This prevents overboosting and engine damage.

• Wastegate stuck open: Allows exhaust gases to bypass the turbine, reducing turbocharger speed and manifold pressure, especially at high altitude where the turbo is needed.
• Wastegate stuck closed: Causes overboost, which can damage the engine.

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2.8 Propeller Control Systems (Module 16.8)

Constant-Speed Propeller Governor

The propeller governor senses engine RPM and adjusts the propeller pitch to maintain a constant RPM set by the pilot. It changes the blade angle to increase or decrease the load on the engine, thereby keeping RPM constant regardless of throttle setting or flight condition.

Governor Faults

Air in the governor oil line can cause a spongy or unresponsive propeller control, as oil is incompressible but air is not. This is a common issue after maintenance. Low oil pressure would affect governor operation but not typically cause a complete lack of response.

Safety Wiring

Safety wiring is performed to prevent loosening of fasteners due to vibration. The wire must be installed so that it pulls the bolt in the tightening direction, and the twist rate is typically 6–8 twists per inch (or as specified).

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2.9 Engine Instruments (Module 16.3)

Exhaust Gas Temperature (EGT)

EGT is used as a leaning aid. A peak EGT corresponds to a specific mixture setting. It helps pilots and maintainers set the correct fuel flow for performance and engine protection.

Manifold Pressure

Manifold pressure indicates the absolute pressure in the intake manifold. It is affected by throttle position, altitude, and turbocharger operation. A drop in manifold pressure when carburettor heat is selected is normal due to the reduced density of heated air.

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3. Important Formulas, Regulations, and Procedures

3.1 Key Formulas

3.2 Regulations and References

• Regulation (EU) No 1321/2014, Annex III (Part-66): Defines the basic knowledge syllabus and licence categories.
• Appendix I, Module 16: Piston Engine – covers sub-modules 16.1 through 16.8.
• Part-145: Maintenance organisation requirements, including the use of approved data (AMM, EMM, SRM).
• Instructions for Continued Airworthiness (ICA): Manufacturer's maintenance instructions, including witness mark checks and re-torquing procedures.

3.3 Key Procedures

1. Magneto Check: Run engine at specified RPM, switch to one magneto, note RPM drop. Excessive drop (> limit) indicates a fault.
2. Compression Test: Apply regulated air pressure to the cylinder, measure leakage. Air from oil filler indicates ring wear.
3. Valve Clearance Adjustment: Set at TDC on compression stroke, use feeler gauge, adjust to specified clearance.
4. Carburettor Heat Check: Select HOT, expect a manifold pressure drop of up to 4 inHg. No drop may indicate a blocked heat duct.
5. Safety Wiring: Install wire so it pulls bolts in the tightening direction, with 6–8 twists per inch.

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4. Common Relationships Between Concepts

4.1 Ignition Timing and Engine Performance

• Over-advanced timing → Detonation, overheating, potential engine damage.
• Over-retarded timing → Loss of power, overheating of exhaust valves.

4.2 Oil Pressure and Temperature

• Oil warms up → Viscosity decreases → Pressure decreases (normal).
• Relief valve stuck open → Low pressure at idle.
• Blocked oil cooler → High temperature.

4.3 Compression and Leakage Paths

• Air from oil filler → Piston ring wear.
• Air from intake → Intake valve leak.
• Air from exhaust → Exhaust valve leak.
• Air from adjacent cylinder → Head gasket failure.

4.4 Carburettor Heat and Manifold Pressure

• Heat ON → Air density decreases → Manifold pressure drops (up to 4 inHg, normal).
• Heat OFF → Air density increases → Manifold pressure rises.

4.5 Turbocharger and Manifold Pressure

• Wastegate open → Less exhaust through turbine → Lower boost → Lower manifold pressure.
• Wastegate closed → More exhaust through turbine → Higher boost → Higher manifold pressure.

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5. Typical Exam Focus Points

Based on the source questions, the following topics are frequently examined:

1. Carburettor heat: Purpose, operation, normal manifold pressure drop, ice formation conditions.
2. Lubrication system faults: Relief valve operation, oil cooler blockage, oil pressure/temperature relationships.
3. Ignition system faults: Magneto checks, RPM drop interpretation, 'P' lead safety, points gap, impulse coupling.
4. Compression testing: Interpretation of readings, leak-down testing, leakage path identification.
5. Fuel system faults: Accelerator pump, float bowl contamination, pressure carburettor AMC, fuel flow divider.
6. Propeller governor: Purpose, air in oil line, safety wiring.
7. Engine construction: Counterweights, baffles, valve guides, reduction gearbox.
8. Maintenance practices: Oil seepage assessment, metallic particles, witness marks, re-torquing after run.
9. Turbocharging: Wastegate function and failure modes.
10. Engine instruments: EGT, manifold pressure.

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6. Summary of Key Maintenance Actions

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This study material provides the foundational knowledge required for EASA Part-66 Module 16 examination success. Candidates should supplement this with manufacturer-specific data from EMMs and AMMs, as the exam may reference specific engine types (e.g., Lycoming O-320/O-360, Continental, Rotax 912).