Powerplant | PPL(A) Aircraft Technical and General Knowledge

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Piston Engines — General Principles

Every reciprocating (piston) aero engine exists to do one job: convert the chemical energy stored in fuel into rotary mechanical work at the propeller shaft. It does this through a repeating thermodynamic cycle — the Otto cycle — acting on a set of interconnected mechanical components.

This section builds the foundation for the rest of the Powerplant syllabus. Get comfortable with the terminology here (TDC, BDC, stroke, compression ratio) — almost every later topic (mixture, detonation, cooling, ignition) refers back to it.

Piston Motion: Stroke, TDC, and BDC

The piston does not spin — it moves in a straight line up and down the cylinder. The crankshaft’s job is to turn that linear motion into rotation (see below).

• Top Dead Centre (TDC) — the piston’s highest point of travel in the cylinder.
• Bottom Dead Centre (BDC) — the piston’s lowest point of travel in the cylinder.
• Stroke — the linear distance the piston travels between TDC and BDC. Together, TDC and BDC define the full range of piston motion, and the stroke length (combined with bore diameter) determines the engine’s displacement.

Note

Exam Tip: EASA and FAA AGK papers frequently test the difference between stroke (a linear distance, measured in mm/inches) and a “stroke” of the four-stroke cycle (an event, e.g. “the compression stroke”). Don’t confuse the two meanings of the word.

Core Reciprocating Components

The Crankshaft

The crankshaft is the backbone of the engine. It converts the pistons’ reciprocating (linear) motion into rotary motion, transmitting torque to the propeller (or rotor system) and driving engine accessories such as the magnetos, alternator, and oil pump.

• It rotates on journals, which are supported by main bearings in the crankcase.
• Crank throw: the distance between the main journal centreline and the crankpin centreline. This distance directly determines the piston stroke — a larger crank throw produces a longer stroke.
• The connecting rod links the piston to the crankpin, transmitting the combustion force to the crankshaft and converting the piston’s linear motion into the crankshaft’s rotation.

The Camshaft

The camshaft controls valve timing. It is gear- or chain-driven directly from the crankshaft.

• Each valve is controlled by its own cam lobe, machined at a fixed angular position on the shaft.
• Because valve timing (lead, lag, and overlap — covered below) is set mechanically by the shape and position of the lobes, it remains constant regardless of engine RPM.
• Valve opening and closing angles are always referenced back to crankshaft rotation (measured in degrees of crankshaft rotation before/after TDC or BDC), not camshaft rotation.
• Camshaft-to-crankshaft speed ratio: in the Otto cycle, each valve opens only once per full cycle (two crankshaft revolutions), so the camshaft turns at half crankshaft speed — one camshaft revolution for every two crankshaft revolutions.

Valves and Valve Springs

• Inlet valves open to admit the air-fuel mixture into the combustion chamber.
• Exhaust valves open to expel burnt combustion gases.
• Valves are opened by camshaft rotation, transmitted mechanically via the cam lobe and, depending on engine layout, a pushrod and rocker arm (or directly, in overhead-cam designs).
• Valve springs close the valves. They hold the valve firmly shut when not actuated by the cam, maintaining cylinder compression and preventing combustion gas backflow. They also dampen the valve’s return, preventing “valve bounce” off its seat.

Caution

Common Exam Trap: Valves are opened by the camshaft (via the lobe, and pushrod/rocker arm where fitted) and closed by the valve spring — never the reverse. A question stating “valves are opened by the spring and closed by the camshaft” is testing whether you know this is backwards.

Cylinder Volumes and Compression Ratio

Three volumes define how a cylinder behaves through the cycle:

• Total Volume — the full volume above the piston when it is at BDC (swept volume + clearance volume).
• Swept Volume — the volume displaced by the piston as it travels one stroke (Cross-sectional area × Stroke).
• Clearance Volume — the volume remaining above the piston at TDC, i.e. the combustion chamber volume.

The Compression Ratio (CR) compares total volume to clearance volume:

Compression Ratio = Total Volume / Clearance Volume

Tip

Exam Tip (EASA/FAA): A higher compression ratio improves thermal efficiency and fuel economy by extracting more work from each combustion event — but it also increases peak cylinder pressures and temperatures. This raises mechanical stress on the piston, rings, and connecting rod, and increases the risk of detonation (uncontrolled, explosive combustion) if the wrong fuel grade (octane/performance number) is used. This is precisely why your POH specifies a minimum fuel grade — always confirmed by engine manufacturer limitations, not pilot preference.

The Otto Cycle — Four Strokes in Two Revolutions

The Otto cycle is the thermodynamic cycle describing operation of the four-stroke spark-ignition engine fitted to almost all light training aircraft. It produces one power stroke for every two crankshaft revolutions (720° of crankshaft rotation), and is often remembered informally by the mnemonic “suck, squeeze, bang, blow.”

1. Induction (Intake): The intake valve opens and the piston moves down the cylinder, drawing in a fresh air-fuel mixture.
2. Compression: The intake valve closes and the piston moves back up, compressing the mixture into the clearance volume ahead of ignition.
3. Power: Near TDC, a spark ignites the compressed mixture. The resulting high-pressure combustion gases force the piston back down, delivering the stroke that produces usable work.
4. Exhaust: The exhaust valve opens and the piston moves up again, expelling the burnt gases from the cylinder.

The strict order of these events — Induction, Compression, Power, Exhaust — is fixed and repeats continuously while the engine runs, spanning two full crankshaft revolutions per cycle.

Valve Timing: Lead, Lag, and Overlap

The ideal Otto cycle assumes valves snap open and shut exactly at TDC or BDC. Real engines deviate from this deliberately, because the air-fuel mixture has mass and therefore momentum — it cannot start and stop flowing instantaneously.

Near TDC and BDC, piston movement is minimal even though the crankshaft continues turning at a steady angular rate — this is called the ineffective crank angle. Valve timing is adjusted around this dead zone to keep gas flowing smoothly and maximise volumetric efficiency (how effectively the cylinder fills with fresh mixture).

Valve Lead

The valve opens before its theoretical (TDC/BDC) point.

• Intake valve opens before TDC (while the exhaust stroke is still finishing), so it is fully open the instant the induction stroke begins.
• Exhaust valve opens before BDC, using the last of the power stroke’s pressure to begin expelling gas early.

Valve Lag

The valve closes after its theoretical (TDC/BDC) point.

• Intake valve closes after BDC — the incoming mixture’s momentum keeps it flowing into the cylinder for a brief period even as the piston begins its upward travel.
• Exhaust valve closes after TDC, allowing residual exhaust gas momentum to finish clearing the cylinder.

Valve Overlap

Both intake and exhaust valves are partially open at the same time, around TDC at the end of the exhaust stroke / start of the induction stroke.

• Creates a brief low-pressure “scavenging” effect that helps draw in the fresh mixture while helping expel remaining exhaust gas.
• Because it is set by fixed cam lobe geometry, valve overlap does not change with engine speed.

Purpose of lead, lag, and overlap:

• Maximises the mass of air-fuel mixture entering the cylinder each cycle.
• Improves exhaust scavenging and mixture intake by exploiting pressure differentials.
• Enhances overall engine efficiency and power output.

Note

Key Concept: Valve timing itself is fixed by cam geometry and does not vary with RPM or pilot input. Engine power is controlled instead by the throttle, which regulates the mass of air (and therefore fuel-air mixture) admitted into the cylinder, and hence the pressure rise achieved during combustion.

The Induction Stroke in Detail

The induction stroke is a good example of why valve lead and lag exist in practice:

• Opening the inlet valve before TDC ensures it is already fully open the instant the induction stroke begins, so the piston can start drawing in mixture without delay.
• As the piston descends, the mixture gains momentum. This momentum keeps mixture flowing into the cylinder even slightly after BDC, once the piston has already begun moving back upward.
• The inlet valve finally closes after BDC, once cylinder pressure has risen to match induction manifold pressure — closing any earlier would needlessly cut off still-incoming mixture.

Adiabatic Compression

An adiabatic process is one that occurs with no heat or mass transfer between the gas and its surroundings.

During the compression stroke, the piston moves upward with the intake valve fully closed, compressing the air-fuel mixture into the shrinking clearance volume. Because no mixture can escape and (ideally) no heat is exchanged, this compression sharply raises both the pressure and temperature of the mixture — despite some minor, real-world heat loss to the cylinder walls, which keeps a real cylinder slightly less efficient than the ideal adiabatic model.

Tip

Exam Tip: This rise in mixture temperature purely from compression (before the spark even fires) is exactly why excessive compression ratios or an over-lean mixture can trigger pre-ignition or detonation — the mixture can become hot enough to ignite itself. It is also why cylinder head temperature (CHT) is a monitored engine limitation in flight.

Ignition Timing and Engine RPM

• Pre-TDC Ignition: The air-fuel mixture must be ignited before TDC is reached. Combustion is not instantaneous — it takes a small but finite amount of time to burn fully, so ignition must start early enough that peak cylinder pressure is reached just after TDC, ensuring an efficient power stroke.
• Timing Dependency: As engine RPM increases, the piston moves through TDC faster (in real time), even though the crank angle for ignition timing is fixed in degrees. This is why ignition timing is expressed and adjusted in terms of crankshaft degrees before TDC, so that combustion timing stays correctly matched to piston position across the engine’s operating RPM range.

Caution

Safety Note: Incorrect ignition timing — whether from magneto maladjustment or improper mixture leaning at high power settings — can allow peak combustion pressure to occur at the wrong point in the stroke, risking detonation and potential engine damage. This is part of why light aircraft engines use two independent magneto/ignition systems (dual ignition), primarily for redundancy, but also to ensure a more complete and even burn.

Combustion Phase — Constant-Volume Heat Addition

Because combustion happens so rapidly around TDC, the ideal Otto cycle treats it using the instantaneous combustion assumption:

• Combustion occurs so quickly at TDC that piston displacement during the burn is treated as negligible.
• While combustion takes place, the crankshaft rotates through only a small fraction of the cycle, and cylinder volume remains essentially constant.
• With volume effectively fixed, the energy released by combustion instead drives a sharp rise in pressure and temperature.

This is why the combustion phase is described thermodynamically as constant-volume heat addition — heat from burning fuel is added at (near) constant volume, producing a sudden pressure spike immediately before the piston begins its expansion (power) stroke.

Thermal Efficiency and Engine Losses

Thermal efficiency measures how effectively an engine converts the chemical energy in fuel into useful mechanical work:

Thermal Efficiency = (Heat Converted into Work / Heat Energy Available in Fuel) x 100

• Typical piston aero engines achieve only around 25–30% thermal efficiency.
• Engine design (compression ratio, valve timing, cooling) and correct fuel choice both influence achievable efficiency.
• Power output depends on the mass of mixture induced per cycle and the pressure rise achieved during combustion — tying this back directly to volumetric efficiency and compression ratio, covered earlier.

Where the rest of the energy goes:

• Heat Loss — engine components run extremely hot, but most of that thermal energy is rejected to the cooling airflow and exhaust rather than converted into work.
• Friction — moving parts (pistons, rings, bearings, valve gear) rub against each other, consuming energy as mechanical friction and heat.
• Incomplete Combustion — not all fuel burns perfectly on every cycle, so some chemical energy is expelled unused in the exhaust.

Tip

Exam Tip: A very common AGK question simply asks for the typical thermal efficiency range of a piston aero engine. Commit 25–30% to memory — and remember it means roughly 70–75% of the fuel’s energy is lost, mostly as heat.

Ignition Circuits

Unlike most electrical systems on the aircraft, the ignition system is deliberately built to be independent of the battery. This section covers how that independence works, why it matters, and how it’s checked before every flight.

Magnetos

Aircraft piston engines use two independent magnetos for ignition. Each magneto is a small, engine-driven generator that produces its own high-voltage spark — meaning the ignition system will keep functioning even with a total electrical (battery/alternator) failure.

Propeller Safety

Never manually rotate the propeller with the ignition switch ON. Because magnetos function independently of the battery, turning the propeller with the ignition ON can start the engine unexpectedly, posing a serious injury risk. Always confirm the ignition switch is OFF (and treat the propeller as “live” regardless) before touching it.

Magneto redundancy:

• Two engine-driven magnetos provide redundancy in case one fails.
• Each magneto typically fires its own spark plug in each cylinder — most aero engines run two spark plugs per cylinder, one from each magneto circuit.
• If one magneto fails, combustion becomes less efficient (only one spark plug per cylinder is firing) → lower power output and RPM — but the engine keeps running on the remaining magneto.

Why It's Called a 'Dead Cut' Check

The ignition switch works by grounding (short-circuiting) a magneto to switch it off — it does not cut the magneto’s power supply, because the magneto has none to cut. This is why, during the pre-start “dead cutting” check, briefly selecting OFF should cause a slight RPM drop or the engine to falter, confirming the switch and P-leads are correctly grounding the magneto. No drop at all on this check can indicate a broken P-lead — a hazardous fault, since the magneto may still be “live” even with the switch OFF.

Magneto RPM Drop Check

Conducted at approximately 75% maximum engine speed (per the POH) to verify magneto and spark plug function before flight:

1. Run the engine at approximately 75% power, as specified in the POH.
2. Switch to left magneto only and note the RPM drop.
3. Return to BOTH magnetos and allow RPM to stabilise before the next check.
4. Switch to right magneto only and note the RPM drop.
5. Return to BOTH magnetos.
6. Compare both drops, and the difference between them, against the manufacturer’s published limits.

Why RPM Drops on a Single Magneto

With only one magneto selected, each cylinder fires from a single spark plug instead of two. This reduces ignition efficiency, slows the flame front’s spread through the mixture, and slightly delays peak combustion pressure — causing a small but expected RPM drop. An excessive drop, or a large difference between the two magnetos’ drops, indicates a fouled spark plug, incorrect timing, or a faulty magneto, and is unairworthy.

Tip

Exam Tip: Typical single-magneto RPM drop limits, and the maximum allowable difference between the two drops, are engine-type specific and always published in the POH/AFM — expect exam questions to test the procedure and reasoning, not a single universal RPM figure.

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Propellers

The propeller converts the engine’s rotary shaft power into thrust. It is, in effect, a rotating aerofoil — and like any aerofoil, its efficiency depends heavily on the angle at which the air meets its blades.

Fixed-Pitch vs Constant-Speed Propellers

Fixed-Pitch

A fixed-pitch propeller has a blade angle set (and unchangeable) by design.

• Most efficient only at its one designed operating speed (typically optimised for either cruise or climb, not both).
• Simple, light, and low-maintenance — the standard fit on most PPL trainers.
• Efficiency falls away significantly outside its designed speed range.

Constant-Speed

A constant-speed propeller uses a governor to automatically vary blade pitch in flight, maintaining a selected RPM.

• Maintains high efficiency across a wide range of airspeeds and power settings, by continuously adjusting blade angle to keep the blades’ angle of attack near its optimum.
• Allows the pilot to select RPM independently of manifold pressure/throttle setting.
• Enables feathering (see below) — a capability fixed-pitch propellers do not have.

Note

Static Efficiency: When the aircraft is stationary, propeller efficiency is zero — regardless of propeller type. Propeller efficiency = Thrust Power / Engine (Shaft) Power, and Thrust Power = Thrust × True Airspeed (TAS). At zero airspeed, TAS = 0, so thrust power is 0 even though the propeller is producing static thrust and the engine is delivering full shaft power. This is a classic AGK exam distinction between thrust (a force, present even at zero speed) and propeller efficiency (a ratio involving speed).

Propeller Blade Speed and Geometry

• Speed Variation — Rotational (tangential) speed increases from root to tip, because tip elements travel a much greater circumferential distance per revolution than root elements closer to the hub.
• Blade Geometry — Each blade has a root (nearest the hub), a tip (outer end), leading and trailing edges, and a cambered aerofoil cross-section, generating thrust the same way a wing generates lift.
• Blade Twist (Wash-out) — Blade angle decreases progressively from root to tip. Because the tip moves faster through the air than the root, a twisted (washed-out) blade keeps the angle of attack — and therefore the thrust generated — more even along the whole blade span, while also reducing bending loads that would otherwise concentrate near the tip.

This combination of twist and aerofoil section is what allows a propeller to generate reasonably efficient, evenly-distributed thrust along its full length despite the large speed difference between root and tip.

Angle of Attack on a Fixed-Pitch Propeller

Because a fixed-pitch propeller’s blade angle (a fixed, geometric value set by design) cannot change, its angle of attack (an aerodynamic value, dependent on the resultant airflow the blade actually meets) varies continuously with flight condition:

• At zero airspeed (e.g. static run-up): angle of attack is initially high, since the propeller must generate maximum thrust for acceleration from a standstill.
• As airspeed increases: the resultant airflow direction shifts, aligning more closely with the blade’s chord line — this progressively lowers the angle of attack.
• As RPM increases: the rotational speed vector grows larger, which increases the angle of attack (for a given forward airspeed).

Tip

Exam Tip: Don’t confuse blade angle (fixed, mechanical, set at manufacture on a fixed-pitch prop) with angle of attack (variable, aerodynamic, changes with the resultant of forward airspeed and rotational speed). This distinction is a favourite AGK exam trap.

Feathering

Feathering rotates the propeller blades to align them edge-on with the airflow, minimising drag.

• Definition: Aligns the blades with the airflow, minimising aerodynamic resistance.
• Aerodynamic Effect: Reduces angle of attack toward zero and minimises drag by adjusting blade pitch to a near-90° (edge-on) angle.
• Engine Failure Scenario: On multi-engine aircraft fitted with constant-speed propellers, feathering the propeller on a failed engine minimises drag from what would otherwise be a windmilling propeller.
• Yawing Moment Reduction: By removing the drag of a windmilling propeller, feathering helps reduce the asymmetric yaw the failed engine would otherwise cause, easing the directional control burden on the pilot.

Note

Feathering is a constant-speed propeller feature and generally requires multi-engine architecture to be of practical benefit — most single-engine PPL trainers with fixed-pitch propellers do not have a feathering capability.

Feathering improves both aerodynamic efficiency and aircraft controllability during an engine failure — a key reason constant-speed, feathering propellers are standard on multi-engine aircraft.

Propeller Icing

• Aerodynamic Disruption — Ice accretion alters the blade’s aerofoil shape, increasing drag and reducing thrust efficiency, exactly as wing icing degrades lift.
• Imbalance & Vibration — Ice rarely forms evenly across all blades. This uneven distribution unbalances the propeller, causing vibration that accelerates wear on the engine, mounts, and airframe.
• Performance & Longevity — Reduced thrust efficiency degrades climb performance and increases fuel consumption for a given power setting. Sudden, asymmetric shedding of accumulated ice can also cause a sharp vibration spike, risking fatigue damage to the propeller, engine mounts, and accessories over time.

Caution

Propeller icing degrades performance well before it becomes visually obvious. Where fitted, use propeller anti-icing/de-icing systems proactively in icing conditions rather than waiting for a vibration or performance change to appear.

Limitations on Propeller Diameter

• Tip Speed & Supersonic Limit — As propeller diameter increases, blade tip speed rises for a given RPM. If tip speed approaches or exceeds the local speed of sound, shock waves form, sharply reducing efficiency and increasing noise.
• Efficiency & Shock Waves — Supersonic (or transonic) blade tips generate significant additional drag, degrading propulsive performance. For this reason, propeller designs are constrained to keep tip speeds below Mach 1, factoring in the resultant of rotational speed and the aircraft’s forward speed.
• Blade Twist & Angle of Attack — Important for optimising thrust distribution along the blade, but these are not diameter-limiting factors in themselves.
• Power Absorption Solution — Rather than increasing diameter (and risking supersonic tip speeds) to absorb more engine power, designers add more blades. This increases the propeller’s total blade area and power absorption without increasing tip speed.

Tip

Exam Tip: Blade tip speed is the vector sum of rotational speed (ω × radius) and the aircraft’s forward (true air) speed — not rotational speed alone. This is why tip speed limitations become more restrictive at high forward speeds, and why high-speed propeller aircraft tend to use more, shorter blades rather than fewer, longer ones.

Air Cooling Systems

Most piston aero engines are air-cooled — there’s no radiator or coolant loop. Cooling relies entirely on airflow over finned cylinder heads, which makes airspeed just as important a cooling factor as engine power itself.

Air-Cooled Engine Cooling Factors

Efficient air cooling depends on both airspeed and temperature. High RPM combined with low airspeed is the most problematic combination an engine can face.

Factor	Effect
High RPM	More heat generated — requires effective dissipation
Low airspeed	Reduced airflow over cooling fins — increases overheating risk
Cooling fins & baffles	Less effective at low airspeeds — limited heat exchange
Cowlings & airflow	Designed for cooling, but airflow weakens at low speeds

Ground Operations

During prolonged ground operations at high power (e.g. an extended run-up), monitor CHT (Cylinder Head Temperature) closely. Without forward airspeed, cooling airflow is significantly reduced and overheating can occur rapidly — this is exactly the “high RPM, low airspeed” combination that stresses air cooling the most.

Radial vs Inline/V-Type Cooling

Radial Engines

Cylinders are arranged in a circle around the crankshaft.

• Every cylinder is presented directly to oncoming airflow.
• This gives all cylinders even, equal cooling airflow — the most effective air-cooling arrangement of the common layouts.

Inline / V-Type Engines

Cylinders are arranged in a row (inline) or two angled banks (V-type), one behind another relative to the airflow.

• Rear cylinders receive air that has already passed over, and been heated by, the cylinders ahead of them.
• This produces uneven cooling, with rear cylinders running hotter and at greater risk of overheating than the front ones.

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Mixture

The mixture control lets the pilot adjust the ratio of fuel to air entering the engine, keeping combustion efficient as air density changes with altitude and power setting.

Why Mixture Needs Adjusting

• Critical for Combustion: The fuel-air ratio directly determines how efficiently the mixture burns and how much power is generated.
• Sea-Level Calibration: Carburettors (and injection systems) are set up assuming FULL RICH provides the correct ratio for dense, sea-level air.
• Altitude Effect: As altitude increases, air density decreases. If fuel flow isn’t reduced to match, the same fuel flow is now mixed with less air by mass — the mixture becomes progressively too rich unless the pilot leans it.
• Leaning Process: The pilot reduces fuel flow via the mixture control to restore the correct fuel-to-air ratio for the thinner air, keeping combustion efficient and preventing rough running or fouling.

Note

Prevents rough running and spark plug fouling, avoids unnecessary engine damage, and optimises power and fuel efficiency across the aircraft’s operating altitude range.

Key Mixture Ratios

8:1 — Rich

Rich Mixture (approx. 8 parts air to 1 part fuel by weight)

• Excess fuel relative to the chemically ideal ratio.
• Produces cooler combustion — the extra unburned fuel absorbs heat as it evaporates, helping cool the cylinder.
• Used for maximum cooling margin, e.g. full-rich for takeoff/climb.

15:1 — Stoichiometric

Stoichiometric (Chemically Correct) Mixture

• The exact ratio at which all fuel and all oxygen are theoretically consumed with nothing left over.
• Produces the highest combustion temperatures — ideal in theory for fuel-oxygen balance, but carries the highest risk of high temperatures and detonation in practice.

12.5:1 — Best Power

Best Power Mixture

• Slightly rich of stoichiometric.
• The ratio pilots typically target for the best balance of engine cooling, cylinder lubrication, and usable power output.

Leaning Effects: EGT and CHT

• Leaner Mixture → Less fuel relative to air, generally improving combustion efficiency as mixture approaches best-power/stoichiometric ratios.
• EGT Increase → Exhaust Gas Temperature (EGT) rises as the mixture is leaned toward peak EGT — this is a normal, expected trend.
• Cooling Effect Reduction → Fuel itself has a cooling effect (evaporative cooling of the cylinder head). Leaning reduces the amount of fuel available for this cooling.
• Risk of Excessive Heat → Leaning too close to peak EGT raises combustion temperatures, driving CHT and EGT higher.
• Potential Engine Damage → Sustained overheating can trigger detonation, damaging pistons, rings, and cylinder heads.
• Critical Monitoring → Pilots must track CHT and EGT together and stay within POH limits when leaning.

Caution

Exam Nuance: The hottest, highest-stress operating region is not necessarily “leaned past peak” — CHT typically peaks slightly rich of peak EGT (often called the operational “red box” or “red fin”). Many operating techniques therefore call for running either richer than this zone (for cooling margin) or leaned past peak EGT (lean-of-peak operation, where both EGT and CHT fall again), rather than lingering near peak EGT on the rich side.

EGT as a Mixture Indicator

The EGT gauge tracks mixture efficiency in real time:

• Leaning from full rich increases EGT progressively, up to a peak value.
• Further leaning past peak EGT reduces power output, and prolonged operation near peak (rather than well past it) risks overheating — reinforcing why CHT and EGT should always be monitored together, not EGT alone.

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Performance & Engine Handling

Detonation from High Manifold Pressure at Low RPM

Danger

High manifold pressure combined with low propeller RPM can cause detonation — an uncontrolled, explosive ignition of the fuel-air mixture in the combustion chamber, distinct from normal, controlled flame-front combustion.

Causes:

• Excessively high cylinder pressure and temperature, combined with reduced airflow/cooling during the compression stroke, causes multiple pockets of fuel-air mixture to ignite nearly simultaneously — instead of a single, controlled flame front spreading in one direction from the spark plug.

Risks:

• Engine Damage — the near-instantaneous pressure spikes from detonation place severe stress on pistons, rings, and connecting rods.
• Pre-Ignition — detonation-induced heat can trigger early, uncontrolled ignition of the mixture on a subsequent cycle, compounding the damage.

Prevention:

• RPM Management — avoid combining high manifold pressure with low RPM.
• Proper Mixture — maintain the correct fuel-air ratio; both overly lean and overly rich conditions can contribute to abnormal combustion.

Power Changes on Constant-Speed Propellers

For constant-speed propellers, power changes must follow a set sequence to avoid excessive torque and cylinder pressure — directly avoiding the low-RPM/high-manifold-pressure detonation risk described above.

Increasing Power

1. Enrich the mixture (if not already full rich) to guard against detonation as power and cylinder pressures rise.
2. Increase prop RPM (via the RPM/prop lever) first, reducing engine load per revolution before adding manifold pressure.
3. Advance the throttle (MAP) last, for smooth, controlled power delivery.

Decreasing Power

1. Retard the throttle (MAP) first, avoiding an abrupt pressure shift while RPM is still high.
2. Reduce prop RPM, minimising drag once manifold pressure is already lower.
3. Adjust the mixture as needed to maintain combustion efficiency at the new power setting.

Tip

Exam Tip: The underlying rule is often summarised as “throttle before prop when decreasing power, prop before throttle when increasing power.” This sequencing avoids ever momentarily combining high MAP with low RPM — the exact combination that risks detonation.

Why MAP matters differently by propeller type:

• Constant-speed propellers: RPM is actively regulated by the CSU/governor and does not by itself indicate engine power output — so Manifold Pressure (MAP) becomes the essential power indicator, and is also used to optimise boost performance.
• Fixed-pitch propellers: RPM rises and falls directly with power, so RPM alone reflects power output, making a MAP gauge unnecessary.

Effect of Altitude on Naturally Aspirated Engines

• Air Density Decreases → Higher altitude means less dense air.
• Reduced Air Intake → A smaller mass of air enters the cylinders on each induction stroke.
• Lower Power Output → Less oxygen available for combustion reduces engine efficiency and power — this is why naturally aspirated engines lose power steadily with altitude, unlike turbocharged/supercharged variants.

As altitude increases and air density falls, a fixed fuel flow effectively enriches the mixture (more fuel relative to the reduced air mass) — tying directly back into the leaning process covered above:

• Pulling the mixture control leans the mixture, restoring correct fuel-air balance, improving efficiency, and lowering specific fuel consumption.
• Excessively lean mixtures can cause slow, incomplete burning, overheating, and engine damage.
• Proper mixture adjustment at each altitude ensures safe and efficient operation throughout the climb and cruise.

Summary — Exam Quick Reference

Key Formulas to Remember

Formula	Use
n = L / W	Load factor
V = 9 × √P	Aquaplaning speed (knots), P in psi
P = V × I	Electrical power (Watts)
I = V / R	Ohm’s Law
1/RT = 1/R1 + 1/R2	Parallel resistance
CT = C1 + C2	Parallel capacitance
Compression Ratio = Total Vol / Clearance Vol	Engine compression
Thermal Efficiency = (Work Out / Heat In) × 100	Engine efficiency
F = P × A	Hydraulic force
SFC = Fuel Flow / Power Output	Specific fuel consumption (e.g. lb of fuel / hp / hr)

Exam Traps — Most Commonly Tested

• Carburettor icing can occur above 0°C — the venturi effect drops temperature below freezing even in warm, humid air
• Valves are opened by the camshaft and closed by the valve spring — not the other way around
• Series batteries: voltage adds, capacity stays the same. Parallel batteries: capacity adds, voltage stays the same
• Static wicks do not protect against lightning — they only dissipate triboelectric static
• A trip-free CB will not re-make the circuit if a fault still exists — a non-trip-free CB can be forced closed, which is dangerous
• Fuel injection does not use a venturi — not prone to carburettor icing. It can, however, still suffer impact icing on the intake filter/screen in visible moisture — a different mechanism
• The limit load is the maximum without permanent deformation; the ultimate load = limit × 1.5
• Aquaplaning speed: V = 9 × √P — higher tyre pressure = higher aquaplaning speed
• Monocoque: no internal stiffeners — skin alone carries all load (rare in practice)
• Semi-monocoque: skin plus frames and stringers share the load — this is the standard light-aircraft fuselage construction
• Camshaft rotates at half crankshaft speed