Systems | PPL(A) Aircraft Technical and General Knowledge

Systems for PPL(A) candidates

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Batteries

Aircraft batteries provide the electrical power required for engine starting, standby power, and backup for essential avionics when the generator or alternator is offline. Understanding how batteries behave when connected together — and how their capacity rating actually works — is a core AGK exam theme.

Battery Capacity and the Ampere-Hour (Ah)

A battery’s capacity is expressed in Ampere-hours (Ah) — the amount of current it can theoretically deliver over a specified time before it is fully discharged.

A 60 Ah cell can theoretically provide 6 A for 10 hours, or 60 A for 1 hour.
- 60 Ah = 4 A x 15 h
A 40 Ah cell can theoretically provide 4 A for 10 hours, or 40 A for 1 hour.
- 40 Ah = 5 A x 8 h

Capacity is normally quoted at the 1-hour rate (C1).

Connecting Batteries: Series vs Parallel

How you wire two batteries together determines whether you gain voltage or capacity — this is a classic exam trap.

Series Connection
Parallel Connection

Voltages add up. Capacity stays the same.

Connecting batteries in series is done positive-to-negative, stacking their voltages while the current path (and therefore Ah capacity) remains that of a single cell.

Example: 2 x 12V, 30Ah batteries in series = 24V, 30Ah

Electrical Systems — General & Definitions

Conductors, Current, and Charge Flow

A conductor is a material that allows electrons (negatively charged particles) to flow from atom to atom when a voltage (electric potential difference) is applied across it.

This flow of charge is called electrical current.
Current is the number of electrons flowing past a point per unit time.
The unit of current is the Ampere (A).

Magnetism and the Effects of Current

When current flows through a wire, it generates a magnetic field around the conductor, forming concentric circles in accordance with the right-hand rule. The strength of this field depends on the magnitude and direction of the current flowing.

Attraction and Repulsion Rules — when two bar magnets are aligned end to end:

Like poles (e.g., North-North or South-South) repel each other.
Unlike poles (e.g., North-South) attract each other.

Rule: Like poles repel; unlike poles attract.

Electromotive Force (EMF) and Voltage

EMF (Electromotive Force) is the electrical “pressure” that drives current through a circuit — conceptually similar to water pressure driving flow through a pipe.

Measured in volts (V), EMF represents the electric potential difference between two points.
Higher EMF (higher volts) increases electron flow.
Lower EMF (lower volts) reduces electron flow.
A stable voltage supply is essential to maintain consistent current in a circuit.

ICAO / EASA Convention
FAA / English-Speaking Convention

The symbol U is used internationally (ICAO standard) to represent voltage, particularly in European and EASA technical documentation.

Capacitors and Capacitance

A simple capacitor consists of two metal plates separated by a gap containing a dielectric (an insulating material). Because of this gap, no current actually flows through a capacitor in steady-state DC conditions.

Capacitance — the capacitor’s ability to store electrical charge — is measured in Farads (F) and depends on:

Directly proportional to:
- The dielectric constant (k) of the material in the gap
- The area of the plates
Inversely proportional to:
- The distance between the plates

Dielectric Material	Approximate Dielectric Constant (k)
Air	k ≈ 1
Wax paper	k ≈ 3

In a DC circuit, a capacitor blocks direct current by charging up and storing energy. The dielectric between its plates prevents continuous electron flow, so instead of conducting current, the capacitor stores electrical energy.

Capacitors in parallel: connecting capacitors in parallel effectively increases the total plate area, so total capacitance increases, and is the sum of the individual values:

CT = C1 + C2 + …

Ohm’s Law and Electrical Power

Ohm’s Law states that current is directly proportional to voltage and inversely proportional to resistance:

I = V / R

This can be transposed to solve for any of the three variables:

V = I x R
R = V / I

Worked Example: I = V / R I = 6 / 2 I = 3 Amperes

In an electric circuit, work is done as voltage causes current to flow through a resistance, producing heat, magnetism, or chemical action (as in a battery). The rate at which this work is done is called power, measured in Watts (W):

P (Watts) = V (Voltage) x I (Amperes)

Since P = V x I, and using Ohm’s Law substitutions, power can also be expressed as:

P = V x I
P = I² x R
P = V² / R

Worked Example: P = V x I P = 3 x 4 P = 12 W

Resistors in Series and Parallel — Kirchhoff’s Laws

Series Resistors
Parallel Resistors

Because the resistances lie on the same current path, their values are cumulative. Total circuit resistance is the sum of the individual resistances:

RT = R1 + R2 + R3

If one resistor in a series string fails open, the entire circuit is broken.

Write the reciprocal formula: 1/RT = 1/R1 + 1/R2 (using R1 = 2 Ω and R2 = 3 Ω)
Sum the reciprocals: 1/RT = 1/2 + 1/3 = 1/0.8333
Invert the result to find RT: RT = 1.2 Ω

Kirchhoff’s Laws govern how current and voltage behave in any circuit, and underpin why parallel wiring works the way it does:

Kirchhoff’s First Law (Current Law): The total current entering a junction equals the total current leaving it.
Kirchhoff’s Second Law (Voltage Law): The sum of all voltage drops around a closed loop equals the total applied voltage.

In a parallel circuit, since each resistor shares the same voltage, the total current is the sum of the currents through each individual resistor — a direct consequence of Kirchhoff’s Current Law.

Resistors & Capacitors

Resistors in Series

Resistances are cumulative — the total resistance is the sum of all individual resistances:

Resistors in Parallel

Each resistor receives the same voltage and operates independently. Failure of one does not affect the others. Most aircraft loads are connected in parallel.

Kirchhoff’s Laws (parallel circuits):

First Law (Current): Total current entering a junction equals total current leaving it
Second Law (Voltage): Sum of all voltage drops in a closed loop equals the total applied voltage

Capacitors

A capacitor stores electrical charge using two metal plates separated by a dielectric (insulating material). Because of the gap, no current flows through the capacitor — it stores energy instead.

Capacitance (C) is measured in Farads (F) and is:

Directly proportional to plate area (larger area = higher capacitance)
Directly proportional to dielectric constant (k) (e.g. wax paper k ≈ 3, air k ≈ 1)
Inversely proportional to distance between plates (smaller gap = higher capacitance)

In a DC circuit, a capacitor blocks direct current by charging up — the dielectric prevents electron flow through it.

Capacitors in parallel — total capacitance increases:

Electrical Generation

Every aircraft electrical system starts with generation: turning mechanical energy from the engine into usable electrical power. On a PPL syllabus (EASA Part-FCL 021 — Airframes and Systems: Electrics), this is where examiners test whether you understand how the current is made, not just that it exists.

Both the generator and the alternator physically produce alternating current (AC) inside their windings. The difference is entirely in how each machine converts that AC into the direct current (DC) the aircraft actually uses:

Generator — uses a commutator (a mechanical, rotating switch) to convert AC into DC.
Alternator — uses a rectifier (solid-state diodes) to convert AC into DC.

Internally generates AC, converted to DC by a mechanical commutator.
Commutator and brushes are wearing parts — more maintenance, more friction.
Poor low-RPM output; less efficient at idle/taxi speeds.
Largely superseded by alternators in modern light aircraft.

How an Alternator Converts AC to Usable DC Power

Mechanical drive: The engine turns the alternator’s rotor via a belt or gear drive.
AC generation: The rotating magnetic field induces alternating current in the stator windings.
Rectification: The rectifier (diode pack) converts this AC into DC.
Voltage regulation: The voltage regulator holds the DC output within the aircraft’s operating voltage limits (commonly 13.8–14.2 V for a 12 V system, or 27–28.5 V for a 24 V system).
Distribution: Regulated DC feeds the electrical buses and charges the battery.

Centre-Zero Ammeter

The centre-zero ammeter is one of the most commonly misread instruments on a PPL exam, because its needle can move in either direction from a centred rest position.

Shows the current flow to or from the aircraft battery — not total system current.
Needle at zero: No current flow; battery is neither charging nor discharging.
Needle right of zero (positive): Alternator is charging the battery and powering the system.
Needle left of zero (negative): Battery is discharging; the alternator may not be working.
It helps the pilot monitor both battery status and alternator function at a glance.

Electrical Distribution

Once power is generated, it must be distributed to the systems that need it — reliably, safely, and with the right systems prioritised if something goes wrong.

DC vs. AC in Aircraft

DC (Direct Current)
AC (Alternating Current)

Flows in one direction, providing a stable voltage.
Used in smaller aircraft and for essential systems: avionics, lighting, navigation.
Batteries supply DC — especially critical during engine-off ground operations or emergencies, when no generator/alternator output is available.

Summary:

DC: Reliable, stable power for critical or low-power systems.
AC: Efficient and flexible for high-power loads in large aircraft.
Both systems are frequently used together on larger aircraft to meet different operational needs efficiently — DC for essential/avionics loads, AC for high-power services.

Electrical Bus Bars

A bus (or bus bar) is a central point in the aircraft’s electrical system where power is distributed out to multiple circuits and components. It exists to keep power distribution efficient, organised, and — critically — prioritised if the system loses a generation source.

Bus	Power Source	Typical Loads	Behaviour
Hot Battery Bus (Direct Bus)	Direct from the battery, at all times	Emergency lighting, fire extinguishers	Always live — even with the aircraft fully shut down
Essential Bus	Battery or generator/alternator	Navigation, communication	Stays powered from the battery if the generator fails
Non-Essential Bus	Generator/alternator (normally)	Cabin lighting, entertainment systems	Deliberately shed during a power emergency to preserve battery life
Switched Battery Bus	Battery, only when the battery switch is ON	Pilot-selected systems not always required	Off whenever the battery master switch is off

Parallel Wiring of Electrical Devices

Aircraft electrical devices are wired in parallel, not in series.

Each device receives the same voltage and can operate independently of the others.
This means that if one device fails or is switched off, the others remain powered and unaffected — a critical redundancy feature. (In a series circuit, by contrast, one failed component would break the entire circuit.)

Common Earth (Grounding)

In an aircraft, common earth means all negative terminals are connected to a single ground point — usually the airframe itself, which is typically constructed from conductive aluminium alloy.

This arrangement:

Provides a stable return path for electrical currents.
Simplifies wiring, since a dedicated negative return wire isn’t needed for every circuit.
Reduces overall aircraft weight.
Ensures consistent voltage references across the whole system.
Prevents malfunctions caused by voltage differences between components.
Makes maintenance and fault-finding easier.

Why Small Piston Aircraft Use DC Systems

Small piston-engine aircraft use a DC electrical system because it allows electrical components to run directly from the battery’s direct current supply.

This setup is simpler and more reliable for smaller aircraft, with fewer components and less complexity than an AC distribution system.
Although the alternator itself produces AC internally, its output is rectified to DC to maintain a consistent voltage and power supply suitable for onboard avionics and equipment — tying back directly to the Generation section above.

Electrical Components — Fuses & Circuit Breakers

The final piece of the system is protection: stopping an electrical fault before it causes component damage, overheating, or fire.

Excess current (overcurrent) occurs when a fault — such as a short circuit — lowers the resistance in a circuit. By Ohm’s Law (I = V / R), a drop in resistance (R) for a fixed voltage (V) produces a rise in current (I), which is exactly the overcurrent condition these devices are designed to catch.

Fuses vs. Circuit Breakers

Fuses
Circuit Breakers (CBs)

Blow when current exceeds their rated value.
Act before full fault current is reached, sacrificing themselves to protect the circuit.
Must be physically replaced once blown — there is no reset.

Both devices exist to protect circuits from overcurrent, which can otherwise cause overheating and — in the worst case — an electrical fire. This is also why they are rated in Amperes — the rating defines exactly the current level at which the device is designed to act.

In-Flight Circuit Breaker Reset — Decision Process

Identify the trip: Note which CB has tripped and what system it feeds.
Consult the checklist: Check the relevant abnormal/emergency checklist for that system before touching anything.
Follow checklist guidance: If the checklist specifically directs a reset, follow that procedure exactly.
Command judgement override: If no checklist guidance applies, only reset the CB if the captain/PIC assesses it as essential for the immediate safety of the flight.
Otherwise, leave it tripped: If in doubt, leave the CB out and manage without that system rather than risk reintroducing an active fault.

Trip-Free vs. Non-Trip-Free Circuit Breakers

There are two types of circuit breakers, and the distinction is a common exam trap.

Trip-Free (Safer)
Non-Trip-Free (Riskier)

If pushed back in while the fault is still present, the circuit will not be made — it trips again immediately.
This is the safer design, because it physically prevents you from holding a faulted circuit closed.

Hydraulics

Hydraulic systems use a confined, near-incompressible liquid to transmit force and multiply it — allowing a pilot to move heavy components (landing gear, brakes, and on larger types, flight controls) with a small input force. This corresponds to ATA Chapter 29 (Hydraulic Power) on the wider PPL/CPL systems syllabus.

Pascal’s Law and Hydraulic Force

Pascal’s Law states: a force applied to a confined liquid is felt equally in all directions.

Because of this, hydraulic systems obey a simple, direct relationship between force, pressure, and area:

F = P x A (Force = Pressure x Area)
P = F / A (Pressure = Force / Area)

Worked example — when does pressure actually build?

Pressure in a hydraulic system only arises when fluid flow is restricted.

If oil flows freely through an open tube, there is no resistance to flow, and therefore no pressure.
If the end of that tube is blocked, flow is restricted and pressure builds.

No restriction = no pressure. This is a key concept for understanding how actuators, valves, and brake calipers generate usable force from a moving fluid.

Key Hydraulic Fluid Properties

A hydraulic fluid must satisfy several competing requirements simultaneously:

Property	Why it matters
High flash / fire points	Reduces fire risk near hot engine or brake components
Thermal stability	Maintains consistent performance across operating temperatures
Non-corrosiveness	Protects metal components and seals from chemical attack
Incompressibility	Enables efficient, near-instant power transfer through the system
Lubricant	Reduces wear on pumps, valves, and actuators
Low viscosity	Flows easily, reducing pump workload and system lag
Material compatibility	Prevents damage to seals, hoses, and gaskets
Low emulsifying tendency	Resists mixing with water or air, which would degrade performance

How the Hydraulic System Powers Aircraft Components

Fluid storage: Hydraulic fluid is stored in the reservoir when not in use.
Pressurisation: The pump draws fluid from the reservoir and generates flow, pressurising it.
Directed flow: Pressurised fluid is directed through the system toward the point of use.
Conversion to motion: At the actuator, hydraulic energy is converted into mechanical motion — moving control surfaces or extending/retracting landing gear.
Braking application: Alternatively, pressurised fluid is sent to the brake calipers, applying pressure to the brake discs and giving the pilot both stopping power and directional control on the ground.

Types of Hydraulic Fluid

A phosphate-ester synthetic fluid used in high-performance aircraft.
Wide operating temperature range: -55 °C to +105 °C.
High fire resistance — more cavitation-resistant and less flammable than mineral or vegetable oils.
Most common grade, Skydrol 500B, is light purple; other grades are green.
Hygroscopic (absorbs moisture) and corrosive when overheated.
Damaging to PVC and wiring insulation — a key handling consideration during maintenance.
Poses serious health hazards; must be handled carefully.
Compatible seals: butyl rubber, silicone, or Teflon.

Anti-Icing Systems

Anti-icing and de-icing systems (ATA Chapter 30 — Ice and Rain Protection) protect critical surfaces from ice accretion, which can degrade aerodynamic performance, block sensors, or jam control surfaces.

Electric Heating Elements (Windshield)

Thin heated wires or films embedded in, or applied to, the windshield.
When activated, they warm the surface to prevent ice formation or melt ice that has already formed.
Common on general aviation and small commercial aircraft.
Typically switched on before entering icing conditions, in line with the timing principle above.
Provides consistent, effective heating and is simple for the pilot to operate.

Pitot Tube Anti-Ice

Pitot tubes are essential for measuring airspeed, but in icing conditions they are vulnerable to ice accumulation, which can impair or completely block their function.

Resistance wire installed: A resistance wire is embedded in the pitot tube, covering the areas most prone to ice formation.
Electric current applied: When the pitot heat system is activated, current flows through the resistance wire.
Heat generation: The wire resists this current flow, generating heat as a result (resistive/Joule heating).
De-icing action: The heat melts any existing ice and prevents new ice from accumulating.

Lubrication Systems

The lubrication system (ATA Chapter 79 — Oil) keeps engine internals cool, clean, and free of damaging friction, while also carrying away contaminants and heat generated by combustion and mechanical wear.

Wet vs. Dry Sump Oil Systems

Wet Sump
Dry Sump

Stores oil in the bottom of the engine itself (the sump).
Simpler and lower cost than a dry sump system.
Limited cooling capacity and limited oil capacity overall.
Lighter weight installation.
Because there’s less oil in circulation, it heats up faster.

Pressure Relief Valve (PRV) System

Function: Limits maximum system oil pressure by returning excess oil to the pump inlet.
Operation: The spring-loaded valve responds to pump-generated pressure, opening to divert oil and helping maintain a consistent feed pressure across the engine’s speed and temperature range.
Downside: At high engine speed, a significant amount of oil is spilled back to the tank via the relief valve, reducing overall system efficiency.

Oil Viscosity

Viscosity is a fluid’s resistance to flow, and it’s one of the key properties of any engine oil.

Viscosity decreases as temperature increases — meaning oil flows more easily when hot.
Because of this relationship, oil grade must be chosen to match the aircraft engine’s expected operating temperatures (hence multi-grade oils and seasonal oil selection in some piston fleets).

Oil System Monitoring

Parameter	Detail
Importance	Ensures proper engine function and helps prevent engine failure
Oil pressure measurement	Gauged after the pump, at the outlet side — typically 50–100 psi in normal operation
Sensing methods	Electrical sensors or mechanical direct-reading sensors
Mandatory flight deck indicators	Oil temperature and oil pressure must both be displayed
Oil quantity checks	May be displayed electronically, or checked via dipstick or sight glass before flight
Operational limits	Any deviation in oil temperature or pressure could signal an impending engine failure

Carburettors & Fuel Injection

This topic covers piston engine fuel metering (ATA Chapter 73 — Engine Fuel and Control), comparing the two main systems found on PPL-category aircraft: the float-type carburettor and fuel injection.

Fuel Injection vs. Carburettor Icing Risk

Fuel injection systems are less prone to icing than carburettors, simply because they don’t use a venturi.

In a carburettor, air passing through the venturi causes a pressure and temperature drop, which can lead to ice formation and airflow blockage.
Fuel injection delivers fuel directly into the intake manifold or cylinders without this cooling effect, greatly reducing both the risk of icing and the need for carburettor heat.

Fuel Injection vs. Carburettor Systems

Fuel Injection
Carburettor

Directly delivers fuel into the intake manifold or cylinders.
Uses high-pressure injectors for precise air-fuel control.
Ensures even fuel distribution, improving efficiency, performance, and emissions.
Does not rely on the venturi effect — the mechanism that causes the temperature drop responsible for carburettor icing.

Factor	Fuel Injection	Carburettor
Fuel delivery	Direct into manifold/cylinders	Mixed with air before entry
Precision	Better control, equal fuel amounts to each cylinder	More prone to uneven distribution
Efficiency	Enhances fuel economy and performance, reducing emissions	Lower efficiency under varying conditions
Icing reliability	Not prone to icing	Prone to icing via the venturi effect

How a Float-Type Carburettor Works

Air intake: Air is filtered and directed into the carburettor’s venturi.
Venturi acceleration: As air passes through the venturi, its velocity increases and its pressure decreases (Bernoulli’s Principle), creating a low-pressure area.
Fuel draw: The pressure differential between the air inlet and the venturi throat draws fuel from the float chamber into the airstream.
Atomisation: The venturi effect atomises the fuel, ensuring efficient mixing with the incoming air.
Throttle control: The throttle (butterfly) valve regulates the mixture flow to the engine’s inlet valve, based on the pressure differential and airflow, controlling overall power output.

Additional characteristics of float-type carburettors:

Meter fuel at the venturi throat, where airflow is fastest.
Control the air-to-fuel ratio automatically across all power/throttle settings.
Function in all flight conditions.
Enable easy engine starting.
Allow fuel shutoff (used for mixture/idle cutoff control).

Carburettor Icing: Causes & Conditions

Causes: High humidity (above 50% RH) combined with temperatures between -20 °C and +33 °C can lead to icing in the induction system.
Venturi effect: As air accelerates through the venturi, its temperature drops — this cooling is compounded by the latent heat absorbed as fuel evaporates, so ice can form on the venturi walls and throttle valve even when ambient temperature is above 0 °C.
No visible moisture needed: High humidity alone can cause icing — clouds or visible precipitation are not required.
Low power settings: A partially closed throttle valve restricts airflow, which intensifies the venturi effect and produces greater air cooling — this is why icing is most likely at idle, descent, or approach power settings.
Prevention: Carburettor heat prevents icing at low throttle settings, ensuring smooth engine operation and avoiding unwanted power loss.

Recognising Carburettor Icing

Carburettor icing is indicated by a gradual drop in RPM or engine power (on a fixed-pitch propeller aircraft), caused by ice restricting airflow and disrupting the fuel-air mixture.

As air speeds up through the venturi, its temperature drops.
If moisture is present, it can freeze on the venturi walls and throttle valve, reducing both airflow and fuel delivery.
This leads to rough running and power loss.
If not corrected with carburettor heat, icing can progress to complete engine failure.

Rough running isn’t always icing, though — incorrect fuel/air mixture produces similar symptoms:

Mixture Too Rich
Mixture Too Lean

Excess fuel relative to air.
Burns inefficiently, causing poor engine performance.

Carburettor Heat: Expected Response

Initial RPM/power drop: Selecting carb heat introduces hot, unfiltered air from the exhaust manifold in place of colder, denser air. This creates a richer fuel-air mixture and briefly reduces combustion efficiency — expect an immediate small drop in RPM or power.
Gradual recovery (if ice was present): As the hot air melts any accumulated ice, airflow is restored and RPM/power gradually climbs back toward normal.
No recovery (if no ice was present): If no ice existed, the warm air permanently reduces power slightly, because warm air is less dense and carries lower oxygen content — RPM/power will not recover further.

Symptoms to watch for: engine roughness or misfires. Once ice melts and carb heat has done its job, efficient operation resumes.

Ignition circuits

Aircraft ignition (ATA Chapter 74 — Ignition) is built around the magneto — a self-contained, engine-driven ignition source that is completely independent of the aircraft’s battery and electrical system.

Propeller Handling and Magneto Safety

Confirm ignition OFF: Verify both magneto switches (and the master ignition switch, if fitted) are OFF before approaching the propeller — regardless of battery/master switch state.
Treat the prop as live: Never assume the engine can’t start just because the aircraft’s electrical master is off — magnetos don’t need it.
Stand clear of the arc: Position yourself outside the propeller’s rotational plane whenever possible while confirming switch positions.
Move the propeller deliberately: Only then manually rotate the propeller if required, following the applicable checklist/procedure.

How Magnetos Provide Redundant Ignition

Independent ignition: Aircraft engines are typically fitted with two engine-driven magnetos, each firing its own set of spark plugs, to provide redundancy in case one fails.
Failure effect: If one magneto fails, combustion becomes less efficient, causing lower power output and RPM — but critically, the engine keeps running on the remaining magneto.

Magneto (Mag) Drop Check

Set run-up RPM: Stabilise the engine at approximately 75% of maximum engine speed (or per the manufacturer’s checklist) as part of the pre-flight run-up.
Isolate first magneto: Switch the ignition from BOTH to one magneto only (e.g., LEFT), and note the resulting RPM drop.
Return to BOTH: Switch back to BOTH and allow RPM to restabilise before testing the other magneto.
Isolate second magneto: Switch to the remaining magneto (e.g., RIGHT) only, and note the RPM drop.
Compare against limits: Check both the individual RPM drop and the difference between the two drops against the manufacturer’s published limits.

Why RPM drops: With only one magneto firing, ignition efficiency is reduced — combustion slows and power generation in each cylinder is delayed, producing a measurable RPM loss.

Mixture Control

Fuel/Air Mixture Ratios

The fuel-to-air mixture ratio is critical for efficient combustion. Carburettors are calibrated to full rich at sea level for dense air conditions.

Ratio	Name	Effect
8:1	Rich mixture	Excess fuel; cooler combustion; used for engine cooling at high power
12.5:1	Best power	Best combination of cooling, cylinder lubrication, and optimal performance
15:1	Stoichiometric (chemically correct)	Ideal fuel-oxygen balance; risks high temperatures and detonation

Leaning the Mixture

The pilot reduces fuel flow using the mixture control lever to match the lower air density at altitude, restoring efficient combustion.

Effects of leaning:

Leaner mixture → less fuel relative to air → combustion efficiency improves
EGT (Exhaust Gas Temperature) increases as mixture approaches peak EGT
Fuel also helps cool the cylinder head (CHT), so leaning reduces cooling
If leaned too close to or beyond peak EGT, combustion becomes excessively hot → raises CHT and EGT → risk of detonation and engine damage

EGT Gauge

The EGT gauge tracks mixture efficiency:

Leaning increases EGT until peak EGT is reached
Leaning beyond peak EGT reduces power and risks overheating
Used as the primary reference for mixture adjustment in cruise

Fuel

Fuel properties — grade, octane rating, and combustion behaviour — directly determine how much compression an engine can safely use, and what goes wrong when that limit is exceeded. This corresponds to ATA Chapter 28 (Fuel) on the wider systems syllabus.

AVGAS Types & Identification

Grade	Colour	Lead Content	Notes
AVGAS 100LL	Blue	Low lead	Most common in GA worldwide
AVGAS 100	Green	High lead	Less common; higher octane
AVGAS 80	Red	Very low lead	Older grade; rarely available

Octane Rating

Octane rating measures a fuel’s anti-knock (anti-detonation) capability — higher ratings allow greater compression ratios without detonation.

Benefits of high-octane fuels:

Increased compression ratios → improves thermal efficiency, fuel consumption, and engine power
Higher induction pressure → allows superchargers to boost engine power by increasing the mixture weight burned per unit time
Higher flash point → contributes to better engine performance and efficiency

Detonation

Detonation occurs when the unburnt end-gas in the combustion chamber ignites explosively rather than burning in a controlled flame front. This happens when pressure and heat become excessive — fuel’s anti-detonation limits exist precisely to prevent this.

Normally: The spark plug ignites the mixture, and a flame front moves smoothly across the chamber, heating the gases ahead of it and creating a controlled, progressive pressure wave that pushes the piston down.

Detonation: As the normal flame front advances, it compresses and heats the remaining unburnt fuel ahead of it. If pressure and heat cross a critical threshold, this remaining unburnt end-gas spontaneously combusts all at once — explosively.

Pre-ignition

Pre-ignition occurs when the fuel-air mixture is ignited prematurely by a hotspot inside the cylinder before the spark plug fires. This creates an uncoordinated second flame front while the piston is still moving upward on the compression stroke.

Normally: The spark plug fires at a precise moment near the top of the piston’s stroke, ensuring the resulting pressure wave pushes the piston down during the power stroke.

Pre-ignition: An abnormal hotspot — such as a glowing carbon deposit, an overheated spark plug electrode, or a damaged valve edge — ignites the mixture early.

Alternate Fuels

Fuel	Energy Content vs AVGAS	Vapour Lock Risk	Notes
MOGAS (Motor Gasoline)	Lower — more fuel needed for same power; reduced range	Higher — lower boiling point; vaporises more easily	Cheaper and more available; certified use only; ethanol content reduces energy further
Diesel	Higher per litre/gallon	Low	Compression ignition — not interchangeable with AVGAS; requires a diesel-certified engine

Effects of using alternate fuels on piston engines:

More fuel consumption, shorter range, and potentially lower power — especially during takeoff
MOGAS may be used only if the aircraft is certified for it; performance limitations may apply

Vapour Lock — Why Some Fuels Are More Prone

Vapour lock occurs when fuel vaporises too early within the fuel system, forming bubbles that block liquid fuel flow to the engine — reducing performance or causing outright engine failure.

Why MOGAS is more prone to vapour lock than AVGAS:

Lower boiling point — MOGAS vaporises at lower temperatures than AVGAS, making it inherently more prone to vapour lock.
Altitude effects — higher altitude means lower ambient air pressure, which makes MOGAS boil more easily inside the fuel lines.
Temperature sensitivity — hot weather worsens the tendency toward vapour lock, which can cause engine hesitation during critical flight phases such as takeoff or climb.
Fuel system design — aircraft fuel systems are engineered around AVGAS’s stability characteristics, so running MOGAS reduces the built-in margin against vapour lock.

Specific Fuel Consumption (SFC)

SFC measures how efficiently an engine converts fuel into power:

Fuel System

The fuel system (ATA Chapter 28) stores fuel, delivers it to the engine at the correct pressure, and gives the pilot the means to monitor and manage it throughout the flight.

Unusable Fuel

Unusable fuel is fuel that physically remains in the tank but cannot be safely drawn upon during flight.

This happens because the fuel pickup — which draws fuel from the tank to the engine — is not positioned at the absolute bottom of the tank.
As a result, a small amount of fuel always stays at the bottom of the tank, inaccessible to the fuel pumps.

Fuel Delivery Methods: Gravity Feed vs. Fuel Pressure

Gravity Feed
Fuel Pressure (Pressurised)

Uses gravity to deliver fuel from tanks positioned above the engine.
Simple, lightweight, low-maintenance, and reliable without any external power source — an electric pump may still assist.
Can struggle at high altitudes or during extreme manoeuvres, where fuel may momentarily move away from the feed line.
Common in small, carburetted aircraft.

Fuel Pumps

Electric Pump
Engine-Driven Pump

Acts as the backup/boost/priming pump.
Used during engine start, take-off, and landing, or to help prevent vapour lock.
Often serves as a backup to the engine-driven pump if it fails.

Pump failure: in a pressurised system, pump failure can stop the engine entirely — this is why the electric pump exists as a backup to the engine-driven pump.

Without adequate fuel pressure, fuel can’t reliably reach the carburettor or injectors, and the engine may lose power or fail entirely.

High-Wing vs. Low-Wing Fuel Feed

High-Wing Aircraft
Low-Wing Aircraft

Fuel tanks located in the wings sit above the engine, so gravity alone can feed the engine, often eliminating the need for a fuel pump.
Some high-wing aircraft still fit an electrical auxiliary fuel pump to ensure adequate fuel flow in all conditions.

Air Contamination in the Fuel System

Fuel Tank Venting

The primary purpose of the fuel tank vent system is to allow air into the tank to replace the fuel that’s consumed during flight.

Fuel is consumed: As the engine burns fuel, the volume of fuel remaining in the tank decreases.
Vacuum risk: Without a vent to let air in, this shrinking fuel volume would create a vacuum above the fuel.
Pressure drop: Tank pressure could then fall below atmospheric pressure.
Flow restriction: This pressure drop prevents fuel from flowing freely toward the engine.
Fuel starvation: Left uncorrected, this can starve the engine of fuel and cause it to stop.

Fuel Drain Valve — Pre-Flight Sampling

The drain valve lets pilots or mechanics sample fuel from the lowest point in the system, where water and sediment naturally collect.

Locate the drain: Identify the drain valve at the lowest point of the tank/system (often at the strainer).
Open the valve: Draw a fuel sample into a clear sampling cup.
Check for water: Water is denser than fuel and settles at the bottom of the sample — look for a distinct layer or droplets.
Check colour and clarity: Confirm the fuel is the correct colour for its grade (see AVGAS Types table above) and free of visible contaminants.
Dispose and close: Safely dispose of the sample and ensure the drain valve is fully closed before flight.

Vapour Lock Prevention

Building on the fuel-property causes covered in the Fuel section above, vapour lock at the aircraft system level typically occurs in hot weather, at high altitude, or with low fuel levels — especially when fuel lines run close to hot components, or when the fuel itself has a low boiling point.

Consequences: rough running, power loss, or engine failure — especially dangerous during takeoff or climb.

Prevention includes:

Keeping fuel systems cool
Using submersible low-pressure booster pumps
Avoiding long idling periods in hot weather
Keeping fuel tanks adequately filled
Minimising engine heat during ground operations

Common Fuel System Components

Component	Function
Fuel lines	Deliver fuel through the system with minimal flow resistance
Boost pumps	Assist fuel transfer, particularly during start, take-off, landing, or high-vapour-lock-risk conditions
Fuel pressure valves	Regulate fuel transfer and give the pilot manual control where fitted
Filters	Prevent debris from entering the fuel system and reaching the engine
Strainer	Located at the system’s low point to trap contaminants; fitted with a drain for sampling and draining

Cockpit Fuel Monitoring

Parameter	What It Tells the Pilot
Fuel pressure	Confirms the engine is receiving fuel at the proper pressure; helps detect vapour lock or pump issues; read from a cockpit gauge
Fuel temperature	Monitored on some aircraft (mainly high-altitude or jet-fuel types) to prevent icing or vapour formation — more common on larger aircraft
Total fuel mass	Continuously monitored to ensure sufficient fuel remains; shown as mass or volume
Fuel flow	Shows current consumption rate to help manage efficiency; some systems display both instantaneous and average flow
Remaining endurance	Calculated from current fuel flow and usable fuel remaining; displayed as time remaining at the current burn rate

Fuel Tank Construction Types

Built directly into the aircraft’s structure — typically the wings.
Maximises usable space and minimises added weight.
Common on larger aircraft that need high fuel capacity.

Air Cooling Systems

Most piston aircraft engines are air-cooled rather than liquid-cooled — combustion heat is carried away directly by airflow moving over the cylinders, rather than through a coolant loop and radiator. This makes airflow, not just outside air temperature, the dominant factor in engine cooling.

Factors Affecting Air-Cooled Engine Cooling

Factor	Effect on Cooling
High RPM	Generates more heat, requiring effective dissipation to avoid a temperature rise
Low airspeed	Reduces airflow over the cooling fins, increasing overheating risk
Cooling fins & baffles	Become less effective at low airspeeds, limiting heat exchange with the surrounding air
Cowlings & airflow design	Engineered for cooling, but the airflow they rely on weakens at low speed

Efficient air cooling depends on both airspeed and temperature — which is exactly why high RPM combined with low airspeed is the classic recipe for engine overheating.

How Air Flows Through the Cooling System

Air intake: Cooling air enters through inlets in the cowling, typically around or behind the propeller spinner.
Baffle direction: Internal baffles direct this incoming air downward and around each cylinder’s cooling fins, forcing it through the fin gaps rather than around them.
Heat transfer: Heat moves from the hot fin surfaces into the moving air by convection — the faster the airflow, the more effective this transfer is.
Exit: The now-heated air exits through an outlet or cowl flap at the rear or bottom of the cowling, completing the airflow path and drawing fresh air in behind it.

Cylinder Arrangement and Cooling Uniformity

Engine layout has a direct effect on how evenly each cylinder is cooled — not just how much total airflow reaches the engine.

Radial Engine
Inline & V-Type Engine

Cylinders are mounted in a circle around the crankcase, all facing forward into the airstream.
Every cylinder receives an even, equal share of airflow.
This produces the most effective and uniform cooling of any common piston layout.