Module 2: Physics — Overview

Module 2 of the EASA Part-66 basic knowledge syllabus provides the fundamental physical principles that underpin all aircraft maintenance activities. This module is not merely an academic exercise; it forms the scientific basis for understanding engine operation, structural loads, hydraulic and pneumatic systems, and aerodynamic behaviour. For the B1.2 category (piston-engine aeroplanes), a thorough grasp of mechanics, thermodynamics, fluid dynamics, and basic electricity is essential for safe and effective maintenance practice.

The module is structured around several core areas: matter and SI units, mechanics (statics, kinematics, and kinetics), thermodynamics and heat transfer, fluid dynamics and aerodynamics, and basic electricity. This textbook chapter synthesises the knowledge required for the B1.2 examination, focusing on the practical applications a certifying staff member will encounter daily.

2.1 Matter, SI Units, and Fundamental Quantities

The International System of Units (SI)

The aviation industry, and EASA regulations specifically, mandate the use of SI units for all maintenance documentation and procedures. However, legacy aircraft and American-manufactured components often use imperial units. The certifying staff member must be fluent in converting between these systems.

Base SI Units Relevant to Aircraft Maintenance:

Quantity	Unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Temperature	kelvin	K
Electric current	ampere	A
Frequency	hertz	Hz (1 cycle per second)

Derived Units Commonly Used:

Quantity	Unit	Symbol	Equivalent
Force	newton	N	kg·m/s²
Pressure	pascal	Pa	N/m²
Work/Energy	joule	J	N·m
Power	watt	W	J/s
Torque	newton-metre	N·m	—
Angular velocity	radian per second	rad/s	—

Density and Mass Flow

Density (ρ) is mass per unit volume, expressed in kg/m³. For aviation fuel, density is often given in kg/litre. The relationship between mass, volume, and density is fundamental:

\rho = \frac{m}{V}

Mass flow rate (\(\dot{m}\)) is the mass of fluid passing a point per unit time:

\dot{m} = \dot{V} \times \rho

Where \(\dot{V}\) is the volumetric flow rate.

Pressure Units and Conversions

Pressure is defined as force per unit area. The SI unit is the pascal (Pa), but aviation practice frequently uses bar, psi, and hPa.

Critical Conversion Factors:

1 bar = 100,000 Pa = 100 kPa
1 bar = 14.5038 psi
1 psi = 6.89476 kPa
1 hPa = 100 Pa (hectopascal, used in meteorology)

Example: A hydraulic system pressure of 3000 psi converts to:

3000 \div 14.5 = 206.9 \text{ bar}

Temperature Scales

Three temperature scales are encountered in aviation:

Celsius (°C) — used in most European documentation
Kelvin (K) — the SI unit, used in all thermodynamic calculations
Fahrenheit (°F) — found in American legacy documentation

Conversions:

K = °C + 273.15

°F = (°C \times \frac{9}{5}) + 32

°C = (°F - 32) \times \frac{5}{9}

Critical Point: All gas law calculations must use absolute temperature (Kelvin), never Celsius.

2.2 Mechanics — Statics, Kinematics, and Kinetics

Newton's Laws of Motion

First Law (Law of Inertia): An object at rest remains at rest, and an object in motion continues at constant velocity, unless acted upon by an external force. This explains why an aircraft parked on the apron remains stationary until thrust or towing force is applied.

Second Law: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass:

F = m \times a

Third Law: For every action, there is an equal and opposite reaction. This is the principle behind propeller thrust and jet propulsion.

Weight and Mass

Mass (kg) is the quantity of matter in an object. Weight (N) is the force exerted on that mass by gravity:

W = m \times g

Where \(g = 9.81 \text{ m/s}^2\) at standard sea level conditions.

Example: A connecting rod with a mass of 2.5 kg has a weight of:

W = 2.5 \times 9.81 = 24.525 \text{ N}

Pressure Exerted by a Component

When a component rests on a surface, the pressure exerted is:

P = \frac{F}{A} = \frac{m \times g}{A}

Example: A 120 kg component on a 0.05 m² mount exerts:

P = \frac{120 \times 9.81}{0.05} = 23,544 \text{ Pa}

Torque and Moments

Torque (or moment of a force) is the turning effect produced by a force acting at a perpendicular distance from a pivot point:

T = F \times r

Where \(T\) is torque in N·m, \(F\) is force in N, and \(r\) is the perpendicular distance in m.

Example: A force of 50 N applied at the end of a 0.4 m torque wrench produces:

T = 50 \times 0.4 = 20 \text{ N·m}

This principle is fundamental to all torque-controlled fastening procedures in aircraft maintenance.

Stress

When a component is subjected to axial forces, it experiences direct stress:

\sigma = \frac{F}{A}

Where \(\sigma\) is stress in Pa (or N/m²). Direct stress can be:

Tensile — pulling forces (e.g., connecting rod during induction stroke)
Compressive — pushing forces (e.g., connecting rod during power stroke)

The connecting rod of a piston engine experiences alternating tensile and compressive direct stresses during each operating cycle.

Cross-Sectional Area Calculations

For a circular cross-section (e.g., a bolt):

A = \frac{\pi}{4} \times d^2

Example: An 8 mm diameter bolt has a cross-sectional area of:

A = 0.7854 \times (0.008)^2 = 5.03 \times 10^{-5} \text{ m}^2

2.3 Rotational Motion and Engine Dynamics

Angular Velocity

Rotational speed is commonly expressed in revolutions per minute (RPM). For engineering calculations, angular velocity in radians per second (rad/s) is required:

\omega = \frac{2\pi \times \text{RPM}}{60}

The conversion factor is approximately 0.10472 rad/s per RPM.

Example: A crankshaft rotating at 2400 RPM has an angular velocity of:

\omega = 2400 \times \frac{2\pi}{60} = 251.3 \text{ rad/s}

Frequency

Frequency (Hz) is the number of cycles per second:

f = \frac{\text{RPM}}{60}

Example: A propeller rotating at 3000 RPM has a rotational frequency of:

f = \frac{3000}{60} = 50 \text{ Hz}

Power, Torque, and Rotational Speed

The relationship between brake power, torque, and angular velocity is:

P = T \times \omega

Where \(P\) is power in watts, \(T\) is torque in N·m, and \(\omega\) is angular velocity in rad/s.

Example: An engine developing 120 kW at 2400 RPM produces:

T = \frac{120,000}{251.33} = 477.5 \text{ N·m}

Work and Power

Work is the product of force and distance moved in the direction of the force:

W = F \times d

Power is the rate of doing work:

P = \frac{W}{t}

Example: An engine requiring 20 kW to drive a propeller for 2 hours performs:

W = 20 \text{ kW} \times 2 \text{ h} = 40 \text{ kWh}

2.4 Thermodynamics and Heat Transfer

Temperature and the Gas Laws

The behaviour of gases in engine cylinders is governed by the ideal gas laws. For maintenance personnel, the most important relationships are:

Boyle's Law (constant temperature):

P_1 V_1 = P_2 V_2

Gay-Lussac's Law (constant volume):

\frac{P_1}{T_1} = \frac{P_2}{T_2}

Charles's Law (constant pressure):

\frac{V_1}{T_1} = \frac{V_2}{T_2}

Critical Point: All temperatures in gas law calculations must be in Kelvin.

Example: If cylinder pressure increases from 2 bar to 8 bar at constant volume, the absolute temperature quadruples (from \(T_1\) to \(4T_1\)).

Compression Ratio

The compression ratio of a piston engine is defined as:

r = \frac{V_{\text{BDC}}}{V_{\text{TDC}}} = \frac{V_s + V_c}{V_c}

Where:

\(V_{\text{BDC}}\) = total cylinder volume at bottom dead centre
\(V_{\text{TDC}}\) = clearance volume at top dead centre
\(V_s\) = swept volume
\(V_c\) = clearance volume

Example: With a compression ratio of 8:1 and clearance volume of 0.15 litres:

8 = \frac{V_s + 0.15}{0.15}

V_s = (8 \times 0.15) - 0.15 = 1.05 \text{ litres}

Swept Volume

The swept volume of one cylinder is the volume displaced by the piston moving from TDC to BDC:

V_s = \frac{\pi}{4} \times d^2 \times L

Where \(d\) is the bore (cylinder diameter) and \(L\) is the stroke.

Example: A cylinder with 120 mm bore and 140 mm stroke has a swept volume of:

V_s = 0.7854 \times (0.12)^2 \times 0.14 = 1.58 \times 10^{-3} \text{ m}^3

Heat Transfer Mechanisms

Three modes of heat transfer are relevant to engine cooling:

1. Conduction — Heat transfer through solid materials (e.g., through cylinder walls). This is the primary mode of heat transfer in the metal walls of an air-cooled engine.

2. Convection — Heat transfer between a solid surface and a moving fluid (e.g., oil cooler transferring heat to airflow). This involves both conduction through the wall and convection to the fluid.

3. Radiation — Heat transfer by electromagnetic waves. Negligible at typical engine operating temperatures.

Example: An oil cooler transfers heat by conduction through the metal walls, followed by convection to the surrounding air. Radiation is negligible.

2.5 Fluid Dynamics and Aerodynamics

Bernoulli's Principle

Bernoulli's equation for incompressible, frictionless flow states that the total energy along a streamline is constant:

P + \frac{1}{2}\rho V^2 + \rho gh = \text{constant}

For horizontal flow (neglecting height changes):

P + \frac{1}{2}\rho V^2 = \text{constant}

This principle explains:

Lift generation — faster airflow over the upper surface of an aerofoil creates lower pressure, producing lift
Venturi effect — in a carburettor venturi, increased air velocity at the throat causes a pressure drop that draws fuel

Critical Relationship: The dynamic pressure is proportional to the square of velocity:

q = \frac{1}{2}\rho V^2

Example: If air velocity through a venturi doubles, the pressure drop quadruples.

Angle of Attack

The angle of attack is the angle between the chord line of an aerofoil and the relative wind (the undisturbed airflow direction). This is a critical parameter determining lift and drag characteristics.

Mach Number

The Mach number is the dimensionless ratio of the speed of an object to the speed of sound in the surrounding medium:

M = \frac{V}{a}

Where \(V\) is the object's velocity and \(a\) is the local speed of sound.

International Standard Atmosphere (ISA)

At sea level, the ISA model defines:

Temperature: 15°C (288.15 K)
Pressure: 1013.25 hPa (1 atm)
Density: 1.225 kg/m³

These values are fundamental for engine performance calculations and aerodynamic analysis.

2.6 Electricity and Magnetism

Electromagnetic Induction

The magneto ignition system in a piston engine operates on the principle of electromagnetic induction (Faraday's Law). A rotating magnet induces a high voltage in a stationary coil, which is then delivered to the spark plugs.

Faraday's Law: The induced electromotive force (EMF) in a coil is proportional to the rate of change of magnetic flux through the coil.

2.7 Engine Performance Parameters

Brake Specific Fuel Consumption (BSFC)

BSFC measures fuel efficiency:

\text{BSFC} = \frac{\text{Fuel mass flow rate}}{\text{Brake power}} \quad (\text{kg/kWh})

Example: An engine with BSFC of 0.25 kg/kWh producing 100 kW for 2 hours consumes:

\text{Fuel} = 0.25 \times 100 \times 2 = 50 \text{ kg}

Fuel Consumption Calculations

For volumetric fuel flow with known density:

\dot{m}_{\text{fuel}} = \dot{V}_{\text{fuel}} \times \rho_{\text{fuel}}

Example: An engine consuming 40 L/h of fuel with density 0.72 kg/L has a mass flow of:

\dot{m} = 40 \times 0.72 = 28.8 \text{ kg/h}

Common Relationships Between Concepts

Force → Pressure → Stress: Force produces pressure (force/area) on surfaces and stress (force/area) within materials. The same fundamental relationship applies.
Mass → Weight → Pressure: Mass under gravity produces weight (force), which when distributed over an area produces pressure.
RPM → Angular Velocity → Frequency: Rotational speed can be expressed as angular velocity (rad/s) or frequency (Hz), with conversions involving \(2\pi\) and 60.
Power ↔ Torque ↔ RPM: These three quantities are linked by \(P = T \times \omega\). Increasing RPM at constant power reduces torque.
Pressure ↔ Temperature (Gas Laws): At constant volume, pressure and absolute temperature are directly proportional.
Velocity ↔ Pressure (Bernoulli): In fluid flow, increased velocity produces decreased pressure, with the relationship being quadratic.
Compression Ratio ↔ Volumes: Compression ratio links swept volume, clearance volume, and total cylinder volume.

Typical Exam Focus Points

For the EASA Part-66 Module 2 examination (B1.2 category), candidates should focus on:

Unit conversions — particularly pressure (bar, psi, kPa, hPa) and temperature (°C, K, °F). These appear frequently and require memorisation of conversion factors.
Newton's laws and force calculations — \(F = ma\) and \(W = mg\) applications in engine and structural contexts.
Pressure calculations — \(P = F/A\) with correct unit handling.
Torque calculations — \(T = F \times r\) for fastening applications.
Rotational conversions — RPM to rad/s and Hz, including the \(2\pi/60\) factor.
Engine geometry — swept volume, clearance volume, and compression ratio calculations, including rearranging the compression ratio formula.
Gas laws — understanding pressure-temperature relationships at constant volume, using absolute temperatures.
Heat transfer modes — identifying conduction, convection, and radiation in engine cooling contexts.
Bernoulli's principle — qualitative understanding of velocity-pressure relationships and the quadratic nature of dynamic pressure.
Power and work — \(P = W/t\) and \(P = T\omega\) relationships, including BSFC applications.
ISA standard values — sea-level temperature, pressure, and density.
SI units — identifying correct units for all quantities, particularly frequency (Hz) and pressure (Pa).

Regulatory References

Regulation (EU) No 1321/2014, Annex III (Part-66) — establishes the basic knowledge requirements for aircraft maintenance licences
Appendix I to Part-66 — defines the module structure and knowledge levels:
Level 1: Overview (familiarity with basic elements)
Level 2: General knowledge (understanding of principles and applications)
Level 3: Detailed theory (comprehensive understanding for certification decisions)
AMC (Acceptable Means of Compliance) — provides guidance on examination standards and question types

The B1.2 category requires knowledge levels of 2 or 3 for most Module 2 topics, reflecting the need for certifying staff to understand not just what to do, but why it works.