Module 2: Physics

Module 2: Physics – EASA Part-66 Category A (Turbine)

1. Module Overview

Module 2 of the EASA Part-66 basic knowledge syllabus provides the foundational physics principles required for the safe and effective maintenance of turbine-powered aeroplanes. This module is not about abstract theory; it is the applied physics that governs the operation, troubleshooting, and maintenance of aircraft systems. For a Category A certifying staff member, a solid grasp of these concepts is essential for tasks ranging from reading pressure gauges and performing hydraulic system checks to understanding thermal effects on components and calculating basic mechanical forces.

The module is structured to build from fundamental units and matter properties through to mechanics, thermodynamics, and optics. The knowledge levels for Category A are primarily at Level 1 (overview) and Level 2 (general knowledge), meaning you are expected to understand the principles, their applications, and be able to perform standard calculations using the relevant formulas. This study material synthesises the core knowledge from the syllabus, focusing on the practical applications you will encounter in a line or base maintenance environment.

2. Key Concepts and Detailed Theory

This section is structured to follow the logical progression of the Part-66 syllabus, from basic units to complex system applications.

2.1 Matter, Units, and Measurement

This foundational area covers the nature of matter and the standard units used to quantify its properties.

• States of Matter: Matter exists in three primary states relevant to aircraft: solid (e.g., turbine blades, structural members), liquid (e.g., fuel, hydraulic fluid), and gas (e.g., air, nitrogen in accumulators). Changes in temperature and pressure can cause transitions between these states (e.g., fuel vaporisation, water condensation in fuel tanks).
• SI Units: The International System of Units (SI) is the global standard used in aviation documentation and maintenance manuals. You must be fluent in these units and their conversions.
• Length: metre (m)
• Mass: kilogram (kg)
• Time: second (s)
• Temperature: kelvin (K) or degree Celsius (°C)
• Force: newton (N) = kg·m/s²
• Pressure: pascal (Pa) = N/m²
• Energy/Work: joule (J) = N·m
• Power: watt (W) = J/s
• Temperature Scales: The Celsius scale (°C) is commonly used for ambient and operational temperatures. The Kelvin scale (K) is the absolute thermodynamic temperature scale, used in all gas law calculations. The conversion is: K = °C + 273.15. For example, a bearing housing at 50 °C has an absolute temperature of 323.15 K.
• Pressure Units: Pressure is force per unit area. The SI unit is the pascal (Pa). However, aviation documentation often uses other units, making conversion critical.
• 1 bar = 100,000 Pa = 100 kPa
• 1 psi ≈ 6,894.76 Pa ≈ 6.895 kPa
• 1 MPa = 1,000,000 Pa
• A hydraulic system at 3000 psi is equivalent to approximately 20.68 MPa or 206.8 bar.
• Density and Specific Gravity: Density (ρ) is mass per unit volume (kg/m³). Specific gravity (SG) is the ratio of a substance's density to the density of water (1000 kg/m³). It is a dimensionless number. For example, a fuel with an SG of 0.8 has a density of 800 kg/m³. Specific gravity is temperature-dependent; hydrometer readings must be corrected to a standard temperature (typically 15 °C) using manufacturer-provided correction factors.

2.2 Statics: Forces, Moments, and Pressure

Statics deals with bodies at rest or in constant motion, where all forces are balanced. This is crucial for understanding structural loads, hydraulic systems, and torque applications.

• Force and Moment (Torque): A force is a push or pull. A moment (or torque) is the turning effect of a force about a pivot point. It is calculated as the product of the force and the perpendicular distance from the line of action of the force to the pivot.
• Torque (M) = Force (F) × Perpendicular Distance (d)
• If the force is applied at an angle (θ) to the lever arm, the perpendicular component is used: M = F × d × sin(θ).
• For example, a force of 200 N applied at 30° to a 0.5 m lever arm produces a torque of 200 N × 0.5 m × 0.5 = 50 N·m. This principle is fundamental to torque wrenches, control surface actuation, and many other mechanical systems.
• Pressure in Fluids (Hydrostatics): A fluid at rest exerts pressure on any surface it contacts. This pressure increases with depth due to the weight of the fluid above.
• Pressure due to fluid head: p = ρ × g × h
• Where: p = pressure (Pa), ρ = fluid density (kg/m³), g = acceleration due to gravity (9.81 m/s²), h = height of the fluid column (m).
• This is used to calculate the pressure at the bottom of a fuel tank (e.g., a 2.5 m deep tank of fuel with SG 0.8 gives a gauge pressure of 19.62 kPa) or the pressure at a pump inlet due to a fluid head.
• Gauge vs. Absolute Pressure: Gauge pressure is the pressure measured relative to atmospheric pressure. Absolute pressure is the total pressure, including atmospheric pressure.
• Absolute Pressure = Gauge Pressure + Atmospheric Pressure
• A tyre pressure gauge reads gauge pressure. If a pneumatic line reads 6 bar gauge and atmospheric pressure is 1 bar, the absolute pressure is 7 bar (700,000 Pa).
• Pascal's Principle: This principle states that pressure applied to a confined fluid is transmitted undiminished in every direction throughout the fluid. This is the fundamental basis of all hydraulic systems.
• P = F₁/A₁ = F₂/A₂
• This allows for force multiplication. A small force applied to a small-area piston creates a pressure that acts on a larger-area piston, generating a much larger force. For example, a 500 N force on a 20 mm diameter master cylinder can lift a 12,500 N load on a 100 mm diameter slave cylinder. The system pressure remains constant throughout, assuming no losses and a static condition.
• Stress and Strain: When a load is applied to a material, it experiences internal forces (stress) and deforms (strain).
• Stress (σ) = Force (F) / Cross-sectional Area (A) – Unit: pascal (Pa) or N/m².
• Strain (ε) = Change in Length (ΔL) / Original Length (L₀) – This is dimensionless.
• For example, a 40 kN tensile load on a 200 mm² tie rod creates a stress of 200 MPa. A 1.000 m rod stretching to 1.002 m has a strain of 0.002.
• Hooke's Law: Within the elastic limit of a material, stress is directly proportional to strain. The constant of proportionality is the Modulus of Elasticity (Young's Modulus, E) : σ = E × ε. This is a key concept for understanding material behaviour under load.

2.3 Kinetics: Work, Power, and Energy

Kinetics deals with the relationship between forces and the motion they cause.

• Newton's Laws of Motion:

1. A body at rest stays at rest, and a body in motion stays in motion at a constant velocity, unless acted upon by an external force (inertia).
2. The acceleration of a body is directly proportional to the net force acting on it and inversely proportional to its mass: F = m × a.
3. For every action, there is an equal and opposite reaction.

• Work: Work is done when a force moves an object over a distance.
• Work (W) = Force (F) × Distance (d) – Unit: joule (J).
• Lifting a 25 kg pump 1.5 m against gravity requires work equal to m × g × h = 25 × 9.81 × 1.5 = 367.9 J.
• Energy: Energy is the capacity to do work. It exists in many forms, including kinetic and potential.
• Kinetic Energy (KE): Energy of motion. KE = ½ × m × v². A 2 kg turbine blade moving at 150 m/s has a kinetic energy of 22,500 J.
• Potential Energy (PE): Stored energy. Gravitational potential energy is PE = m × g × h.
• Power: Power is the rate at which work is done or energy is transferred.
• Power (P) = Work (W) / Time (t) – Unit: watt (W) = J/s.
• For hydraulic systems, a common formula for power output is: Power (kW) = (Pressure in bar × Flow in L/min) / 600. A pump delivering 20 L/min against 210 bar has a power output of 7.0 kW.
• Momentum and Thrust: Momentum is the product of mass and velocity (p = m × v). Newton's second law can be expressed as force being the rate of change of momentum. This is the fundamental principle behind jet propulsion.
• Thrust (F) = Mass Flow Rate (ṁ) × Exhaust Velocity (v)
• For a stationary engine producing 50,000 N of thrust with an exhaust velocity of 600 m/s, the mass flow rate is 83.33 kg/s.
• Similarly, the force on a compressor blade can be calculated from the change in momentum of the air: F = ṁ × (v₂ - v₁).

2.4 Thermodynamics: Heat, Temperature, and Gas Laws

Thermodynamics is the study of heat, work, and energy. It is essential for understanding engine operation, cooling systems, and the behaviour of fluids and gases.

• Heat Transfer: Heat is energy in transit due to a temperature difference. It is transferred by three modes:
• Conduction: Transfer through a solid material (e.g., heat conducted through a turbine blade or a heat exchanger fin).
• Convection: Transfer by the movement of a fluid (liquid or gas) (e.g., heat transferred from a finned-tube oil cooler to the surrounding air).
• Radiation: Transfer via electromagnetic waves (e.g., heat from the sun on a parked aircraft).
• Thermal Expansion: Most materials expand when heated and contract when cooled. This is quantified by the coefficient of linear expansion (α).
• Change in Length (ΔL) = Original Length (L₀) × α × Change in Temperature (ΔT)
• A 0.20 m turbine blade with α = 12 × 10⁻⁶ /K will expand by 1.44 mm when heated from 20 °C to 620 °C.
• Thermal expansion is critical for understanding why hydraulic fluid levels rise in a vented reservoir when temperature increases, and why pressure increases dramatically in a sealed, constant-volume system (e.g., a hydraulic system after a pressure test). This is why thermal relief valves are fitted.
• The Ideal Gas Laws: These laws describe the relationship between pressure (P), volume (V), and absolute temperature (T) for a fixed mass of gas.
• Boyle's Law (Constant Temperature): P₁V₁ = P₂V₂. Pressure is inversely proportional to volume.
• Charles's Law (Constant Pressure): V₁/T₁ = V₂/T₂. Volume is directly proportional to absolute temperature.
• Gay-Lussac's Law (Constant Volume): P₁/T₁ = P₂/T₂. Pressure is directly proportional to absolute temperature. This explains why tyre pressure increases when an aircraft is parked in direct sunlight.
• The General Gas Law: Combines all three: (P₁V₁)/T₁ = (P₂V₂)/T₂. This is used for a wide range of calculations involving gases.

2.5 Fluid Dynamics

This area covers the behaviour of fluids (liquids and gases) in motion.

• Flow Rate and Continuity: For an incompressible fluid in a pipe, the mass flow rate must be constant. This is the principle of continuity.
• Volume Flow Rate (Q) = Cross-sectional Area (A) × Velocity (v)
• Mass Flow Rate (ṁ) = Density (ρ) × Volume Flow Rate (Q) = ρ × A × v
• If a fuel line has a diameter of 12 mm and fuel flows at 2 m/s, the mass flow rate is 0.181 kg/s. If the pipe narrows, the velocity must increase to maintain the same flow rate.
• Dynamic Pressure: In a moving fluid, dynamic pressure (q) is the pressure associated with the fluid's kinetic energy.
• Dynamic Pressure (q) = ½ × ρ × v²
• This is a key component of Bernoulli's equation and is fundamental to pitot-static systems for airspeed indication. If air density halves at altitude but true airspeed remains constant, dynamic pressure also halves.

3. Important Formulas and Relationships

The following is a summary of the key formulas you must be able to apply. Ensure you are comfortable with unit conversions before using them.

4. Common Relationships Between Concepts

• Pressure, Force, and Area: These are intrinsically linked. A small force on a small area can create a high pressure, which can then be used to generate a large force on a larger area (hydraulic multiplication). This relationship is central to hydraulic and pneumatic systems.
• Work, Energy, and Power: Work is the transfer of energy. Power is the rate of that transfer. A hydraulic pump does work to move fluid; its power rating tells you how quickly it can do that work. The kinetic energy of an exhaust jet is a form of energy that is converted from the chemical energy of the fuel.
• Thermal Expansion and Pressure: In a confined space (constant volume), thermal expansion of a liquid or gas leads to a direct increase in pressure (Gay-Lussac's law for gases; similar principle for liquids). In an open or vented system, thermal expansion leads to a change in volume (e.g., rising fluid level). This is why vented reservoirs and thermal relief valves are essential.
• Newton's Laws and Propulsion: Newton's second law (F = ma) and third law (action-reaction) are the foundation of jet propulsion. The engine accelerates a mass of air and fuel (increasing its momentum), and the reaction to this change in momentum is the thrust force. This connects directly to the concepts of mass flow rate and velocity.
• Fluid Flow and Pressure: The continuity equation (Q = A × v) shows that for a constant flow rate, velocity increases as area decreases. Bernoulli's principle relates this increase in velocity to a decrease in static pressure. This is the basis for measuring airspeed and understanding the operation of carburettors and certain fuel metering systems.

5. Typical Exam Focus Points

For the EASA Part-66 Module 2 exam at Category A level, you should focus on the following:

• Unit Conversions: Be highly proficient in converting between Pa, kPa, MPa, bar, and psi. Also practice converting between m³, litres, and cm³, and between °C and K. Many exam questions are based on this.
• Core Formula Application: You will be expected to perform direct calculations using the formulas for:
• Hydrostatic pressure (p = ρgh)
• Torque (M = F × d)
• Work and kinetic energy (W = Fd, KE = ½mv²)
• Hydraulic power (P = (P_bar × Q_L/min)/600)
• Stress and strain (σ = F/A, ε = ΔL/L)
• Flow rate (Q = A × v, ṁ = ρAv)
• Thermal expansion (ΔL = L₀αΔT)
• Gas laws (especially Gay-Lussac's law for tyre pressure)
• Understanding Principles: You must be able to explain the why behind the calculations. For example:
• Why does a blocked reservoir vent cause the reservoir to collapse? (Vacuum creation)
• Why does hydraulic fluid level rise when temperature increases? (Thermal expansion)
• Why does tyre pressure increase in sunlight? (Gay-Lussac's law)
• Why is a hydraulic system's pressure the same throughout? (Pascal's principle)
• Interpreting Scenarios: Questions are often framed in a maintenance context (e.g., "During a line maintenance shift..."). Read the scenario carefully and identify the underlying physical principle being tested before applying any formula.
• Precision and Units: Always include the correct units in your final answer. Pay attention to whether the question asks for gauge or absolute pressure. Use the standard value of g = 9.81 m/s² unless otherwise stated.