FAA General Written TestChapter 10 · 40 practice questions

Chapter 10: Physics for Aviation

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Chapter: Physics for Aviation

Overview

This chapter provides the foundational principles of physics that are essential for the aircraft maintenance engineer (AME). It bridges the gap between theoretical physics and the practical, hands-on application required for safe and effective maintenance. The material covers the core areas of fluid mechanics (hydraulics and pneumatics), gas laws, thermodynamics, electricity, and basic mechanics, all viewed through the lens of the aircraft maintenance environment. Mastery of these concepts is critical for troubleshooting, performing inspections, and ensuring the airworthiness of an aircraft, as outlined in advisory circulars like AC 43.13-1B and FAA handbooks.

Key Concepts Explained in Detail

1. Fluid Mechanics and Hydraulics

This is arguably the most critical area for an AME, as it governs the operation of landing gear, brakes, flight controls, and many other systems.

Pascal's Principle

Pascal's Principle - Hydraulic Jack Mechanical Advantage Pascal's Principle — Hydraulic Jack Mechanical Advantage AC 43.13-1B / FAA A&P Input Piston A₁ = 1 in² F₁ = 100 lb Hydraulic Fluid (incompressible) Output Piston A₂ = 10 in² F₂ = 1,000 lb ▲ Load: 1,000 lb Pascal's Law: P₁ = P₂ F₂ = F₁ × (A₂ / A₁) F₂ = 100 lb × (10 in² / 1 in²) = 1,000 lb P Mechanical Advantage A₂/A₁ = 10:1 Pressure transmission P = F/A = const. 14 CFR Part 65 — Aviation Maintenance Technician General Curriculum

Pascal's Principle states that pressure applied to a confined, incompressible fluid is transmitted undiminished in every direction throughout the fluid. This is the fundamental basis for all hydraulic systems.

  • Hydraulic Jacks & Actuators: A hydraulic jack is a classic example. A small force applied to a small piston creates a pressure (P = F/A). This same pressure acts on a larger piston, generating a proportionally larger force. The mechanical advantage is the ratio of the piston areas.
  • Formula: F₂ = F₁ × (A₂ / A₁)
  • Example: A force of 100 lbs on a 1 in² piston creates 100 psi. This pressure acting on a 10 in² piston produces a force of 1,000 lbs.
  • Compression Testing: When performing a compression test on an engine, the air pressure applied to the cylinder acts on the piston crown. Per Pascal's Principle, this pressure is transmitted uniformly, creating a force that can rotate the crankshaft and propeller. This is a direct application of the principle.

Fluid Dynamics and Viscosity

Fluid Dynamics and Viscosity - Hydraulic Fluid Temperature vs Viscosity Fluid Dynamics & Viscosity — Temperature Effects on Hydraulic Systems FAA A&P General — Physics for Aviation (ACS G.1.B, 14 CFR 43.13-1B) COLD WEATHER OPERATION High viscosity — thick fluid FLUID TEMP -20°F VISCOSITY: HIGH HYDRAULIC LINE — SLUGGISH FLOW LANDING GEAR RETRACTION — SLOW SLOW ⚠ TROUBLESHOOTING NOTE Cold weather increases fluid viscosity, causing slow gear retraction and sluggish actuator response. WARM WEATHER OPERATION Low viscosity — thin fluid FLUID TEMP +80°F VISCOSITY: LOW HYDRAULIC LINE — RAPID FLOW LANDING GEAR RETRACTION — FAST FAST ✓ NORMAL OPERATION Warm fluid has lower viscosity, providing faster actuator response and efficient power transfer. INVERSE VISCOSITY-TEMPERATURE RELATIONSHIP VISCOSITY TEMPERATURE → Cold = Thick Warm = Thin

Viscosity is a measure of a fluid's internal resistance to flow. It is the "thickness" of the fluid.

  • Temperature Effects: Viscosity is highly temperature-dependent. As temperature decreases, viscosity increases (fluid gets thicker). As temperature increases, viscosity decreases (fluid gets thinner).
  • Troubleshooting Application: A landing gear system that retracts slowly in cold weather is likely experiencing the effects of increased fluid viscosity, which creates more resistance to flow through lines and valves.
  • Pressure Drops: Fluid flowing through a system will experience a pressure drop due to friction against the pipe walls, fittings, and restrictions. This is a normal and expected phenomenon.
  • Troubleshooting Application: A small pressure drop (e.g., 50 psi on a 1,500 psi system) between the pump and an actuator is normal. A large, unexpected drop indicates a problem such as a leak, a blockage, or a faulty component.

Bernoulli's Principle

Bernoulli's Principle states that for a fluid in steady flow, an increase in the fluid's speed occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.

  • Restrictions and Collapses: If a fuel line is collapsed or restricted, the fluid must accelerate to pass through the narrowed section. This increase in velocity causes a significant decrease in pressure, which can lead to fuel starvation or pump cavitation. This principle explains why restrictions cause pressure loss downstream.

Fluid System Troubleshooting Scenarios

  • Soft Pedal (Brakes): A "soft" or "spongy" brake pedal is almost always caused by air in the hydraulic system. Air is compressible, unlike hydraulic fluid. When pressure is applied, the air compresses, absorbing the force and reducing the pressure transmitted to the brake pads. The correct repair is to bleed the brakes to remove the trapped air.
  • Foaming Fluid: Foamy hydraulic fluid indicates that air is being introduced into the system, typically at the pump suction line. This can cause cavitation and erratic operation. The correct action is to locate and repair the leak and then bleed the system.
  • Vibration-Induced Aeration: Excessive vibration in a hydraulic line can cause the fluid to aerate, creating foam. This reduces the fluid's incompressibility, leading to a spongy control feel. Proper line clamping and support are essential to prevent this.
  • System Leaks: A leak in a pressure line creates an unintended path for fluid to flow, which is a lower-resistance path than continuing to the actuator. This results in a drop in system pressure because the pump cannot maintain the required pressure if flow is being lost.

2. Gas Laws and Pneumatics

These laws govern the behavior of gases in pneumatic systems, engines, and tires.

Boyle's Law

Boyle's Law - Gas Compression and Propeller Resistance Boyle's Law — P₁V₁ = P₂V₂ (Inverse Pressure-Volume Relationship) Gas Compression in Cylinder Piston moves left → volume decreases → pressure increases V₁ = 200 in³ PRESSURE P₂ = 14.7 psi P₁V₁ = P₂V₂ 14.7 × 200 = 29.4 × 100 2940 = 2940 ✓ COMPRESS Propeller Compression Test Resistance felt during rotation indicates cylinder compression CYLINDER RESISTANCE HIGH AC 43.13-1B / FAA A&P Standard Per 14 CFR Part 43, Appendix A: Compression test verifies piston ring and cylinder wall condition. Boyle's Law in Aviation Maintenance P₁V₁ = P₂V₂ — As volume decreases, pressure increases proportionally (inverse relationship) Application: Cylinder compression testing, pneumatic systems, oxygen system calculations

Boyle's Law states that for a fixed amount of gas at a constant temperature, the pressure and volume are inversely proportional.

  • Formula: P₁V₁ = P₂V₂
  • Engine Compression: During a compression test, the piston moves from Bottom Dead Center (BDC) to Top Dead Center (TDC), decreasing the volume of air in the cylinder. According to Boyle's Law, this decrease in volume causes a corresponding increase in pressure. The increased torque required to turn the propeller on the compression stroke is a direct result of this pressure increase.
  • Pneumatic De-icing Boots: The inflation and deflation cycle of pneumatic de-icing boots is governed by Boyle's Law. Pressurized air inflates the boot (increasing pressure, increasing volume), and a vacuum deflates it (decreasing pressure, decreasing volume).

Gay-Lussac's Law

Gay-Lussac's Law - Tire Pressure vs Temperature Gay-Lussac's Law — Tire Pressure vs Temperature P₁/T₁ = P₂/T₂ (absolute units) — Why cold tire pressures are specified COLD TIRE T₁ = 293 K P₁ = 210 kPa HOT TIRE T₂ = 333 K P₂ = 239 kPa PRESSURE GAUGE 239 kPa T₂ = 333 K (60°C) GAY-LUSSAC'S LAW — PRESSURE-TEMPERATURE PROPORTIONALITY P₁/T₁ = P₂/T₂ → P₂ = P₁ × (T₂/T₁) 210 kPa × (333 K / 293 K) = 239 kPa — Absolute temperature required! ⚠ AC 43.13-1B: Check tire pressure when cold — operating temps can raise pressure 10-15% HEATING COLD PRESSURE 210 kPa T₁ = 293 K (20°C) Specification value ! CHECK HOT

Gay-Lussac's Law states that for a fixed volume of gas, the pressure is directly proportional to its absolute temperature.

  • Formula: P₁/T₁ = P₂/T₂ (where T is in absolute units like Kelvin or Rankine)
  • Tire Pressure: When an aircraft taxis on a hot day, the friction and heat cause the temperature of the air inside the tires to increase. Since the tire volume is relatively constant, the pressure increases proportionally. This is why tire pressures are specified for "cold" tires.

Pascal's Principle in Pneumatics

Pascal's Principle applies to confined gases as well as liquids. During a compression test, the applied air pressure acts uniformly on the piston head, generating a force that can rotate the crankshaft and propeller.

Pneumatic System Testing

When pressure testing a pneumatic system, the test pressure is often a multiple of the maximum operating pressure. It is critical to use a gauge with an appropriate range. The test pressure should fall within the middle half of the gauge's full-scale reading for optimal accuracy and to prevent damage to the instrument. For example, a 225 psi test requires a gauge with a range of at least 0-300 psi, not a 0-200 psi gauge.

3. Thermodynamics and Heat

This section covers the principles of heat generation, transfer, and its effects on aircraft systems.

Joule's Law

Joule's Law - Internal Resistance and Heat Generation Joule's Law — Internal Resistance & Heat Generation P = I²R — Doubling current quadruples heat I (current) + − Battery R_internal Load (e.g. starter) 200°F 150°F 100°F Temp P = I² × R Power (heat) = Current² × Resistance If I doubles → P quadruples Current: 2A → 4A Heat: 4W → 16W AMPS 4.0 E = 12V Why internal resistance matters: All real batteries have internal resistance. Current through R_internal generates heat. High current → excessive heat → component damage FAA A&P Application (AC 43.13-1B): • Check for hot spots in electrical circuits • Loose connections increase resistance → heat • Heat = accelerated insulation failure

Joule's Law describes the relationship between heat generated by an electrical current flowing through a resistor.

  • Formula: P = I²R (Power in watts = Current² × Resistance)
  • Battery Charging: When charging a battery, the internal resistance of the battery converts some of the electrical energy into heat. If a battery is excessively hot to the touch, it indicates high internal resistance, which can be caused by sulfation or a shorted cell. This is a key safety check.

Effects of Heat on Materials

  • Electrical Resistance: For most metallic conductors (copper, aluminum), resistance increases with temperature. This is due to increased atomic lattice vibration, which scatters electrons more frequently. This is critical when troubleshooting electrical systems, as a resistance reading that is out of tolerance on a hot day might not indicate a fault.
  • Overheating Bearings: Discoloration of wheel bearings from overheating indicates excessive friction, often caused by over-tightening. The correct action is to replace the damaged bearings and set the axle nut to the manufacturer's specified torque.

Gas Turbine Engine Performance

The performance of a gas turbine engine is heavily influenced by thermodynamics.

  • Compressor Efficiency: A loss of compressor efficiency reduces the mass of airflow delivered to the combustor. With the same fuel flow, the fuel-to-air ratio increases, raising combustion temperatures and EGT.
  • Turbine Efficiency: Normal fuel flow with a rising EGT can indicate that the turbine is not extracting enough energy from the gas, often due to damaged blades or seals. This reduces efficiency and increases exhaust temperature.

4. Mechanics and Materials

This section covers the physical principles governing forces, motion, and the behavior of aircraft materials.

Hooke's Law

Hooke's Law in Cable Tensiometer Measurement Hooke's Law & Cable Tensiometer Operation FAA A&P Physics — AC 43.13-1B §7-12 | F = kx Cable Tensiometer (Mechanical) Control cable deflection measurement F = kx x (deflection) Force vs. Deflection — F = kx Deflection x (in) Force F (lb) slope = k Hooke's Law — Tensiometer Principle F = k × x F = spring force (lb) — cable tension k = spring constant (lb/in) — stiffness x = deflection (in) — displacement k = 50 x = 0.5 AC 43.13-1B §7-12: Tensiometer calibration verified before each use CALIBRATED

Hooke's Law states that the force needed to extend or compress a spring by some distance is proportional to that distance.

  • Formula: F = kx (Force = Spring Constant × Displacement)
  • Cable Tensiometers: A cable tensiometer operates on this principle. It uses a spring or calibrated beam to measure the force required to deflect a cable a set amount. This deflection force is then read as tension.

Static Electricity and the Triboelectric Effect

Static electricity on an aircraft is primarily generated by the triboelectric effect, which is frictional charging that occurs as the aircraft moves through the air. It is also generated by the impact of charged particles (like dust and ice crystals) on the aircraft's surfaces. Static wicks provide a controlled path for this charge to bleed off into the atmosphere, preventing interference with radio communications and navigation systems.

Weight and Balance

Weight and balance is a critical safety-of-flight calculation.

  • CG Shift Formula: CG shift = (Weight Moved × Distance Moved) / Total Weight
  • Example: Moving a 20 lb battery 20 inches forward on a 2,000 lb aircraft results in a CG shift of 0.2 inches forward.
  • Aft CG: An aft CG reduces the aircraft's longitudinal stability, making it less stable in pitch. This can cause the aircraft to be extremely sensitive to control inputs and, in a stall, the nose may tend to pitch up, making recovery difficult.

Corrosion

Corrosion is the deterioration of a material due to a chemical or electrochemical reaction with its environment.

  • Galvanic Corrosion: This occurs when two dissimilar metals are in contact in the presence of an electrolyte (such as moisture). The more active metal (the anode) corrodes. On aluminum aircraft, this often appears as a white powder and pitting, especially where aluminum is in contact with steel fasteners.
  • Treatment: The correct treatment is to remove the corrosion, treat the area with a chemical conversion coating, and apply a corrosion-inhibiting primer.

Stress and Fatigue

  • Stress Risers: A dent in a propeller blade leading edge is a stress riser, which can lead to cracking or failure. Significant dents require blade replacement.
  • Minimum Bend Radii: Fluid lines must be installed with a minimum bend radius to prevent excessive stress and work hardening. A sharp bend creates a localized stress concentration, which can lead to fatigue cracking over time.
  • Cable Chafing: Control cables must not contact any structure or other components. Chafing produces metallic dust, indicating active wear that could lead to cable failure. The correct action is to eliminate the interference by rerouting or installing a chafe guard.

Important Formulas and Regulations

Key Formulas

  • Pascal's Law (Hydraulic Force): F₂ = F₁ × (A₂ / A₁)
  • Pressure: P = F / A
  • Boyle's Law: P₁V₁ = P₂V₂
  • Gay-Lussac's Law: P₁/T₁ = P₂/T₂ (T in absolute units)
  • Joule's Law: P = I²R
  • Hooke's Law: F = kx
  • CG Shift: CG shift = (Weight Moved × Distance Moved) / Total Weight

Key Regulations and Standards

  • AC 43.13-1B: This advisory circular is the primary reference for acceptable methods, techniques, and practices for aircraft inspection and repair. It covers everything from safety wiring and control cables to hydraulic systems and corrosion control.
  • 14 CFR Part 43: This regulation governs the maintenance, preventive maintenance, rebuilding, and alteration of aircraft. It dictates the requirements for inspections, documentation, and return to service.
  • 14 CFR 91.411: This regulation requires the pitot-static system and altimeter to be tested and inspected within the preceding 24 calendar months.

Common Relationships Between Concepts

  • Pascal's Principle & Hydraulic Systems: The foundation of all hydraulic power transmission, from jacks to flight controls.
  • Boyle's Law & Engine Compression: Explains the pressure-volume relationship that creates compression and the resistance felt when turning a propeller during a compression test.
  • Gay-Lussac's Law & Tire Pressure: Explains why tire pressure increases with temperature and why "cold" tire pressures are specified.
  • Viscosity & Temperature: The inverse relationship is a primary factor in troubleshooting slow or erratic hydraulic system operation in cold weather.
  • Bernoulli's Principle & Fluid Restrictions: Explains how restrictions and collapses in fluid lines cause pressure drops and system malfunctions.
  • Joule's Law & Electrical Heat: Explains heat generation in electrical components, such as batteries, due to internal resistance.
  • Hooke's Law & Tension Measurement: The principle behind the operation of cable tensiometers.
  • Triboelectric Effect & Static Wicks: The cause of static charge buildup and the purpose of the system designed to dissipate it.
  • Stress Risers & Material Failure: The relationship between physical defects (dents, sharp bends) and the potential for fatigue cracking and structural failure.

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