Comprehensive ATPL Airframes and Systems Study Guide

FUSELAGE, WINGS AND STABILIZING SURFACES

Basic Loading Definitions

  • Tension: A load that tends to stretch a structural member. Components designed to resist this are called ties.
  • Compression: A load tending to shorten a member. Components designed to resist this are called struts.
  • Shear: A force tending to slide one face of material over an adjacent face. Riveted joints are designed to resist shear.
  • Bending: A combination of tension (outer edge), compression (inner edge), and shear.
  • Torsion: Twisting forces producing tension at the outer edge, compression in the center, and shear across the structure.

Stress, Strain, and Elasticity

  • Stress: Internal force per unit area inside a part (
    N/mm2N/mm^{2}
    or
    MN/m2MN/m^{2}
    ).
  • Strain: Deformation caused by stress, usually expressed as a percentage of original dimensions.
  • Young's Modulus: The constant relationship between stress and strain for an elastic material.
  • Elastic Limit: The point beyond which deformation becomes permanent (plastic deformation).
  • Buckling: Occurs in thin sheet materials under end loads or in ties subjected to compression.

Design Philosophies

  • Design Limit Load (DLL): Maximum load expected in service.

    • Transport:
      +2.5g/−1.0g+2.5g / -1.0g

    • Utility:
      4.4g4.4g

    • Aerobatic:
      6g6g

  • Design Ultimate Load (DUL): DLL multiplied by a safety factor (minimum
    1.51.5
    ). The structure must withstand DUL without collapse.

  • Safe Life: Minimum life during which no catastrophic damage should occur. Items are replaced/overhauled after cycles (landings, pressurization events).

  • Fail-safe / Damage Tolerant:

    • Fail-safe: Redundant load paths ensure a single failure is compensated for by adjacent parts until the next inspection.
    • Damage Tolerant: Spreads loads over larger areas to eliminate heavy redundant members; relies on detection of damage before failure.

  • Fatigue: Failure due to cyclic reversals of loading at stress levels lower than ultimate stress.

  • Station Numbers: Locating components via zero datum lines.

    • Fuselage Stations: Inches forward (negative) or aft (positive) of datum.
    • Water Line (WL): Vertical dimension from a horizontal datum.

Fuselage Construction

  • Pressurization Stresses:
    • Axial (Longitudinal): Tends to elongate the fuselage.
    • Hoop (Radial): Tends to expand the cross-section. Internal pressures can reach
      65.5kN/m2(9.5psi)65.5 kN/m^{2} (9.5 psi)
      .
  • Shapes:
    • Circular: Ideal for spreading hoop stress evenly.
    • Double Bubble: Efficient use of space for passengers and cargo.
  • Construction Types:
    • Monocoque: Skin takes all loads; internal frames give shape. Weak against skin damage.
    • Semi-monocoque (Stressed Skin): Uses stringers (longitudinal stiffeners) and longerons (main bending load bearers) to assist the skin.
  • Firewalls: Separate engine from cabin. Made of stainless steel or titanium (can withstand
    3000oC3000^{o}C
    temporarily).

Wings (Mainplanes)

  • Cantilever Monoplane: Wing built around spars (main load-bearing members).
  • Bending Stress Relief: Achieved by mounting engines on wings, using 'Aileron Up-float', and positioning fuel tanks in wings (wing fuel is used last).
  • Torsion Box: Formed by spars, skin, and ribs to resist bending and twisting.
  • Components:
    • Ribs: Maintain aerofoil shape and pass concentrated loads into skin/spars.
    • Stringers: Stiffen skin in compression.
  • Flutter: Catastrophic uncontrolled oscillation. Prevented by mass balancing control surfaces (moving C of G closer to the hinge).

BASIC HYDRAULICS

Physical Laws

  • Pascal’s Law: "If a force is applied to a liquid in a confined space, then this force will be felt equally in all directions."
  • Formula:
    Force=Pressure×AreaForce = Pressure \times Area
    ;
    Pressure=ForceAreaPressure = \frac{Force}{Area}
    .
  • Bramah’s Press: Relates work done:
    Force×Distance(PistonA)=Force×Distance(PistonB)Force \times Distance (Piston A) = Force \times Distance (Piston B)
    .

Hydraulic Fluids

  • DTD 585 (Mineral Based): Red color. Uses Neoprene (synthetic rubber) seals.
  • SKYDROL (Phosphate Ester): Purple or Green. Fire-resistant. Uses Butyl rubber seals. Irritant to skin/eyes and damages paint.

System Components

  • Reservoirs: Store fluid, compensate for leaks/thermal expansion, and provide head pressure to pumps. Usually pressurized by engine bleed air to prevent cavitation and air bubbles.
  • Accumulators: Store fluid under pressure using compressed gas (Nitrogen). Functions: dampen fluctuations, allow thermal expansion, provide emergency pressure, and prolong ACOV cycles.
  • Pumps:
    • Constant Delivery: Requires an Automatic Cut-out Valve (ACOV) to offload the pump when pressure is reached.
    • Constant Pressure (Variable Volume): Varies pump stroke (via a swash plate/yoke) to maintain constant pressure regardless of demand.
  • Priority (Pressure Maintaining) Valve: Ensures primary services (e.g., flight controls) get pressure before secondary services.
  • Fuses: Shut off flow if a serious leak/pressure drop occurs to prevent total fluid loss.

LANDING GEAR, WHEELS, AND TYRES

Shock Absorbers

  • Oleo-pneumatic Strut: Uses oil (to dampen movement by passing through an orifice) and compressed gas (acting as a spring).
  • Fescalized Metal: Hard outer coating of the inner strut.

Operation and Safety

  • Shimmy: Rapid sinusoidal vibration of the nose wheel. Controlled by shimmy dampers, twin contact wheels, or hydraulic locks.
  • Retraction Mechanics: Uses Sequence Valves to ensure doors open before gear moves and vice versa.
  • VLO: Maximum velocity for landing gear operation.
  • VLE: Maximum velocity with landing gear extended.
  • Air/Ground Logic: Proximity sensors or microswitches on oleos send signals for system inhibiting (e.g., preventing gear retraction on ground).

Wheels and Tyres

  • Fusible Plugs: Melt under excessive heat to deflate tubeless tyres safely, preventing blowouts.
  • Creep: Slippage of the tyre on the wheel rim. Monitored by white creep marks. Can tear out inflation valves on tubed tyres.
  • Aquaplaning Formula:
    V<em>p=9PV<em>{p} = 9 \sqrt{P} (where PP is psi, V</em>pV</em>{p}
    is knots).
  • Tyre Pressure: Checks should account for
    4%4\%
    increase due to aircraft weight-on-wheels and up to
    10%10\%
    heat-related increase.

FLIGHT CONTROLS AND FBW

Control Surfaces

  • Primary: Ailerons (Roll), Elevator (Pitch), Rudder (Yaw).
  • Secondary: Flaps, Slats, Spoilers, Trims.
  • Adverse Yaw: Corrected by differential ailerons, Frise ailerons, or aileron-rudder coupling.
  • Speed Brakes: Increase drag; usually wing-mounted spoilers or fuselage-mounted surfaces.

Balancing and Feel

  • Aerodynamic Balance: Horn balance, balance tabs, or set-back hinges reduce stick forces.
  • Artificial Feel: Necessary for irreversible (fully power-operated) controls. Uses 'Q' pots to make feel proportional to
    12ρV2\frac{1}{2} \rho V^{2}
    (dynamic pressure).

Fly-By-Wire (FBW)

  • Converts mechanical inputs into electrical signals processed by computers (ELAC, SEC, FAC).
  • Advantages: Weight reduction, flight envelope protection (prevents stalls/overstressing), and automated turn coordination.

PNEUMATICS, PRESSURIZATION, AND ICE PROTECTION

Air Conditioning

  • Bleed Air System: Uses HP/LP compressor tappings. LP air is used normally; HP air is regulated by a High Pressure Shut-Off Valve (HPSOV) during low RPM.
  • Cold Air Unit (CAU/Bootstrap): Uses air cycle cooling. Charge air is compressed (heating it further), cooled in a secondary heat exchanger, then expanded across a turbine (Work done = Temperature drop).

Pressurization

  • Max Differential Pressure:
    8−9psi8-9 psi
    .
  • Safety Devices: Outwards relief (prevents overstressing) and Inwards relief (prevents negative differential).
  • Cabin Altitude: Usually maintained at
    8000ft8000 ft
    equivalent or less.

Ice Protection

  • Systems:
    • Bleed Air Thermal: Continuous anti-icing for wing leading edges.
    • Pneumatic Boots: Cyclic de-icing by inflating rubber tubes to break ice bond.
    • Electrical: Used for pitot probes, windshields, and propeller leading edges/spinners.
  • Rain Repellent: Must not be used on dry windshields as it ruins visibility.

FUEL, SMOKE, AND FIRE SYSTEMS

Fuel Systems

  • AVGAS:
    • 100LL: Blue color.
    • 100: Green color.
  • JET A1: Kerosene based. Flash point
    38oC38^{o}C
    ; Freezing point
    −47oC-47^{o}C
    .
  • Storage: Integral tanks (structures), flexible bags, or rigid tanks. Baffles prevent surging.
  • Jettison: Required if MTOM significantly exceeds MSLM.
  • Measurement: Capacitance-type probes measure fuel mass (weight) rather than volume to compensate for density changes.

Detection and Protection

  • Smoke Detection: Optical (light refraction), Ionization (radioactive bombardment), or Semiconductor (toxic gas absorption).
  • Fire Detection: Fire Wire (Resistive or Capacitive elements) or gas-filled tubes.
  • Extinguishants:
    • Halon 1211 (BCF): Best for electrical/liquid fires in cabins.
    • Halon 1301 (BTM): Used in engine/APU fixed systems.
    • Dry Powder: Used for wheel/brake fires (no cooling effect prevents explosions).