Airframes: Sheet Metal, Hydraulics & Electrical

• These flashcards cover various aspects of airframes, including sheet metal work, laminated structures, shock struts, hydraulic systems, air conditioning, oxygen systems, fuel quantity indication, and electrical systems.

Sheet Metal Forming

• For maximum strength, bends in sheet metal fittings should be made across the grain.
• The minimum bend radius is determined by the alloy, thickness, and hardness of the material.
• The natural axis is a plane within the metal that neither stretches nor shrinks during bending.
• A mold line is an extension of the flat sides beyond the radius in a flat pattern.
• The bend tangent line is where the bend starts in a flat pattern layout.
• Setback refers to the distance the brake jaws are set back from the mold line for bending.
• Bend allowance is the actual amount of metal in a bend, measured between bend tangent lines.
• A sight line is used during bending to ensure the bend starts at the tangent line.
• Throatless shears are used for cutting irregular curves in mild and stainless steel.
• Bumping is a metal forming technique used for creating compound curves.
• When forming a curved angle, flanges must be stretched (convex) or shrunk (concave).
• Hand-forming a concave curve starts at the edges, working towards the center.
• A joggle is a small offset that allows a sheet to overlap another.

Laminated Structures

• Laminated construction offers light weight and rigidity compared to riveted sheet metal.
• Foam and honeycomb are popular core materials.
• Polyester and epoxy resins are common matrix materials.
• Polyester matrix consists of resin and a catalyst.
• Epoxy matrix consists of resin and a hardener.
• Fiberglass, Kevlar, and graphite are used to reinforce matrix materials.
• Unidirectional fabric has major fibers running in the same direction.
• Ribbon direction in honeycomb is parallel to the length of the core strips.

Landing Gear and Hydraulic Systems

• Compressed air or nitrogen absorb taxi shocks in an oleo shock strut.
• Servicing oleo struts requires specific air/nitrogen levels per the maintenance manual with the aircraft's weight on the strut.
• Exerciser jacks help remove air from the oil during strut servicing.
• A squat switch prevents landing gear retraction on the ground and is in the torsion links.
• Precision microswitches sense the condition of retractable landing gear.
• Centering cams align the nose wheel before retraction.
• Initial landing impact is absorbed by oil transfer through a metered orifice in oleo struts.
• An orifice check valve allows full flow in one direction, restricted flow in the other.
• Pressure regulators control system pressure, relief valves prevent overpressure.
• Unloading valves control hydraulic system pressure by diverting pump output to the reservoir when pressure is high.

Air Conditioning and Oxygen Systems

• Warm air for cabin heating in large jets comes from engine compressor bleed air.
• The primary heat exchanger is the first point where hot bleed air loses heat in an air-cycle cooling system.
• Air-cycle machines cool compressed air by expansion.
• Water separators are needed to remove moisture from air-cycle systems, preventing fogging and icing.
• Temperature control valves regulate air temperature by mixing hot bleed air with cold air.
• Supplemental oxygen can be stored as high-pressure gas, liquid, or chemical candle.
• Continuous-flow oxygen systems deliver a constant flow of oxygen.
• Pressure-demand systems deliver oxygen only during inhalation above a set altitude.

Fuel and Electrical Systems

• Electronic fuel quantity indicating systems use tubular capacitors to sense fuel levels.
• The dielectric constant of fuel is approximately twice that of air, affecting capacitance.
• Older systems use a variable resistor moved by a float to sense fuel level.
• Drip gauges allow mechanics to check fuel level from the bottom of the tank.
• Fuel quantity indicators must read "zero" when fuel equals unusable supply during level flight.
• Generator size must exceed the connected load; if monitoring is impractical, the load should be 80% or less of the generator rating.
• "Flashing" a generator restores residual magnetism by passing battery current through field coils.
• Paralleling generators means adjusting voltage regulators for equal load sharing.
• Voltage regulators include vibrator, carbon-pile, and solid-state types.
• Trip-free circuit breakers cannot be closed while a fault exists.
• Slow-blow fuses allow temporary current surges before opening the circuit.
• Circuit breakers operate on magnetic or thermal principles.
• Starter motor circuits do not require circuit protection.
• Automatic-reset circuit breakers are not approved for aircraft use.
• Vibrator-type voltage regulators use electromagnetic relays and resistors to maintain constant voltage.
• Vibrator voltage regulators often include a current limiter and reverse-current cutout relay.
• Wire size selection considers current carrying capability and voltage drop.
• A maximum of four wires should connect to a single stud in a terminal strip.
• Wire bundles are protected from chafing with grommets and secured with cushioned clamps.
• Cushioned clamps are used to secure wire bundles.