Airframes, Structures and Aircraft Systems
This domain tests how the primary structure carries and transmits load, and how the major systems - hydraulic, pneumatic, fuel, pressurization and ice protection - generate, distribute and control power. Questions often ask you to identify load paths, system components, normal indications and the effect of a failure.
Structure
The semi-monocoque fuselage combines skin, stringers and frames. Skin carries shear and part of the bending load; stringers stiffen the skin and carry axial bending; frames maintain shape and support concentrated loads. A stressed-skin wing uses spars for bending, ribs to preserve aerofoil shape and skin to resist torsion. Fail-safe structure provides an alternative load path after damage; safe-life components are retired at a calculated life. Limit load is the maximum expected in service. Ultimate load is limit load multiplied by the design factor, normally 1.5, and the structure must not fail at ultimate load.
Systems
Hydraulic systems commonly operate near 3,000 psi. Because pressure equals force divided by area, actuator force depends on piston area. Reservoirs allow thermal expansion and relief valves protect against overpressure. Pneumatic systems supply engine starting, anti-icing and pressurization; the outflow valve regulates cabin pressure. Fuel systems use boost pumps, crossfeed and tank vents; a failed boost pump or an unported pump produces low-pressure warning.
Worked example
A hydraulic actuator has a piston area of 2 in² and the system pressure is 3,000 psi. Force = pressure × area = 3,000 × 2 = 6,000 lbf. If the piston area is halved, force halves at the same pressure; if pressure doubles, force doubles.
Traps
Do not confuse gauge pressure with absolute pressure: hydraulic gauges read gauge pressure. Do not assume the skin carries no load in a semi-monocoque structure. Relief-valve cracking pressure is not the same as normal system operating pressure. The outflow valve controls cabin pressure; the pressurization controller commands it.