Airframes, structures and hydraulic systems
Scope
This domain tests how the airframe carries and transmits loads, and how hydraulic power is generated, controlled and distributed. Expect structural design philosophy, load paths, fluid types, pumps, accumulators and protective valves.
Structures
- Semi-monocoque construction: the skin carries shear and tension; stringers and frames stiffen it and carry bending and concentrated loads.
- Fail-safe design allows a damaged element to remain load-limited until inspection. Safe-life parts are retired at a calculated life. Damage tolerance evaluates crack growth.
- Pressurisation is cyclic: fatigue cracking starts at cut-outs, door corners and rivet rows. Corrosion is a separate inspection driver.
- Loads include flight, ground, pressurisation and control-surface loads; the structure must redistribute them after a failure.
Hydraulics
- Pascal's law: pressure is equal throughout a confined fluid; force = pressure x area.
- Transport aircraft typically use 3,000 psi (207 bar). Phosphate-ester fluid is fire-resistant but attacks incompatible seals and paint; mineral fluid is used on smaller types.
- Pumps: engine-driven, electric, power transfer unit (PTU) and ram air turbine (RAT). Accumulators damp pressure pulses and supply limited emergency volume.
- Valves: check, pressure relief, priority and hydraulic fuse, which isolates a major leak.
Worked example
An accumulator has a 1.0 L gas volume and 1,000 psi precharge. System pressure is 3,000 psi. Isothermal compression gives V = 1.0 x (1,000/3,000) = 0.33 L gas volume, so about 0.67 L of fluid is stored. If pump pressure falls, the accumulator delivers fluid until pressure reaches precharge.
Traps
Hydraulic fluid is not compressible; the accumulator gas is the spring. Precharge is not system pressure. A hydraulic fuse is not a circuit breaker. Fail-safe is not the same as fail-passive.