Airframe Structures and Systems
This domain tests how the airframe carries load and how the major systems generate and control force. Expect questions on semi-monocoque construction and on hydraulic and pressurization fundamentals.
Structure
- Semi-monocoque construction uses skin, stringers, frames and bulkheads. The skin carries tension and shear; stringers and spar caps carry bending; frames maintain shape and support the skin.
- Fail-safe design provides multiple load paths so a single failure does not cause loss of the aircraft. Damage tolerance requires inspection before a crack grows to critical size.
- Repairs must follow the structural repair manual or an approved design. Stop-drilling is acceptable only where the SRM or an engineer permits it.
Systems
- Flight controls use cables, pulleys, turnbuckles, bell cranks and push-pull rods; check cable tension and safety turnbuckles.
- Hydraulic systems have a reservoir, pump, relief valve, accumulator, selector valve and actuators.
- Pressurization uses engine bleed air, an outflow valve, and pressure bulkheads; cabin altitude and differential pressure are the controlled variables.
- Landing gear oleo struts use nitrogen and hydraulic fluid; torque links resist rotation and the shimmy damper stops nosewheel oscillation.
Worked example
A hydraulic actuator has a piston area of 1.5 in2 and the system is at 3,000 psi. Force = pressure x area = 3,000 x 1.5 = 4,500 lbf. If the load needs 5,000 lbf, the actuator will not move it even though pressure is at the relief setting; the fix is a larger piston area, not higher pressure beyond limits.
Traps that cost marks
- Stop-drilling a crack in pressurized skin without engineering approval.
- Confusing the outflow valve with the pressure relief valve.
- Over-servicing an oleo strut.
- Ignoring cable tension and turnbuckle safety after rigging.