Airframe Systems and Components: Hydraulics, Air Data, and Utility Systems
Hydraulics and pneumatics
Fluid does not create pressure; resistance to flow does. The pump moves volume, and pressure rises only as the system resists. Use F = P x A for force questions, and remember that a small pump piston driving a large actuator piston multiplies force while reducing travel. The relief valve protects the system, the accumulator stores energy (nitrogen precharge, never oxygen), and check valves set flow direction. Fluid identification is heavily tested: MIL-PRF-5606 mineral-base fluid is red, MIL-PRF-83282 synthetic hydrocarbon is also red, and phosphate-ester fluid is purple or blue-green. Phosphate-ester systems use EPR seals, mineral systems use Buna-N; mixing fluids or seals causes swelling or hardening and system failure.
Gear, brakes, fuel
Shock struts are serviced to a specified fluid level and nitrogen pressure; over-servicing gives a harsh ride and false high pressure readings. Torque links maintain wheel alignment; shimmy dampers stop nosewheel oscillation. A spongy brake pedal means air in the lines; bleed per the manual and never reuse fluid bled from a system. Fuel systems: vents prevent tank collapse, sump drains are sampled for water, and unusable fuel is part of empty weight.
Air data, pressurization, protection
With the pitot line blocked but the drain hole open, the airspeed indicator acts as an altimeter: it indicates a climb when the aircraft climbs. With the static line blocked, the altimeter freezes, the VSI reads zero, and the ASI overreads in a descent. The outflow valve controls cabin altitude. Bonding, circuit protection, fire detection (thermal, rate-of-rise, smoke), and Halon for occupied compartments round out the domain.
Worked example
A pump piston of 0.5 in2 pushes at 500 psi into an actuator piston of 3 in2. Output force = 500 x 3 = 1,500 lbf, input force = 250 lbf, mechanical advantage = 6, and the actuator moves one-sixth as far.