The systems of an A320-family jet, simplified: three hydraulic systems and the PTU, fly-by-wire control laws and their protections, the AC and DC electrical network, the brakes, and fuel, bleed air, ice protection and fire protection. Fail things and see what is left.
Three independent systems at about 3000 psi. Green is driven by engine 1, yellow by engine 2 (with an electric pump as backup), and blue by an electric pump, with the ram air turbine (RAT) as the emergency source. The power transfer unit (PTU) lets green and yellow help each other without exchanging fluid.
Consumers shown are typical of the A320 family and simplified. Where a consumer has two or three systems, it keeps working on any one of them, sometimes slower (flaps and slats run at half speed on one motor).
The sidestick does not move the surfaces directly. In normal law, pitch input is a load factor demand and roll input is a roll rate demand; the computers move the elevators, the trimmable horizontal stabiliser (THS), the ailerons and the spoilers to deliver it, inside a protected envelope. As failures add up, the law degrades and the protections go away one by one.
Two engine-driven generators (each with an integrated drive generator, IDG, that gives constant frequency) normally feed AC bus 1 and AC bus 2. Transformer rectifiers (TRs) turn AC into 28 V DC. The essential buses carry what you need to fly safely, and they are the last to lose power.
A tyre brakes best with a little slip, typically 10 to 20 percent. Push harder and the wheel goes past the peak, the friction falls, and a locked wheel gives no cornering force at all. Anti-skid keeps each wheel near the peak; autobrake picks a deceleration and lets anti-skid do the rest.
On standing water deeper than the tyre tread can drain, typically more than about 3 mm, a water wedge lifts the tyre off the runway and braking and cornering friction almost disappear. The speed at which a rotating tyre starts to hydroplane follows a rule of thumb: about 9 × √p kt, with p the tyre pressure in psi. A tyre that is not yet turning, such as at touchdown before spin-up, hydroplanes at a lower speed, about 7.7 × √p kt. Reverted-rubber hydroplaning, on a locked wheel whose rubber melts from the steam it creates, can last down to very low speeds. That is one more reason never to land with the brakes on and to keep anti-skid working.
Three systems that every engine failure touches. The fuel system feeds the engines from five tank cells in a fixed order. The bleed air system takes hot compressed air from the engines or the APU for the packs and the wing anti-ice. The fire protection system detects a fire with two loops and puts it out with agent bottles. Choose a system, fail something and follow the logic.
Teaching model of a generic A320-family aircraft. System layouts, pressures, consumers, control law logic, protection limits, electrical priorities and brake values are simplified and illustrative, not FCOM data. The aircraft FCOM, QRH and your operator’s procedures give the real logic and limits.