The A320 power transfer unit
· Matt Horn
Lose an engine and you lose the hydraulic system its pump drove, along with everything running on it. So two of the A320’s hydraulic systems, green and yellow, are joined by one shaft through a sealed bulkhead, with a bent-axis piston unit at each end. Whichever system holds the higher pressure turns its unit as a motor and drives the other as a pump, so green can feed yellow or yellow can feed green. No fluid crosses.
Both systems run at 3000 psi. When the difference between them exceeds about 500 psi the shaft breaks away and spins up, pressurizes the low side until the difference collapses, drops out, and crosses the threshold again a second later. That cycling is the barking you hear from your seat during pushback, when only one engine is turning.
I built the barking dog to watch that cycle run. The 500 psi threshold is the aircraft’s; the flow rates and time constants are not, and are slowed down so one cycle is watchable. Drop an engine pump offline, drag the flight-control demand, and watch the pressure difference cross 500 psi. Turn the sound on.