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Simplified — not for operational use. Click pumps, reservoirs, the PTU, or actuators for a summary.
Click any component above to see what it does →
PTU — the one that trips people up
POWER ONLY
NO FLUID CROSSES
NO FLUID CROSSES
The Power Transfer Unit links the Green and Yellow systems hydraulically — it's a pump-motor pair, not a valve, so it moves power between the two systems without ever mixing their fluid. It activates automatically whenever pressure between Green and Yellow differs by roughly 500 psi, in either direction. Blue is never connected to the PTU. On the ground, it's inhibited unless certain conditions are met (e.g., during cargo door operation), specifically to avoid nuisance activation while parked.
Quick reference
GREEN
3000 psi
- Pressurized by EDP on Engine 1
- Landing gear, nose wheel steering
- Normal braking
- Shares power (not fluid) with Yellow via PTU
BLUE
3000 psi
- Pressurized by an AC electric pump
- Backed up by the RAT in emergencies
- Not connected to the PTU
- Smallest reservoir of the three systems
YELLOW
3000 psi
- Pressurized by EDP on Engine 2
- Electric pump for ground ops (cargo doors)
- Alternate braking, rudder
- Hand pump available for cargo door backup