No pressure: the spring pushes the by-pass actuator down. Its internal gallery now joins the A line to the B line, so chambers A and B of the power actuator (5) are interconnected and the actuator is free to move. At the same time the actuator's foot presses on the end of the lever: the roller is jammed and becomes the anchorage point (1). Each movement of the pilot is transmitted directly to the power actuator through the lever pivoting on that point. The stirrup (3) and distributor (4) do not move and all lines are unpressurised.
Step 1 – Hydraulic pressure lifts the by-pass actuator: its gallery no longer reaches the B line, so A and B are isolated, the power actuator locks and becomes the anchorage point; the roller (2) is free.
Step 2 – Through the set of levers and the anchor point, the pilot's action moves the stirrup (3) and hence the distributor (4). The distributor opens the P port to one groove and the return to the other: one chamber is fed, the other drained, the actuator moves, and its movement brings the distributor back to neutral. The power rod follows the pilot input with hydraulic power.
Without AFCS orders the flapper is centred: both calibrated orifices leak equally, the pressures in D and E are equal and the distributor is centred inside the stirrup with P closed. A signal from an APM moves the flapper toward one nozzle, the pressure on that side rises, and the distributor moves against one stirrup spring. P is opened to one groove and one chamber is fed while the other drains to return; the power actuator moves, and through the linkage the stirrup follows and recentres the distributor. The pilot's controls do not move. When the signal stops the spring returns the distributor and the actuator comes back to its original position.
1 Anchorage point
2 Roller
3 Stirrup
4 Distributor actuator
5 Power actuator
A, B Actuator chambers
Red frame Calibrated orifices
Hatched By-pass actuator and its sleeve
Dashed blue Neutral position of the bars
APM 1, 2 Autopilot modules