These three servo cylinders send flight control movements to the stationary swashplate of the rotary-wing head. If the primary hydraulic system is operating, the servo cylinders hydraulically aid flight control movement. If the power fails, they function only as control rods. See figure 10-8. This is accomplished by the spring-loaded bypass valve, which prevents hydraulic lock and a sloppy link pilot valve connection. The pilot valve and the lower clevis of the power piston connect to the flight control linkage by the same bolt. There is a very close tolerance in the pilot valve connection. This tolerance causes the pilot valve to operate before the power piston clevis. The power piston is then mechanically displaced.

Fluid under pressure entering the servo cylinder upper port closes the bypass valve and enters the upper chamber. With the pilot valve in neutral, fluid cannot escape from the lower chamber, and the piston remains motionless. If the pilot valve moves upward, fluid flows into the lower chamber. The piston will rise because of a pressure area differential. If the pilot valve moves down, the fluid in the lower chamber flows to return. The piston will be forced downward by upper chamber pressure.

When flight control movements stop, the piston will continue to move until the ports of the pilot valve close. The pilot valve clevis will be in the center of the sloppy link. When pressure is off, the bypass valve will open, preventing hydraulic lock.