The Boeing 727 jet has two operating hydraulic systems plus a third, standby system. All three use Skydrol 500A and operate at 3000 psi.

System A is the utility system which operates the landing gear retraction system, nose gear steering and brakes, leading edge slats and flaps, trailing edge flaps, ground spoilers, and half of the flight spoilers. In addition, it provides one of the hydraulic power sources for the ailerons, rudder and elevators. Two engine-driven pumps installed on accessory pads of the number one and number two engines supply hydraulic power. The reservoir is pressurized to 45 psi by engine bleed air routed through a combination pressure regulator/relief valve. Case drain cooling fluid from the two engine-driven pumps is brought together and routed through a fluid-to-fuel heat exchanger in the right fuel tank, then returned to the reservoir through the return system filter.

The B system, primarily a flight control power system, provides one of the power sources to operate the ailerons, elevators, and rudder, as well as inboard flight spoilers, main wheel brakes, and aft passenger entry stairs. Two electric-motor-driven pumps supply power. Cooling fluid flow from the pumps runs through case drain filters, then through a fluid-to-fuel heat exchanger in the left fuel tank and back to the reservoir. All components associated with the B system are conveniently located in a compartment with access through a large door.

Reservoirs Connected — The A and B reservoirs are connected by a balance line from the top of the B reservoir to a point above the minimum operating level on the A reservoir. This permits both reservoirs to be pressurized simultaneously. On the ground only, the balance line also serves as a return line, permitting the B system pumps to pressurize all A system functions, by means of a system interconnect valve.

A pressure fill in the B system compartment services all three reservoirs. Fluid may be pumped into any reservoir through a selector valve with a hand-operated pump, or from a filling cart through a disconnect adjacent to the hand pump. Fluid from either source passes through a filter before entering any system reservoir.

The standby system equipment is accessible from the aft passenger entry stairs. It is a second power source for the leading edge slat and flap system. It is the third power source for the rudder, which cannot be operated manually. The system contains a motor pump, reservoir, fluid level indicating system, filters, and a pressure indicating system.

James E. Klansnik is Hydraulic System Group engineer at The Boeing Co., Seattle.

This article was originally published as "727 jet hydraulics: MORE and BETTER," in the July 1964 issue of Hydraulic & Pneumatics. It is reprinted here for its archival value and reader interest.