F-15 Hydraulic System
By ROBERT S. ANDREWS/Senior Engineer, Hydraulic Design
The F-15 Hydraulic System incorporates some of the latest hydraulic design concepts from the standpoint of safety, survivability, and maintainability. McDonnell has been able to incorporate these many design improvements because of the high learning curve obtained from design and operation of the highly successful F-101 Voodoo and F-4 Phantom II.
The Hydraulic Systems consist of three independent systems: Power Control 1 (PC-1), Power Control 2 (PC-2), and Utility. PC-1 and PC-2 systems power the primary flight controls and the Utility system supplies all other requirements, plus back-up for stabilator longitudinal and roll control, aileron roll control, and rudder directional control. Hydraulic power is available to adequately and safely maintain control for flight and landing with any one of the three systems operational.

The block diagram shows the various subsystems in the "A" and "B" circuitry of the PC-1, PC-2, and Utility systems. In the Utility system, the "A" circuit lines are primarily on the left side of the aircraft and the "B" circuit is primarily on the right-hand side. This improves survivability from a gunfire standpoint.
Since any one of the three hydraulic systems can maintain a supply of hydraulic pressure to the control system, it is obvious, as you refer to the illustration, that the crisscross of hydraulic supply to the flight controls from left and right engine driven pumps, through RLS circuitry and switching valves, gives multiple redundancy of hydraulic supply to the F-15 primary flight control components. Here is what will happen during several emergency situations:

  • When all electrical power is lost,
    control is maintained with ailerons
    and differential stabilator for roll,
    stabilator for pitch, and two rudders.

  • When either PC hydraulic system
    plus the Utility hydraulic system, and
    all electrical power are lost, control is
    maintained with ailerons on one wing
    and differential stabilator for roll, stabilator
    for pitch, and one rudder. (If PC-2 and Utility are lost, the Control Stick Boost and Pitch Compensator will be inoperative.)

  • When all mechanical controls are
    lost, control is maintained by the
    Control Augmentation System driving
    the differential stabilator for roll and
    pitch, and both rudders.

  • When both PC-1 and PC-2 hydraulics are lost, control is maintained with the Utility hydraulic system supplying power to all primary flight controls.

For ease of maintenance, the F-15 pump was designed as a plug-in type. The intake, outlet, and case drain fluid flows are directed to the spline-drive end of the pump where they pass through quick disconnect couplings. These connect the pump to an aircraft mounted manifold which has rigid tubing attached, allowing the pumps to be installed and removed without disconnecting hoses and tubes. Doing away with hoses eliminates the possibility of chafing and there are fewer leakage points. Self-sealing checks were incorporated to prevent line drainage during replacement. The pump also incorporates fast-response compensator shutoff to lower hydraulic system pressure spikes. Basic system accumulators found in most aircraft have been eliminated (these are high replacement items and can be responsible for introducing air into a hydraulic system).

Each of the three systems (PC-1, PC-2, and Utility) has a single filter module which incorporates pressure and return filters, system relief valves, pressure switches, pressure transmitters, and pump outlet check valves. As a result, there is one module and one door per system, simplifying servicing. All pressure and return elements are non-collapsible at 4500 PSI ΔP and are in one size and type (15 micron absolute with an approximate 8 gram dirt capacity) for commonality and good logistics control. The filters have self-sealing checks incorporated to prevent line drainage, and there are delta-P indicators at the bottom of the bowl to reveal a dirty element. The bowls must be removed to reset the indicator and the bowl cannot be replaced without an element inside. The bowls feature self-locking ratchets, and are non-interchangeable pressure-to-return to assure murphy-proof maintenance. The relief valve is a fast-response type backing up the fast-response pump compensator allowing elimination of accumulators. The pressure filter is non-bypass while the return filters are dual purpose. They filter the system return oil (bypass) and the pump case drain (non-bypass). This allows the pump case drain (which carries particles from the hardest-working most wear-producing component in the system) to have a large, high-dirt capacity filter with no danger of particle recirculation to accelerate pump wear. This also prevents wear particles from a failing Utility pump from contaminating the second Utility system pump.

Each of the three F-15 bootstrap type reservoirs incorporates reservoir level sensing (RLS). RLS works on the principle that a leak developed in the aircraft will cause the reservoir level to sink. As the level decreases, RLS sensing mechanically operates a valve which shuts off half the system (designated "A"). If this stops the leak, the reservoir level will stop sinking and the other half of the system (designated "B") will be retained.
On the other hand, if the leak •continues, the reservoir will continue to deplete until a second valve shuts off the "B" half of the system. When "B" shuts off, the "A" system returns, reactivating one-half of the system. This is accomplished by mechanical linkage between the "A" and "B" shutoff valves. Leaks in the pump or filter circuit are not protected by reservoir level sensing. However, as you can see, RLS improves the survivability of the aircraft.
The gaging system on the F-15 reservoirs is also unique as the gaging is temperature compensating to allow for volume increase or decrease due to oil temperature changes. Automatic overflow occurs if the reservoir is overfilled, preventing reservoir damage.

Another new type of hydraulic component found in the F-15 is a "switching valve." Four of these are used to further improve the survivabil-ity of the primary flight control systems. Two switching valves are in the aileron circuit; two others are in the tandem stabilator/rudder circuits.
  These valves allow the normal operating pressure from the "B" RLS circuits of both PC-1 and PC-2 to pass directly through the switching valves to the left and right ailerons, to one side of each tandem stabilator, and to each rudder. Should a "B" circuit lose pressure for any reason (leak, pump failure, etc.), the switching valves will move to a test position to assure that the system downstream of the switching valve is intact. If system integrity is verified, the Utility system will be switched into the downstream flight control actuators. This test position prevents loss of Utility oil should the break be downstream of the valve.

F-15 Hydraulic System Diagram
In the F-15, the hydraulic selector valves have a design feature called return pressure sensing (RPS) which was incorporated to improve hydraulic system reliability. Selector valves with RPS will not operate if there is a leak in the selected lines or in the return line to the first check valve. This prevents the pilot from switching into a failed hydraulic circuit where the oil would be directed overboard, thus losing the entire system, or half a system if the failure was in one of the RLS branches.
Return pressure sensing blocks the pressure to the solenoid pilot-operated section of the selector valve. The block is achieved by sensing the loss of return line pressure in the subsystem lines which have failed. Subsystems which must be operated after failure have emergency back-up provisions.
In selector valves, care was also taken to design out "man traps" such as doors or surfaces that are hydraulic-ally positioned open or closed upon removal of electrical power. An example is the F-15 speed brake valve which remains in a full trail position (both selected lines become common to return if electric power is removed from the aircraft).

F-15 check valves are designed so that they can be installed in only one direction. Therefore, it is impossible to install one backwards during maintenance. The secret lies in the different size end fittings.
The return check valves in each subsystem have been installed as far downstream as possible, just prior to entering the main return trunk line. This gives the maximum line protection against losing reservoir oil from back-flow into a leak in a return line.

Hoses and most swivels have been eliminated in the F-15 through use of coil tubes. Some of the common problems of the past (including chafing, installation in a twist which accelerates failures, cross-connection which is dangerous, and weepage through hose liner imperfections) have been avoided. In addition, swivels with rotating dynamic seals are at a minimum in the Eagle.

F-15 flight control and engine inlet components use dual external dynamic shaft seals. This design utilizes two seals in series with the center area vented to return through a restrictor which reduces system internal leakage in event of a first stage seal failure. The second stage atmospheric seal is normally subjected to return pressure but is capable of withstanding full pressure should the first stage seal fail. This allows increased seal life and component survivability as the first stage dynamic seal is lubricated on both sides. Should the first seal fail, the second seal can act as a back-up.


The F-15 plumbing uses a new, permanently swedged fitting in some locations, eliminating many potential inline tube connector leak points. The tube connectors used at valves, and at remaining inline connectors, are of the latest design, stay tight, and require less maintenance. (The DIGEST took a closer look at the Dynatube fittings in Volume 22, Number 3, 1975.)
Air in hydraulic systems is an age-old problem. The F-15 components have been specially designed to eliminate this possibility. The canopy accumulator is the only unit in the hydraulic systems where pressurized air leaking by a seal can enter the hydraulic system. In this case, space dictated the use of a smaller standard accumulator with a single dynamic seal. Air problems such as overflow or bursting of reservoirs, excessive bleeding after emergency operations, and cavitated pumps with momentary loss of system pressure have been minimized during design of the Eagle.
Here are some of the applications that minimize on-board air problems.
  • Basic system accumulators have
    been eliminated.

  • Dual vented seals are used in
    components which have air chambers.
    Typical of these are jet fuel start
    accumulators, arresting gear cylinder,
    and canopy counterbalance actuators.
    Dual seals allow the air to be vented
    overboard instead of into the hydraulic
    system should air leak by a dynamic

  • Emergency air systems have been
    eliminated. The landing gear, brakes,
    and steering emergency systems use
    oil from the jet fuel start accumulator.
    The aerial refueling emergency system
    uses a pyrotechnically operated

With all these new features, we feel that the F-15 exhibits a giant step ahead in hydraulic system design. The results - improved system maintenance, reliability, and aircraft survivability. Things that make a product better, and a weapon more effective.