April 1, 2002
By Chris Grosenick
Top: A Boeing 757 No. 1 engine pylon hydraulic pressure and thrust reverser modules.

Middle and bottom: Boeing 757 left system hydraulic service center.

Hydraulic/pneumatic systems are used in modern aircraft because, for the weight, they efficiently transmit power to subsystems that require large operating forces to remote locations on the airframe. Research is currently underway to replace hydraulic systems with lightweight powerful electrical systems, but hydraulic power will be the primary force used in aircraft until electrical technology matures.

Figure 1. Pascal's Law
Figure 2. Bernoulli's Law

Two laws fundamentally describe the ways in which hydraulic systems accomplish work. For this discussion, "work" is defined as the movement of control surfaces, cargo doors, landing gear, and the other hydraulically operated systems on an aircraft. Also, the term fluid describes both liquids (oils) and gases. Pascal's Law states that fluid flow is equal and undiminished in all directions within the confines of a container (Figure 1). The "container" in an aircraft is the system's tubing, valves, actuators, reservoirs, pumps, and other components. Bernoulli's Law states that as the velocity of a fluid increases, its pressure decreases (Figure 2). This law describes the pressure-flow relationship and the operation of an aircraft wing.

Output force in a hydraulic system is calculated by multiplying system pressure by cylinder piston area. In Figure 3, the 10 psi produced in the left hand cylinder is multiplied by the 20-in. piston area in the right hand cylinder to generate 200 pounds of force on the piston rod. The 10 psi is generated by applying 20 pounds of force on the left hand cylinder piston and spreading that force over the 2-in. piston area (force ÷ area). On the working cylinder; F = P x A, and on the work producing cylinder; P = F ÷ A (where P is pressure, F is applied or generated force and A is piston area).

There are several types of fluid flow within a system: laminar and turbulent. Flow types are important because turbulent flow generates heat and decreases the system's ability to perform efficient work. Hydraulic systems incorporate as much laminar flow as possible by the design of tubing runs, and the internal construction of components.

The main difference between liquids and gases is that gases are highly compressible. Liquids are theoretically uncompressible, but they compress a very small amount because entrained air is trapped in the fluid due to the effect of atmospheric pressure. When fluid is pressurized, the air is forced into solution and becomes difficult to detect, but these microscopic pockets of air cause wear and erosion within the hydraulic system, mainly in the pump. When the pressure is relieved in a hydraulic system, air forced into solution bubbles out and causes air pockets in the system. This is similar to opening a soda bottle and having the gas escape out the partially opened top (along with half the soda). A venting reservoir on engine shutdown or pump cut-off is a good indication that there is too much air in the hydraulic system. At high altitude, fluid will foam in an unpressurized reservoir, and this causes problems with supplied pressure and system operation, since hydraulic systems are designed to work with fluid and not foam. High performance aircraft have pressurized reservoirs for this reason, and this is why it is important to bleed hydraulic systems after a component is changed.

Figure 3. Force, Area, Pressure Relationship
Figure 4. Laminar and Turbulent Flow

The interaction between pressure and fluid flow (volume) is sometimes confusing; what makes actuators move, pressure or flow? The answer is both depending on the context of the question, and whether force or movement is the issue. Aircraft hydraulic systems are usually described by pressure, psi, and volume in gallons per minute, gpm. Hydraulic pumps are rated at a certain psi and gpm output, for example, 3,000 psi at 25 gpm. This means the pump will produce 25 gpm of flow at 3,000 psi. As pressure increases to 3,000 psi, flow decreases to zero with no flow demand in the system. Thus, the pressure-flow relationship is an inverse one. Pressure is controlled in a hydraulic system by a pressure-sensitive regulating device such as a pump compensator, pressure regulator valve, pressure-operated relief valve or a pressure switch (electrically operated pump).

Given a pressurized, static, closed hydraulic system, closed meaning no bleed-through or bypass built into any system components, fluid flow is zero and pressure is regulated maximum. Operating the landing gear on jacks, the force to overcome is gravity, and with the gear selector positioned to "up", fluid is ported to the gear actuators. There now exists a demand for fluid flow by the gear actuators prompted by opening the gear selector valve, so flow increases and pressure decreases. As the pressure drops, the regulating device in the system senses this drop and positions the pumping element to increase fluid flow. As this volume of fluid flowing into the system increases, it "crowds" fluid moving already, and resistance (pressure) builds. This delicate balance shifts back and forth many times throughout the entire travel of the actuator stroke, until the actuator movement, and thus fluid flow, stops. Fluid flow moves actuators, and pressure works to overcome gravity and airloads. Both properties must exist for the system to do any meaningful work. What about when something operates and the pressure does not drop? Hydraulic systems are designed taking into account volume requirements for all components, and then using a pressure source that can operate those components at the system rated pressure with some margin for safety and operating efficiency. For example, our 3,000-psi, 25-gpm pump mentioned above could be used in a system that has a volumetric demand of up to 25 gpm, and still maintain 3,000 psi. It's important to remember that pressure will drop past the 25-gpm point, despite the pressure regulator device.

Figure 5. General gas law relationships

Gases are considered fluids in a strict physical sense. They are also highly compressible, which leads to another set of laws to describe their behavior. Pascal's and Bernoulli's laws hold true for gases, but because they are more compressible, several more specific gas laws are used. Charles' Law and Boyles' Law deal with the relationship between temperature, pressure, and volume of a gas, and their respective laws can be summarized in the General Gas Law. These laws state three fundamental properties of gases. With the temperature remaining constant, pressure varies inversely with volume (A). As the temperature of a confined gas increases, the pressure of that confined gas increases as well, the temperature-pressure relationship being directly proportional (B). With the pressure held constant, the volume of a gas increases in direct proportion to the temperature (C).

This article only touches the surface of the physical principles relating to aircraft hydraulic and pneumatic systems. Hydraulic systems are presently the predominant workhorses of the aircraft structure, and knowing and understanding the basis of their operation is crucial for the effective troubleshooting and maintenance of these systems.

Hydraulic Fluids

There are two general categories of hydraulic fluids: petroleum and synthetic. Petroleum-based fluids were developed first, and are used primarily in military aircraft, light aircraft, and almost universally in landing gear shock struts. Synthetic oils are used mainly in the commercial fleet and military aircraft derived from civilian designs. The main reason for developing synthetic fluids was the increase of system operating temperatures and fire retardancy. The latest revision MIL-H-83282 synthetic/petroleum blended fluid has an operating range of -40 degrees F to 275 degrees F, with a flashpoint of 445 degrees F. Phosphate ester based (synthetic) fluids like BMS 3-11, Skydrol® and Hyjet® have ranges of up to 275 degrees F and a flashpoint of 320 degrees F. Phosphate ester based fluids and petroleum fluids are not interchangeable, and require O-rings made for that particular fluid type. There are synthetic O-rings that work with synthetic oils, and have universal application in hydraulic, engine oil and fuel systems. Check the parts book and maintenance manual for the correct packing application.

"Petroleum-based fluids are used primarily in military aircraft, light aircraft, and almost universally in landing gear shock struts. Synthetic oils are used mainly in the commercial fleet and military aircraft."

Chris Grosenick is Quality Assurance Specialist at NASA Langley Research Center, Hampton, VA. He holds an A&P certificate and private pilot's license.

Photos by Chris Grosenick