Boeing 757: Aircraft Hydraulic Power Units (HPUs) & Hydraulic Mules

Hydraulic power refers to the use of hydraulic systems to transmit force, motion, or energy by circulating a controlled, pressurized fluid to a motor which then converts it into a mechanical output capable of performing certain functions. In aviation, hydraulic power plays a crucial role in various aircraft systems, including flight controls, landing gear, brakes, thrust reversers, and auxiliary systems.

A hydraulic power unit (HPU) is a self-contained system that generates and supplies hydraulic pressure to power various aircraft systems during maintenance and other critical ground operations without requiring the aircraft's engines to be running. HPUs and hydraulic mules are considerably preferable over other energy sources because they can transmit large amounts of electric energy into hydraulic power with minimal losses and over long distances. Portable hydraulic power units are known for their reliability and can be used for demanding applications related to aviation hydraulic systems.

The primary components of a hydraulic power unit include:

• Power Source: The HPU's power source – typically either an electric motor or an internal combustion engine – drives the hydraulic pump to generate fluid flow under pressure.
• Hydraulic Pump: This crucial component converts mechanical energy into hydraulic energy by pressurizing the fluid.
• Reservoir: The reservoir stores the hydraulic fluid used in the system. It serves multiple purposes: it supplies fluid to the pump, helps dissipate heat, allows for the settling of contaminants, and compensates for fluid volume changes due to temperature fluctuations.
• Filtration System: Filters remove contaminants from the hydraulic fluid. Clean fluid is essential for the system's reliability and longevity, as contamination can damage the components.
• Pressure Relief Valve: This critical safety feature prevents excessive pressure build-up in the hydraulic system by redirecting fluid back to the reservoir or a low-pressure circuit when the system pressure exceeds a predetermined limit.
• Control Valves: These valves regulate the flow of hydraulic fluid to the aircraft's systems, controlling the speed, direction, and force of hydraulic actuators.
• Heat Exchanger (Cooler): In systems where hydraulic fluid may become excessively hot, a heat exchanger or cooler helps maintain the fluid within the desired temperature range, ensuring the system operates efficiently and preventing damage from overheating.
• Pressure Gauges and Sensors: These components monitor the pressure within the hydraulic system, providing essential feedback for safe and efficient operation. They help diagnose system performance and preemptively identify potential issues.
• Hoses and Connectors: These are the conduits through which the hydraulic fluid flows. High-quality hoses and connectors ensure that there are no leaks and that the hydraulic fluid reaches its intended destination under the required pressure.

No, not all hydraulic power units use electric motors. While electric motors are common in many HPUs, especially those used in hangars or maintenance facilities, other types of motors can be used as well. Diesel engines or gasoline engines are often employed in portable or mobile HPUs where electric power may not be readily available.

The output of a hydraulic power unit is rated based on three key parameters:

• Flow Rate: The volume of hydraulic fluid the HPU can deliver per minute, typically measured in gallons per minute (GPM) or liters per minute (LPM). This determines the speed at which hydraulic systems operate.
• Pressure: The maximum hydraulic pressure the unit can generate, usually measured in pounds per square inch (PSI) or bar.
• Power: The amount of power the motor consumes, often rated in horsepower (HP) or kilowatts (kW), influences the pump's performance.

Depending on your aircraft testing requirements, some aircraft require two pressures to be run simultaneously to conduct full testing. A split-system hydraulic power unit adds another set of pressure and return hoses to the aircraft. The ability to run two pressures out of one unit eliminates the need to disconnect the hoses and reattach them to another location on the aircraft. For more information about single, split, and true dual HPU systems, please visit our blog.

Customizing your Tronair HPU by adding these optional features ensures optimal performance and longevity.

• Hourmeter (Option F): This 60 Hz option adds a monitor to help schedule filter changes and other preventative maintenance measures.
• Electric Filter Clogging Indicator (Option R): This sensor detects the pressure differential between the filter input and output. When a clog occurs, a panel warning light alerts you to change the filter.
• Drip Pan (Option 4): This stainless steel pan covers the base of the unit to keep fluid from dripping on your hangar floor.
• Hand Pump (Option M): This 5,000 psi (345 bar) Hi/Lo hand pump, equipped with a 2-micron filter and a 10,000 psi (689 bar) pressure gauge, is perfect for reservoir servicing or component testing. (NOTE: Some units exclude the pressure gauge.)
• Self-Cleaning Circulation Kits: These kits connect pressure and return hoses, allowing hydraulic fluid to circulate through internal 2-micron and 5-micron filters, keeping your HPU pristine.

The overflow return dump hose – Option P in Tronair HPU models – is mainly used on smaller hydraulic power units. It allows an unrestricted flow of fluid from the aircraft reservoir to the HPU's reservoir.

No, the 220V/50 Hz/3PH is not applicable for the 58, 59, 5A, and 5L series of HPUs. For all D-frame HPUs, the voltage must be 380V or higher. In this case, the hertz does not matter.

Yes! We offer new HPUs in yellow, red, green, white, and gray. For Tronair units, our recommendation is the common and popular Tronair Blue.

Yes! Adding a towing/trailer package to your Tronair HPU means your unit will have larger rubber tires, leaf spring suspension, automatic brakes, and a small towbar. Simply ask your aviation specialist to add Option N to your quote.

Tronair's smaller 50 Series HPUs are often lower to the ground, which can cause connectivity issues with the aircraft. Adding Option X to your new hydraulic power unit can help prevent future issues of this nature.

No. For aircraft requiring a single system to function but using a dual-split-system HPU, you will only need one of the coupling kits (one pressure, one return coupler) to use on the dual/split HPU. For aircraft that require a dual/split system, you will need two of the coupling kits: two pressure couplers and two return couplers.

Yes. Tronair hydraulic power units are equipped with a clearly visible sight gauge located on the front of the unit, making it easy to monitor hydraulic fluid levels during filling and maintenance. This feature helps ensure accurate reservoir filling, preventing overfilling or running the unit with insufficient fluid – both of which can impact performance and component life.

When commissioning a Tronair hydraulic power unit, the selector valve should be set to the horizontal position. This ensures proper fluid flow and system functionality during the initial setup process. Correct selector valve positioning is essential to achieve accurate testing results and avoid potential hydraulic system damage.

Yes. After completing testing with a Tronair dual hydraulic power unit, the bypass valves must be opened to relieve any residual hydraulic pressure in the system. This step helps protect both the HPU and the aircraft’s hydraulic components, ensuring safe disconnection and preventing unintentional system pressurization.

Yes. Tronair dual hydraulic power units feature a built-in reservoir alarm system that alerts operators when hydraulic fluid levels are either too high or too low. This safeguard helps maintain optimal fluid volume, preventing potential system damage, performance issues, and costly downtime.