Posted on September 30, 2021

Maintainable Hydraulic System Design: Redundant Systems

John Joyce
Written by

John Joyce

Posted in
Hydraulic Maintenance

Safe system design and component placement in hydraulics are critical to maintainability and long service life. The proper location of components allows for easy access where they can be readily serviceable. And hydraulic systems are only as good as the components and fluid selection for the application.

Strict adherence to Cybersecurity Requirements, AS9100D Certification and Nadcap Accreditation for fittings and other critical hydraulic parts are essential to safe selection during design and maintenance. However, to ensure safe system operation, designers, fabricators, and maintenance personnel must specify the correct fluids to be used (mineral oils, synthetic or semi-synthetic coolants, etc.).

In addition to the right fluid selection, correct placement of fluid filters, hose connections under frequent activation or vibration, and any component that requires frequent changeover or maintenance must be specified in the design. Being proactive in the design and fabrication for ease of accessibility to components assures longer system life, reduced maintenance costs and protection of workers. Using 3D-CAD solid modeling and other design software prior to assembly helps fabricators find the best component arrangement.

The Necessity of Aircraft Hydraulic Redundancy Systems

An aircraft’s flight control system, flaps, landing gear and brakes are dependent on fluid power in order to take off, fly, and land the plane safely. Hydraulics in today’s aircraft are significantly more secure than even a couple of decades ago, largely due to built-in redundancies which combine two or more hydraulic subsystems.

For instance, in November 2010, a Qantas Airbus A380-842 took off from Singapore to Sydney, Australia, with 440 passengers and 26 crew members. Just as it was climbing out of Singapore over the Indonesian island of Batam, only six minutes into the flight, passengers heard a loud bang as a rotor disc exploded, spraying metal into the number 2 engine. A piece of debris was shot into a wing and cut the hydraulic line, triggering the hydraulic circuit to lose its fluid. The explosion even sent some debris with the Qantas logo tumbling onto a road below.

Almost immediately the Airbus dumped its fuel, turned back to Singapore, and landed safely with its hydraulic control system intact. Even during a seemly catastrophic system failure, the flaps extended, and slats retracted, with the gear doors opening as normal. What could have been a horrifying incident was instead avoided due to the backup provided by redundant hydraulic and electrical systems.

Redundant systems are achieved by two primary methods: multiple systems or multiple pressure sources within the same system.

  • Multiple Hydraulic Systems - Flight control surfaces are typically actuated hydraulically. As a backup, multiple actuators are on each surface which is powered by multiple hydraulic systems. These are central to ensuring that the failure of one hydraulic system will not cause loss of control. In many of today’s commercial aircraft, flight control surfaces are powered by three independent hydraulic systems, allowing for the failure of two systems while being able to maintain control through the third system.
  • Multiple Pressure Sources - Hydraulic systems are often designed with more than one pump to pressurize the system. It is also quite common for a system to have more than one engine-driven pump, plus one or more electric pumps as a backup.

Backup Systems Can’t Always Overcome Faulty Design or Maintenance

On May 16, 2019, an F-16 Fighting Falcon crashed into a warehouse near March Air Reserve Base in Riverside County, California. The plane had lost hydraulic pressure, leaving the pilot unable to control the aircraft. Fortunately, the pilot ejected and parachuted to the ground suffering only minor injuries.

An investigation later concluded that two inlet check valves were improperly installed, leading to a leak of hydraulic fluid that rendered the pilot unable to control the $24 million aircraft, which was subsequently destroyed in the crash. The report was not clear in identifying whether the OEM of the component that contained the check valves was at fault or the crew who did the overhaul. However, in either case, the extreme importance of proper design and maintenance cannot be overstated.

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