Why Aircraft Hydraulic Systems Are Critical to Flight

 When you picture an airplane flying through the air, you most likely think of powerful engines, streamlined wings, and high-tech avionics. But behind the polished surface and high-tech gadgetry is a system of unheralded heroes: the hydraulics. And far from being a luxury item, the hydraulic system is an absolutely essential piece of equipment, facilitating nearly every significant function of an airplane.

The Power of Fluid

Fundamentally, a hydraulic system uses an incompressible fluid (often a specialty hydraulic oil) to transfer force from one location to another. Imagine a fluid lever. A small amount of force applied over a small surface can create an enormous force over a larger surface, or simply pass force around without loss through complex systems. In an airplane, that translates to pilots being able to control huge power with modest effort.

What Do Hydraulics Do in an Aircraft?

The list is quite long, but here are some of the most important functions:

• Flight Control Surfaces: This is perhaps the most important task. Ailerons, elevators, rudders, flaps, and slats – all of these surfaces must travel with phenomenal precision and power to maneuver the aircraft's attitude, altitude, and heading. Without hydraulic assistance, traveling these against the tremendous air pressure at speed would be beyond a human pilot. Think about trying to turn a car without power steering, but at 500 mph!
• Landing Gear Operation: Lowering and retracting landing gear is a work of weight. The gear is itself heavy, and the forces used in descending and locking into place, or retraction neatly into the fuselage, require a lot of power. Hydraulics take care of this smoothly and dependably.
  Brakes: Aircraft braking systems today are very powerful, capable of stopping multi-ton vehicles safely and efficiently, even under emergency conditions. Hydraulic pressure is central to energizing these high-performance disc brakes.
• Nose Wheel Steering: On the ground, accurate steering is necessary for taxiing. Hydraulics deliver the power and control necessary for the nose wheel to guide the airplane around the airport.
• Thrust Reversers: They are used after landing to assist in slowing down the aircraft by reversing engine thrust. Deployment and retraction are usually hydraulically powered.
• Cargo Doors and Stairs: Even routine operations such as opening large cargo doors or deploying integrated airstairs tend to depend on the smooth, controlled power of hydraulics.

Redundancy: A Matter of Life and Death

Aircraft hydraulic systems are designed with enormous redundancy because of their critical roles. Commercial airliners have more than one independent hydraulic system (usually three or more), and each one can drive critical functions. That is, if one system collapses due to a leak or pump failure, there can be another system that will drive through, allowing safe operation to continue. Flight training for hydraulic system failure is practiced intensively, and such failures are rare but severe.

The Maintenance Challenge

Although extremely durable, hydraulic systems need to be carefully maintained. Leaks, fluid contaminants, or failed pumps may cause system deterioration or total loss of function. Frequent inspection, fluid analysis, and component replacement are critical to the reliability of these essential systems.

Conclusion

The next flight you're on, pause for a moment and appreciate the quiet, monumental efforts being made by an airplane's hydraulic system. It may not be as glamorous as a jet engine, but without its steadfast reliability, the intricate dance of take-off, flight, and landing simply wouldn't happen. It is the true behind-the-scenes hero, taking a pilot's instructions and turning them into the literal motions that keep us securely in the air.