What happens when a plane suffers a hydraulic failure?

What are hydraulic systems, and why do aircraft have them?

An aircraft has many movable surfaces.

Among them are ailerons (‘little wing’ or ‘fin’ in French), which are located on the wing’s trailing edge and near the wingtip. The ailerons move up and down, and the pilots can roll or bank an aircraft by moving its ailerons.

Then, there are elevators, which are mounted on an aircraft’s tail. The elevators, like the ailerons, also move up or down. Pilots control an aircraft’s pitch, that is, they raise or lower its nose to climb or descend, by moving the elevators.

The rudder, or the vertical fin mounted on the aircraft’s tail, moves left or right. Pilots control an aircraft’s yaw, or right or left movement, by moving the rudder. On the ground too — while rolling down the runway for takeoff or after touching down — pilots maintain directional control with the help of the rudder.

The ailerons, elevators, and rudder are critical flight control surfaces. Without them, pilots won’t be able to ‘steer’ a plane through the air, bank, climb, or descend. And this is where aircraft hydraulics come in — in the bigger and heavy jets, the ailerons, elevators, and rudder are all moved by hydraulics.

Hydraulics are used for other critical movements as well.

These include the extension or retraction of the aircraft’s landing gear; the movement of the flaps and slats, which are mounted on the wings’ trailing and leading edges and are extended during takeoff and landing to increase ‘lift’; the movement of the spoilers, which are mounted on top of the wings and are deployed to slow an aircraft; and activating the wheel brakes, thrust reversers, and horizontal stabiliser (which is the small wing-like structure on an aircraft’s tail on which the elevators are mounted).

How do aircraft hydraulics work?

A pilot controls the movement of the ailerons, elevators, and rudder by moving the sidestick/ yoke (in Airbus and Boeing aircraft respectively) and rudder pedals in the flight deck, and by moving a lever or switch for other control surfaces and landing gear.

But the force of a pilot’s physical inputs cannot move the huge and heavy movable surfaces and components. The ailerons on a big jet are huge and quite heavy. On top of this, air pressure acts on them during flight.

An aircraft’s hydraulic system uses pressurised fluid to magnify the pilot’s physical inputs several times over, and then transfers the ‘magnified force’ to actuators, which move the control surfaces.

Whether an aircraft is being hand-flown or it is on autopilot, the control surfaces are moved by actuators driven by hydraulic fluids.

While many small aircraft do not have hydraulically-operated control surfaces and components — or have partial or limited hydraulics, meaning only one or two components are driven by hydraulics — all big passenger jets have hydraulically-operated control surfaces and components.

What happens when an aircraft’s hydraulics fail?

There can be loss of pressure of the fluid for a variety of reasons — a leak, overheating, a faulty or old component breaking down, or physical damage to the hydraulic lines. Any of these can cause the system to fail during a flight.

Aircraft systems are designed with multiple redundancies for safety. Modern planes have multiple hydraulic systems. If one fails, there is always a backup to fall back on. And it is unlikely that all the systems will fail at the same time.

Some aircraft systems can be operated manually. For example, in the event of hydraulic failure, the landing gear can be extended manually using an alternative system that allows it to drop down under the force of gravity.

With the evolution of aircraft hydraulic engineering, catastrophic failures have become rare. But in case of a failure warning, depending on the pilot’s assessment of the situation and what backup hydraulics are available, aircraft often divert to the nearest airport and land as quickly as possible for the sake of safety and passenger comfort.

Indeed, there have been incidents when leaking hydraulic fumes have entered the aircraft cabin, requiring evacuation.

Has a significant airline disaster been attributed to hydraulic failure?

The case of United Airlines Flight 232 is well known. On July 19, 1989, a McDonnell Douglas DC-10 operating United flight 232 took off from Denver for Chicago with 296 passengers and crew on board.

The DC-10 had three hydraulic systems independent of each other. But at 37,000 ft, a cracked fan disc in the tail-mounted engine disintegrated mid-air, causing the engine to fail, and the debris ruptured all three hydraulic systems. Captain Al Haynes and First Officer William Records lost control over the plane, and it started rolling to the right. Flight Engineer Dudley Dvorak and Dennis Fitch, a check pilot who was flying as a passenger, joined the crew to help.

Captain Haynes controlled the aircraft’s direction by manipulating the thrust from the two working engines and managed to line up the plane for landing at Sioux City Gateway Airport, Iowa. As the aircraft touched down, its right wing struck the ground. The plane broke apart as it skidded and cartwheeled to a stop. As many as 111 people on board died, but the fact that 184 others survived is regarded as a signal tribute to teamwork in an aviation emergency.

Captain Chesley ‘Sully’ Sullenberger, hero of the “miracle on the Hudson” who landed on a river in New York City in 2009 after both engines of his plane were disabled by a bird hit, mentioned the Flight 232 incident in his book ‘Highest Duty’. “This type of emergency was so rare that there was no training for it, no checklist. It would later be determined that the odds of a simultaneous failure of three hydraulics systems approached a billion to one… In essence, they (the crew) had forty minutes to learn a new way of flying an airplane,” Sullenberger wrote.