The Consequences of Hydraulic System Malfunctions During Aircraft Takeoff and Landing

Aircraft hydraulic systems are the unsung workhorses of flight, powering everything from flight control surfaces and landing gear to brakes and thrust reversers. When these systems fail, the effects cascade through the most critical phases of flight: takeoff and landing. Understanding the specific impacts, the engineering designed to mitigate them, and the rigorous procedures pilots follow is essential for aviation safety. This article explores the full scope of hydraulic system failures, their influence on takeoff and landing procedures, and the measures that keep flying one of the safest modes of transportation.

How Aircraft Hydraulic Systems Work

Modern aircraft rely on hydraulic power to move heavy components with precision. A typical system consists of pumps (engine-driven or electric), reservoirs, accumulators, valves, actuators, and a network of high-pressure lines. The fluid—usually a fire-resistant phosphate ester-based oil—is pressurized to between 3,000 and 5,000 psi. This pressure operates linear actuators for landing gear retraction/extension, rotary actuators for flight control surfaces (ailerons, elevators, rudders), and brake calipers.

Redundancy is built into every commercial airliner. Large jets like the Boeing 787 or Airbus A350 have three or four separate hydraulic systems, each powered by a different engine or electric pump. This design philosophy ensures that a single leak or pump failure rarely disables all hydraulic capability. However, even with redundancy, failures can create complex scenarios that demand immediate and coordinated pilot response.

Common Causes of Hydraulic Failures

  • Fluid leaks: The most frequent failure mode. Leaks can occur due to worn seals, hose ruptures, or component cracks. A slow leak may go unnoticed until a low-fluid-level warning appears; a sudden, large leak (e.g., from a burst line) can cause rapid pressure loss.
  • Pump malfunctions: Engine-driven pumps can fail due to bearing seizures, cavitation, or loss of drive. Electric backup pumps may also fail if electrical power is disrupted.
  • Fluid contamination: Water, debris, or incorrect fluid types can degrade seals, clog filters, and cause valve sticking. Contamination often leads to multiple component failures over time.
  • Accumulator failure: Accumulators store pressurized fluid for emergency actuation. A failed accumulator can reduce response time or eliminate the ability to operate brakes or landing gear after engine shutdown.
  • Valve or actuator jamming: Mechanical binding or debris can prevent a control surface or landing gear from moving, creating asymmetric conditions during flight.

Impact on Landing Procedures

Landing is the most demanding phase of flight, requiring precise control of speed, descent rate, and aircraft attitude. Hydraulic failures during this phase can affect multiple critical subsystems, increasing pilot workload and potentially compromising safety.

Landing Gear and Braking

Landing gear normally extends and locks using hydraulic pressure. If the primary system fails, pilots can often use an alternate extension system, which may rely on gravity, springs, or a hand pump. For example, on the Boeing 737, the landing gear can be manually cranked down from the cockpit using a mechanical handle, though this requires significant effort and coordination. On larger aircraft, an electric-powered backup hydraulic pump (such as the Power Transfer Unit on the A320) may be used. If all hydraulic pressure is lost, the gear may not fully lock, forcing the crew to perform a “gear-up landing” or use an emergency extension procedure that could take several minutes.

Braking systems are equally reliant on hydraulics. On most airliners, the brakes are applied via a hydraulic servo valve. A total hydraulic failure typically causes loss of normal braking, leaving only an accumulator-powered emergency brake system (which provides a limited number of brake applications) or the parking brake. In such cases, the aircraft’s stopping distance can increase significantly. On wet or contaminated runways, the risk of a runway excursion becomes much higher. Pilots must also consider the use of reverse thrust, which itself may require hydraulic pressure to deploy the thrust reverser doors.

Anti-skid protection, which prevents wheel lock-up, is also lost without hydraulic pressure. Without anti-skid, pilots must brake manually, applying light intermittent pressure to avoid skidding, a technique that demands excellent feel and skill.

Control Surfaces and Flaps

Flaps and slats are extended during approach to increase lift at low speeds. Their actuation is hydraulically powered on nearly all transport category aircraft. A hydraulic failure may prevent full flap extension, limiting the aircraft to a cleaner configuration (e.g., flaps 15 instead of flaps 30). This requires higher approach speeds and increases landing distances. Similarly, spoilers (which assist in speed reduction and load transfer to the wheels) may not deploy, further reducing braking effectiveness and increasing stopping distance.

Control surface movement (ailerons, elevator, rudder) can be affected. While most fly-by-wire aircraft have multiple hydraulic actuators per surface, a total hydraulic failure forces reliance on manual reversion—a system available on some older types like the Boeing 727 or 737 Classic. In those aircraft, cable-driven control surfaces require significantly more physical force. For example, in a Boeing 737 with no hydraulic pressure, moving the control column requires effort comparable to pushing a heavy shopping cart. This dramatically degrades handling precision, especially during flare and touchdown.

Impact on Takeoff Procedures

Takeoff is equally vulnerable to hydraulic issues. Failures that occur before V1 (decision speed) can lead to a rejected takeoff, while those after V1 commit the aircraft to fly despite the malfunction.

Before Takeoff: Pre-Flight Checks

Crews verify hydraulic system pressures and fluid levels before every flight. If a leak is detected on the ground, the flight may be delayed or canceled. However, some failures manifest only after engine start or during taxi. For instance, a pump that fails to pressurize may force the crew to return to the gate for maintenance. The decision to take off with a known hydraulic issue is rare and only allowed per Minimum Equipment List (MEL) provisions for specific, low-risk failures (e.g., one of three hydraulic systems inoperative with extensive operational limitations).

During Takeoff Roll

If a hydraulic failure occurs below V1, the standard procedure is to reject the takeoff. This can be dangerous if braking capability is compromised. With a total hydraulic failure, normal brakes are lost, and the crew must rely on the emergency/parking brake. That brake may not provide enough deceleration to stop within the remaining runway, especially on a wet surface. Pilots must also be aware that all brakes may be inoperative—the emergency brake typically only applies to the main gear wheels, and its effectiveness is limited. In such cases, they may need to steer off the runway or use overrun areas.

For failures after V1, the aircraft must continue the takeoff. Hydraulic power may be needed to raise the landing gear. If gear retraction is impossible, drag increases, reducing climb performance. Flaps may also fail to retract, further penalizing climb rate. The crew must calculate a revised climb gradient and possibly a weight-limited flight plan. On some aircraft, a total hydraulic failure prevents normal flap operation, forcing the aircraft to climb in a dirty configuration. Noise abatement procedures are abandoned, and the flight diverts to a suitable alternate.

Thrust Reversers and Control After Takeoff

Thrust reversers are not used during takeoff itself, but their availability affects rejected takeoff performance. Some rejected takeoff calculations rely on reverse thrust to stop the aircraft. If hydraulic failure prevents reverser deployment (typical of most transport-category jet engines), stopping distance increases measurably. After takeoff, the inability to use reverse thrust may also affect landing performance at the destination or alternate.

Redundancy and Backup Systems

Given the severity of hydraulic failures, aircraft designers have developed multiple layers of protection. Understanding these systems is critical for appreciating how pilots manage emergencies.

Multiple Independent Systems

Most airliners have three separate hydraulic systems, each with its own reservoir, pumps, and lines. For example, the Airbus A320 has Green, Blue, and Yellow systems. The Boeing 777 has Left, Right, and Auxiliary systems, with the Auxiliary system powered by a RAT (Ram Air Turbine) that deploys in flight if both engines fail. This segregation means that a leak in one system will not affect the others, though a single catastrophic event (e.g., rotor burst striking all three lines) could theoretically cause total loss.

Power Transfer Units (PTUs)

PTUs allow one hydraulic system to power the pumps of another without fluid exchange. On the A320, the Green system can power the Yellow system’s pump via a PTU. This provides backup for flight controls and landing gear even if the Yellow engine-driven pump fails. However, PTUs themselves can fail, and their operation creates an additional load on the working system.

Ram Air Turbine (RAT)

On many fly-by-wire aircraft, a RAT deploys from the fuselage and uses the airstream to drive a hydraulic pump and/or electric generator. The RAT provides sufficient pressure to control the aircraft and extend landing gear, but at reduced capacity. For example, the Bombardier Global 6000’s RAT powers a single hydraulic pump that can operate flight controls at reduced rates and extend gear. The RAT is not designed for sustained high-demand operations like repeated full flap extension.

Manual and Mechanical Backup

Some aircraft retain mechanical linkages for critical functions. The Boeing 757 and 767 have cable-operated rudder and elevator backups, though ailerons require hydraulic pressure. On the 737, the stabilizer trim can be operated manually via a wheel in the cockpit. Landing gear can be manually extended with a hand crank on many types. These backups demand high crew strength and coordination, especially under time pressure.

Pilot Response and Emergency Procedures

Crews train extensively for hydraulic failures. The specific response depends on the aircraft type and the nature of the failure (low fluid, pump failure, system pressure loss). Common steps include:

  1. Identify and confirm: Use the cockpit indications (pressure gauges, fluid quantity, ECAM/EICAS messages) to determine which system has failed and whether it is a leak (fluid loss) or pump failure (pressure loss with stable fluid). Leaks are more dangerous because they can lead to total loss if fluid empties.
  2. Isolate the failure: Shut down the affected pump or isolate the system via valve closures to prevent fluid cross-feed and preserve remaining fluid.
  3. Activate backup systems: Engage PTUs, electric pumps, or deploy the RAT as needed. On aircraft like the A320, the crew may press the “Yellow pump” or “Blue pump” pushbutton to bring electric pumps online.
  4. Evaluate landing condition: Calculate landing distance with degraded flaps, no spoilers, and limited braking. The crew must consider the runway length, wind, and weight. They may request a longer or wider runway or divert to an airport with better facilities.
  5. Perform abnormal checklists: For total hydraulic failure, the QRH (Quick Reference Handbook) provides instructions for manual gear extension, flap setting, and brake usage. The crew will brief a landing “hot” (faster) and may plan to use maximum reverse thrust (if available) and aerodynamic braking.
  6. Execute a safe landing: The landing flare may be more difficult due to reduced control authority. The pilot flying must avoid hard touchdowns that could cause structural damage or bounce. After touchdown, braking is carefully modulated to avoid wheel lock-ups.

Maintenance and Prevention

Preventative maintenance is the first line of defense against hydraulic failures. Airlines follow rigorous schedules for:

  • Fluid sampling and analysis: Periodic testing detects contamination and chemical breakdown.
  • Hose and seal inspection: Replacements are performed at intervals determined by flight hours or cycles.
  • Filter replacement: High-pressure filters are changed regularly to keep fluid clean.
  • Component overhaul: Pumps, valves, and actuators are overhauled or replaced at specified life limits.
  • One-time inspections after events: After a sudden pump failure or pressure spike, the entire system is flushed and inspected for debris.

Despite the best maintenance, failures still occur due to factors like manufacturing defects, foreign object damage, or extreme operating conditions. The key is that crews are prepared to handle them.

Real-World Case Studies

Several notable incidents illustrate the criticality of hydraulic failures:

  • United Airlines Flight 232 (1989): A catastrophic engine failure severed all three hydraulic lines on a McDonnell Douglas DC-10. The crew lost almost all flight control, using differential thrust to maneuver and crash-land at Sioux City with 185 survivors. This accident drove improvements in hydraulic system separation and flight control design.
  • Air Canada Flight 624 (2015): A hydraulic failure during landing at Halifax caused the aircraft to touch down hard, and a series of subsequent events led to a runway excursion. While no fatalities occurred, the incident highlighted the importance of approach speed management with degraded systems.
  • Qantas Flight 7 (2002): A Boeing 747 experienced a hydraulic fluid leak from a failed pump coupling. The crew diverted to Bangkok and landed with one of four hydraulic systems disabled. The aircraft suffered a brake fire after landing due to a overheated brake unit, later linked to a fault in the brake hydraulic control valve.

These events demonstrate that even with redundancy, hydraulic failures can cascade if not properly managed.

Conclusion

Hydraulic system failures represent one of the most challenging emergencies an aircrew can face during takeoff and landing. The loss of normal braking, flap control, or flight control authority demands quick thinking, extensive knowledge of backup systems, and precise technique. Modern aircraft are designed with multiple layers of redundancy, but the human element—pilot training, judgement, and crew coordination—remains the ultimate safety net. By understanding the impacts of hydraulic failures and continuously improving maintenance and training standards, the aviation industry ensures that even when hydraulic power is lost, safe outcomes remain achievable.

For further reading, refer to FAA Advisory Circulars on Hydraulic Systems, Boeing Aero Magazine articles on system design, and NTSB accident reports for detailed case studies.