The Boeing 737 Hydraulic System | Redundancy, Design Philosophy and Why Hydraulics Matter.

Written by Pooleys Ambassador Angela Donnelly.

Behind the scenes, the hydraulic system provides the power that allows many of the aircraft’s most important components to operate. Understanding how it works reveals a considered design philosophy built around one simple priority: keeping the aircraft controllable.

I recently had the opportunity to present on an aircraft system and chose the Boeing 737 hydraulic system. It is one of those subjects that might initially sound quite technical, but once you begin studying how the system is designed and examining the hydraulic schematic, with its network of pumps, reservoirs and control lines, it quickly reveals itself as a remarkably well-designed system. The 737 hydraulic architecture demonstrates how modern aircraft are engineered around a clear objective: keeping the aircraft controllable, even when systems do not operate exactly as expected.

Hydraulic systems provide the power behind many of the aircraft’s most important functions. On smaller aircraft, flight controls can often be operated using mechanical linkages or cables because the aerodynamic forces involved are relatively low. Larger aircraft operate at higher speeds and use significantly larger control surfaces. Moving those surfaces requires greater force, and purely mechanical systems quickly become heavy and inefficient.

Hydraulics solve this problem by allowing force to be generated in one location and transmitted efficiently elsewhere through pressurised fluid. In the Boeing 737, hydraulic power drives flight controls, landing gear, braking systems and high lift devices.

The Boeing 737 uses three hydraulic systems: System A, System B and the Standby system.

Each system has its own pumps, reservoir and hydraulic lines. The systems are physically separated, meaning that a failure in one system is contained and does not automatically affect the others. More importantly, critical flight controls are powered by multiple systems, ensuring redundancy where it matters most.

Both System A and System B have two power sources: an engine driven pump and an electric motor driven pump. The engine driven pump is the primary source during flight, providing both pressure and a higher flow rate. The electric pump acts as an alternate source and is particularly useful during ground operations when the engines are not running.

Both pumps deliver approximately 3000 PSI of hydraulic pressure, providing the capability required to move large aircraft components smoothly and reliably.

Although Systems A and B both contribute to primary flight controls, they support different aircraft functions. A useful way to remember the distinction is that System A supports landing and deceleration, while System B supports lift and configuration.

System A powers the landing gear, ground spoilers and Autopilot A.
System B powers trailing edge flaps, leading edge devices, the yaw damper and Autopilot B.

Because both systems share responsibility for the primary flight controls, the loss of one hydraulic system does not result in loss of aircraft control.

The Standby hydraulic system is smaller than Systems A and B but plays an important safety role.

Rather than powering a wide range of aircraft systems, it supports a small number of critical components:

The rudder receives triple hydraulic redundancy from Systems A, B and the Standby system. This ensures that directional control remains available even during significant hydraulic failures. If System B is lost, the Standby system can also provide hydraulic power to extend the leading edge devices, helping to ensure appropriate high lift protection during the approach.

Another interesting feature of the 737 hydraulic system is the Power Transfer Unit (PTU).

A common misconception is that the PTU transfers hydraulic fluid between systems. In reality, it transfers mechanical power.

If System B loses its engine driven pump, pressure from System A can drive a hydraulic motor within the PTU. This motor then drives a pump on the System B side, restoring pressure without mixing fluids between the systems.

In practice, the PTU primarily assists System B during high demand situations, such as the operation of leading edge devices or during take-off when additional hydraulic flow may be required. Many pilots recognise the PTU not by sight, but by its distinctive sound when it operates.

Hydraulic failures are most commonly caused by downstream leaks, where fluid escapes from the system after the pump. As fluid quantity decreases, the system eventually loses pressure.

A pump failure appears differently. In this case, fluid quantity remains stable but pressure drops. Understanding the difference allows crews to quickly identify the nature of the problem and respond appropriately.

Loss of System A primarily affects landing related systems. Ground spoilers become unavailable and the landing gear must be extended using the manual gravity extension system.

Loss of System B mainly affects configuration systems such as flaps and leading edge devices. Flaps can still be extended using an alternate electric system, although this operates more slowly and increases crew workload.

The Worst Case Scenario

Even the loss of both primary hydraulic systems does not mean loss of control.

The Boeing 737 is not a fly by wire aircraft. Control inputs are transmitted mechanically through cables and linkages. Without hydraulic assistance the aircraft reverts to manual reversion, meaning pilots are effectively flying the aircraft using physical control forces.

Control forces increase significantly and manoeuvres must be gentle, but the aircraft remains flyable. The rudder continues to receive hydraulic assistance through the standby system, preserving directional control.

A System Designed for Resilience

Hydraulic failures in commercial aviation are rare, and the complete loss of all hydraulic systems is extremely unlikely. The system architecture is intentionally designed with multiple layers of redundancy and physical separation to minimise risk.

The Boeing 737 hydraulic system demonstrates how aircraft are engineered with resilience in mind. Even when systems fail, the aircraft remains controllable and predictable, allowing the crew time to manage the situation calmly and methodically.

References:

Boeing 737 Flight Crew Operations Manual (FCOM), ATA 29 – Hydraulic Power
Boeing 737 Quick Reference Handbook (QRH) – Hydraulic System Abnormal Procedures
Standard airline aircraft systems training materials