737 MAX Hydraulics Questions

How many independent hydraulic systems does the aircraft generally have, and what are they commonly designated?

• Three: System A, System B, and a Standby system
• Two: a single main system and a backup electric system only
• Four fully independent systems, one per major flight control surface
• One central system with no independent backup

Explanation: The type generally uses three hydraulic systems - System A and System B as the two main systems, plus a Standby system that can be brought online to back up specific functions if A or B is lost.

Hydraulics - General/Description

How are hydraulic Systems A and B each normally pressurized?

• Each is normally pressurized by its own engine-driven pump, backed up by an electric pump on the same system
• Both systems share a single engine-driven pump between them
• Only electric pumps are used, with no engine-driven pumps at all
• Compressed bleed air alone pressurizes both systems

Explanation: System A is normally driven by an engine-driven pump on one engine with an electric pump as backup, and System B mirrors that arrangement on the other engine - giving each main system both an engine-driven and an electric pressure source.

Hydraulics - System A & B Pumps

If normal hydraulic gear extension is unavailable, how is the landing gear generally extended as a backup?

• Via an alternate extension method that releases the uplocks and allows the gear to free-fall into place under gravity and airloads, independent of hydraulic pressure
• The Standby hydraulic system always takes over gear extension automatically
• The aircraft cannot land until hydraulic pressure is restored
• The APU is used to hydraulically drive the gear down

Explanation: The alternate gear extension method is deliberately independent of any hydraulic system - releasing the mechanical uplocks lets gravity and aerodynamic loads drive the gear down and into its locked position, so gear extension remains available even with a total hydraulic failure.

Hydraulics - Landing Gear Alternate Extension

Why is aircraft hydraulic fluid generally a fire-resistant synthetic fluid rather than a simple petroleum-based oil?

• Because hydraulic lines run through areas with hot surfaces and potential ignition sources, so a fluid resistant to catching fire reduces that risk if a leak occurs
• Because synthetic fluid is significantly cheaper to manufacture than petroleum-based oil
• Because fire resistance has no bearing on the choice of hydraulic fluid
• Because synthetic fluid is required only for aesthetic (color) reasons

Explanation: Hydraulic lines and components are routed through areas that can include hot engine or APU surfaces, so using a fire-resistant synthetic fluid (rather than a more flammable petroleum-based oil) reduces the risk that a hydraulic leak turns into a fire hazard.

Hydraulics - Hydraulic Fluid Properties

Why are the three hydraulic systems kept physically separate, with their own reservoirs and fluid lines, rather than sharing one common reservoir?

• So that a fluid leak in one system doesn't drain fluid from (and disable) the other systems as well, keeping the loss contained to just the affected system
• Sharing a single reservoir would make the aircraft lighter with no downside
• Physical separation is done purely for ease of manufacturing, with no safety benefit
• It is required only so each system can use a different color of fluid

Explanation: Keeping each hydraulic system's reservoir and fluid lines physically separate from the others means a leak in one system can only drain that system's own fluid - it can't also drain the other systems - which is exactly the kind of containment that keeps a single leak from turning into a total hydraulic failure.

Hydraulics - System Separation

If the crew observes a hydraulic system's fluid quantity steadily decreasing in flight, what is a generally sensible precaution regarding use of that system?

• Minimize unnecessary use of hydraulically-powered functions on that system to conserve the remaining fluid for functions that will still be needed later, such as landing gear or flight controls
• Immediately shut down both engines
• Continue using every hydraulically-powered function exactly as normal, since fluid quantity has no operational significance
• Manually vent the remaining fluid overboard to prevent a fire

Explanation: A steadily decreasing quantity points to an ongoing leak, so a sensible precaution is to reduce non-essential use of that system's hydraulically-powered functions, conserving whatever fluid remains for functions - like extending the landing gear or operating the flight controls - that will still be needed later in the flight.

Hydraulics - Low Quantity Precautions

What is the general concept behind a hydraulic priority valve protecting a critical function?

• If system pressure or fluid quantity drops low, the priority valve automatically reduces or cuts off flow to a less critical function so that pressure is preserved for a higher-priority one, such as flight controls or gear braking
• It always shuts down the highest-priority function first to save fluid
• It has no effect on how fluid is distributed among functions
• It only operates while the aircraft is on the ground

Explanation: A priority valve is a way of automatically rationing a limited hydraulic supply: if pressure or quantity falls into a degraded range, it cuts back or isolates flow to a lower-priority user (such as ground-only equipment) so that a higher-priority, safety-critical function keeps getting the fluid it needs.

Hydraulics - Priority Valve Concept

What is a general ground-operations use for an electric motor-driven hydraulic pump, separate from its role as an inflight backup?

• It can pressurize a hydraulic system for maintenance checks or to operate certain systems (such as flight controls) while the aircraft is parked with engines shut down
• It can only be used while both engines are running at high power
• It exists solely to charge the aircraft battery
• It has no ground-operations use of any kind

Explanation: Beyond backing up engine-driven pump output in flight, an electric hydraulic pump is also useful on the ground - letting maintenance personnel pressurize a system to check or exercise flight controls or other hydraulically-powered equipment without needing an engine running.

Hydraulics - Motor-Driven Pump Ground Use