Aircraft Electrical Systems: A320/A321 Briefing

 

Understanding the A320 Dual Hydraulic Failure: Blue + Yellow Systems

Introduction: A Failure That's Less Scary Than It Sounds

To an aspiring pilot, losing two of the three hydraulic systems on an aircraft like the Airbus A320 "sounds catastrophic." After all, hydraulics power nearly every critical system, from flight controls to brakes.

However, in the specific case of a combined Blue and Yellow system failure, the reality is far less dire. The A320 is designed with such intelligent redundancy that the aircraft remains safe, fully controllable, and—most importantly—retains its highest level of flight law protection, Normal Law.

This document will explain why this is possible by breaking down the A320's intelligent design and the key reasons you remain in complete control.

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1. The Two Reasons You Remain in Complete Control

The aircraft's safety in this scenario isn't a matter of luck; it is the direct result of deliberate and robust engineering design. There are two primary reasons why the flight control computers maintain full authority.

1. The Green System Survives The Green hydraulic system is the most powerful of the three. By design, it powers the complete set of surfaces required for safe flight:
   • Ailerons
   • Elevators
   • Rudder
   • Spoilers
   • Stabilizer
2. Smart Electrical Backups The Airbus architecture is intelligent. Key components that would normally rely on Blue or Yellow hydraulics, such as the RAT-driven Blue pump or Yellow's role in providing alternate brakes, have their own redundancies. The flight control computers are aware of this design and know that the Green system can handle the essential flying tasks alone, which is why they do not degrade the flight laws.

Because these core flight controls are still powered, the aircraft can provide the pilot with its full suite of safety protections.

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2. The Biggest Relief: Staying in Normal Law

The most significant benefit of this design is that the aircraft remains in Normal Law. Because the flight control computers have full authority via the Green system, they do not need to downgrade to the less-protected Alternate or Direct Laws.

For the pilot, the key takeaway is simple and stress-reducing: You don’t need to relearn how to fly the jet.

You continue to benefit from the critical flight envelope protections that make the Airbus so safe. These protections remain active:

• High Angle-of-Attack Protection Prevents the aircraft from entering an aerodynamic stall.
• Load Factor Limitation Protects the airframe from being over-stressed by excessive G-forces.
• Bank Angle Protection Keeps the aircraft from banking too steeply, ensuring controlled flight.
• Overspeed Protections Helps prevent the aircraft from exceeding its maximum safe airspeed.

While the core flying remains protected, the failure does have significant consequences for other systems that you must now manage.

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3. Managing the Consequences: What You've Lost

With the aircraft flying normally, the pilot's job shifts from control to management. You must now account for the systems that are no longer available due to the loss of the Blue and Yellow hydraulics.

Lost  System/Capability	What This Means for the Pilot
Thrust Reversers	They are inoperative. You must reassess your landing distance and rely only on brakes and spoilers.
Normal Brakes/Anti-Skid	Normal braking and anti-skid are lost. You are left with a limited number of braking applications powered by accumulator pressure from the Green system.
Nosewheel Steering	It is completely lost. You will need assistance from a tug on the ground as taxiing will be difficult.
Cargo Doors	They are inoperative. This is a concern for ground operations after landing.

Why There’s No Backup Pressure

It is also important to note that the electric pumps for both the Blue and Yellow systems are inoperative in this failure. This means that other components, such as the Power Transfer Unit (PTU), cannot be used to restore pressure to the failed systems.

With these systems in mind, the pilot's focus shifts from worrying about control to planning a safe landing and preparing for what happens on the ground.

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4. Your New Mission: Fly, Plan, Prepare

In this scenario, the pilot's responsibilities can be distilled into a new, three-step mission.

1. Fly the Airplane Thanks to the protections of Normal Law, this is the easy part. The aircraft will handle just as you expect it to.
2. Plan Your Landing You must plan for a longer landing roll. This involves recalculating your landing distance without thrust reversers and with reduced braking performance.
3. Prepare for the Ground Be ready for limited ground handling. With no nosewheel steering, you will need to coordinate with ground crews for a tug to move the aircraft off the runway.

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5. Conclusion: The Airbus Philosophy in Action

A Blue and Yellow dual hydraulic failure is a serious event, but it is one the A320 is specifically designed to handle safely and logically. The aircraft’s systems ensure that even with two-thirds of the hydraulics gone, flight control is never compromised.

This scenario is a perfect example of the Airbus design philosophy, which is focused on "keeping the pilot focused on flying, not fighting" the aircraft.

It highlights the importance of understanding not just what fails, but why the aircraft's design still protects you.