A320 RAT Deployment: The Engineering Marvel Saving Dead-Stick Flights

Last Updated: 3 days ago

The sheer silence of a dual-engine failure at 35,000 feet is something no pilot ever wants to experience. The deep, resonant hum of the CFM56 or V2500 engines fades, leaving only the rushing sound of the wind against the windshield. All primary electrical generators drop offline, and the glass cockpit screens flicker and die. At this exact moment, survival depends entirely on a successful A320 RAT deployment. Without it, the aircraft transitions from a highly computerized marvel of modern aviation into an 80-ton glider with zero mechanical authority.

This is where the Ram Air Turbine (RAT) enters the stage. It acts as the absolute ultimate backup, a mechanical guardian angel stowed quietly within the belly of the aircraft, just forward of the left main landing gear bay.

The Anatomy of an A320 RAT Deployment

Let’s dissect the hardware. The A320 RAT is a twin-bladed wind turbine, measuring roughly 80 centimeters (31.5 inches) in diameter. When the aircraft computers sense a total loss of power to AC BUS 1 and AC BUS 2 in flight, and the airspeed is above 100 knots, the A320 RAT deployment sequence triggers automatically. The bay doors snap open, and spring tension forces the turbine out into the rushing slipstream.

It takes exactly 8 seconds from the absolute loss of power for the RAT to fully deploy, spin up, and come online. During those agonizing 8 seconds, the aircraft relies entirely on its two main 24V, 23Ah Ni-Cd batteries to keep the most critical systems alive. Once the turbine bites into the rushing air, it spins at a ferocious speed, converting kinetic energy into vital hydraulic and electrical lifeblood.

Pumping Life Back In: The Blue Hydraulic Network

The true genius of the Airbus emergency architecture lies in its integration with the aircraft’s triple-redundant hydraulic system. The RAT does not spin an electrical generator directly. It drives a hydraulic pump dedicated exclusively to the Blue Hydraulic System, pressurizing the fluid up to the standard 3,000 PSI.

Why the Blue system? The A320 relies on Green, Blue, and Yellow systems to move its flight surfaces. As we discussed in our detailed breakdown of the A320 Fly-By-Wire system architecture, the Blue network acts as the critical bridge for emergency control. With it pressurized, pilots regain mechanical authority over crucial flight control surfaces:

• Elevators (Left and Right)
• Ailerons
• Rudder
• Spoiler 3

This setup restores basic aerodynamic control, allowing the flight crew to pitch, roll, and yaw the aircraft safely toward a diversion airport.

Electrical Power Generation: The CSM/G

Pressurizing the hydraulics is only half the battle. The pilots need their displays to navigate, and their radios to communicate with air traffic control. This is achieved through a brilliant component known as the Constant Speed Motor/Generator (CSM/G).

The newly pressurized Blue hydraulic fluid is routed through the CSM/G, which acts as a hydraulic motor spinning a dedicated emergency electrical generator. This ingenious setup ensures that even if the aircraft’s airspeed fluctuates wildly during descent, the CSM/G output remains perfectly stable. It delivers a steady 5 kVA of 115/200V, 3-phase, 400Hz AC power.

This 5 kVA output is a tiny fraction of the 90 kVA provided by a standard Engine Driven Generator (IDG). Yet, it is precisely enough to power the AC Essential Bus and DC Essential Bus. The captain’s Primary Flight Display (PFD) and Navigation Display (ND) flicker back to life, along with VHF 1 for critical radio communication.

Flight Control Degradation: Surviving in Alternate Law

Flying on emergency power fundamentally changes the very nature of the A320. The sophisticated fly-by-wire system, normally operating in “Normal Law” with full flight envelope protection, degrades instantly.

The aircraft drops into “Alternate Law.” The robust pitch protections—like hard limits for stall and overspeed—are lost. The flight control computers, namely the ELACs (Elevator Aileron Computers) and SECs (Spoiler Elevator Computers), operate on bare-minimum logic. Pilots must rely on raw airmanship, manually managing their speed, pitch, and bank angle without the invisible electronic safety net the Airbus normally provides. For an excellent external resource on handling these degraded states, Airbus Safety First magazine offers invaluable case studies.

Modernizing the Backup: A Generational Shift

Let’s look at how the standard A320 architecture compares with older-generation widebodies and modern aerospace marvels to trace the evolution of emergency power.

In older aircraft, the turbine simply provided hydraulic muscle to physically move heavy steel control cables. The A320 revolutionized this concept by using the hydraulic flow to generate electricity, sustaining the fly-by-wire computers. Newer aircraft have simply scaled this brilliant architecture up to massive proportions.

The Critical Approach Phase

As the crippled airliner descends for an emergency landing, a new physical challenge emerges. The turbine relies entirely on airspeed. As the aircraft slows down on final approach, the kinetic energy in the slipstream decays rapidly. Below 140 knots, the RAT struggles to maintain the 3,000 PSI required by the CSM/G to generate uninterrupted electrical power.

The AC Essential Bus will eventually trip offline, throwing the aircraft back onto the batteries for the final moments before touchdown. The hydraulic pressure will also begin to drop as speed bleeds off, right when the pilots need maximum control authority to flare the aircraft. This demands flawless energy management. The crew must fly the approach faster than normal, bleeding off that crucial airspeed only at the very last second over the runway threshold.

It is a raw, visceral form of flying, a stark contrast to the highly automated cruise just minutes prior. The A320 RAT deployment sits at the absolute center of this survival equation. It is the bridge between disaster and a safe return to earth, an engineering failsafe that remains one of the most fascinating components in modern aviation.

Frequently Asked Questions (FAQ)

1. Can the pilots stow the RAT back into the fuselage during flight?

No. Once the turbine is deployed, either automatically by the computers or manually by the overhead pushbutton, it cannot be retracted in the air. It must be manually restowed by maintenance crews on the ground using a specialized hydraulic hand pump and winch system.

2. What happens if the RAT fails to deploy automatically?

The flight deck overhead panel features an “EMER ELEC PWR” panel. Pilots can press the manual “MAN ON” pushbutton. This bypasses the automatic circuitry and forcefully unlatches the doors, allowing gravity to pull the turbine into the airstream.

3. Does deploying the landing gear affect the RAT’s performance?

Yes, slightly. Because the housing is located near the main landing gear doors, dropping the massive gear struts disrupts the smooth aerodynamic airflow into the turbine blades. This turbulent wake can cause minor fluctuations in hydraulic pressure and electrical frequency, though the system is certified to handle these variations during the final approach phase.

Join the Conversation!

Have you ever dug deep into the systems of the A320, or perhaps experienced a grueling simulator session dealing with a total electrical failure? The sheer workload of managing energy while relying on an A320 RAT deployment is mind-bending! Drop a comment below and let me know your thoughts on Airbus’s emergency architecture, or share your favorite aviation survival story. Don’t forget to share this deep dive with your fellow avgeeks!

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