Staudruckturbine (RAT: Ram Air Turbine)

What Is a Ram Air Turbine?

A Ram Air Turbine (RAT) is a small, retractable turbine connected to a hydraulic pump and/or electrical generator that deploys automatically — or manually — into the airstream when an aircraft suffers a complete loss of normal hydraulic and electrical power generation. The device exploits ram air pressure created by the aircraft's forward velocity to drive its propeller, converting kinetic energy into hydraulic and electrical power with no dependency on engines, APU, or batteries.

How It Works

The RAT is housed in a retracted position in the fuselage belly or wing root, held in place by a deployment mechanism. Upon detection of total loss of normal power generation — or manual crew selection — a spring-loaded actuator or pyrotechnic release deploys the RAT into the airstream in approximately 8 seconds. The propeller diameter is typically 50–100 cm (20–39 in) depending on aircraft size.

Once deployed and stabilized at 100% RPM, the RAT drives a hydraulic pump producing approximately 25 bar (365 psi) at reduced flow rate, adequate to power essential hydraulic system functions — primarily flight control actuators. Many modern RATs also drive an integrated permanent magnet generator producing 5–15 kVA, feeding the essential electrical bus with power for flight instruments, radio, and flight control computers.

RAT activation is among the most dramatic events in aviation emergencies. On January 15, 2009, US Airways Flight 1549 (Airbus A320 on the Hudson River) deployed its RAT after dual engine loss, providing the hydraulic and electrical power that enabled Captain Sullenberger to control the aircraft to a successful water landing. The event remains the most publicly documented RAT deployment in commercial aviation history.

Key Components

• Propeller Assembly: Two- or three-blade variable or fixed pitch propeller; sized for adequate power at minimum deployment speed (typically 140–160 knots).
• Hydraulic Pump: Provides 25–35 bar output to essential hydraulic circuit; flow rate 10–20 L/min (2.6–5.3 gal/min).
• Generator: Permanent magnet alternator producing 115V AC or 28V DC for essential bus; output 5–15 kVA.
• Deployment Mechanism: Spring-loaded with hydraulic or pyrotechnic release; automatic on most modern aircraft.
• Speed Governor: Maintains constant propeller RPM across varying airspeeds to regulate output power.

Aircraft Examples

• Airbus A320: Ventral RAT producing 5 kVA electrical and hydraulic output; powers Blue hydraulic system and AC essential bus; deployed during Flight 1549 (2009).
• Boeing 787-9: RAT produces 10 kVA electrical output to support more-electric architecture essential bus; no separate hydraulic RAT output.
• Airbus A380: Two RATs (one per wing) each producing 70 kVA — largest commercial RAT installation, reflecting the A380's four-engine electrical generation requirement.
• Airbus A350: Single ventral RAT producing 100 kVA, the highest output of any single commercial RAT, supporting the aircraft's extensive fly-by-wire and avionics loads.