A Ram Air Turbine, often called a RAT, is a small emergency turbine that deploys into the airstream when an aircraft loses normal power. It acts like a compact wind turbine, using the aircraft’s forward motion to spin blades and drive a generator or hydraulic pump. This matters because large aircraft rely on electrical and hydraulic systems for flight controls, instruments, and communication.
The RAT provides a last-resort source of power that can help pilots keep control during serious failures.
Understanding Aviation: The Ram Air Turbine
A RAT is designed around a simple fact of flight. An aircraft moving through still air experiences a strong flow of air relative to its body. When the normal electrical supply or hydraulic pressure is lost, an automatic system may release the RAT from a compartment in the fuselage or wing root.
Some aircraft allow the crew to deploy it manually. A spring, actuator, or gravity starts the extension, then airflow takes over.
The blades must unfold and reach useful speed quickly. This process is carefully tested because emergency equipment must work after long periods without use, in cold air, rain, vibration, or icing conditions.
The amount of useful energy depends heavily on airspeed. Dynamic pressure equals one half times air density times airspeed squared. This describes how strongly the moving air pushes on surfaces.
The power available to a turbine equals one half times air density times turbine area times airspeed cubed. The cubed relationship is important. A modest increase in airspeed can produce much more turbine power.
At low speed, the RAT may provide limited output. During a descent at higher speed, it can produce more. Designers must choose blade size and shape carefully, since a larger turbine creates more drag and takes up more space.
A RAT does not normally restore every system on the aircraft. Its job is to support the systems needed to keep the aircraft controllable and navigable until a safe landing is possible. On one design, the turbine drives a hydraulic pump that pressurises a flight control system.
On another, it turns a generator for selected electrical buses. Essential loads may include parts of the cockpit displays, radio equipment, navigation equipment, control computers, and fuel or engine control functions.
Engineers decide these priorities in advance. They separate essential circuits from nonessential equipment so that limited emergency power is not wasted on cabin services or other high demand loads.
Students should notice that a RAT is an example of energy conversion with real limits. Kinetic energy in moving air becomes shaft rotation, then hydraulic pressure or electricity. Each step loses some energy through blade drag, bearing friction, heat, vibration, and generator losses.
Output power equals efficiency times input power, so the useful output is always below the ideal airflow power. The device also creates drag, which can reduce speed or increase the descent rate when engine thrust is unavailable. This is an engineering tradeoff.
The RAT gives up some aircraft performance to preserve control. It shows why aircraft safety uses layers of protection rather than relying on one power source.
Key Facts
- A Ram Air Turbine converts airflow energy into mechanical rotation during an emergency.
- Dynamic pressure is q = 1/2 rho v^2, where rho is air density and v is airspeed.
- Ideal wind power through a turbine area is P = 1/2 rho A v^3.
- Actual RAT power is lower than ideal because of efficiency losses: Pout = eta Pin.
- RAT output increases strongly with airspeed because available airflow power depends on v^3.
- A RAT may power an electric generator, a hydraulic pump, or both depending on aircraft design.
Vocabulary
- Ram Air Turbine
- A deployable emergency turbine that uses the aircraft’s forward motion through air to produce hydraulic or electrical power.
- Dynamic Pressure
- The pressure associated with moving air, calculated as q = 1/2 rho v^2.
- Hydraulic Power
- Power transmitted by pressurized fluid to move aircraft components such as control surfaces or landing gear.
- Generator
- A machine that converts mechanical rotation into electrical energy.
- Airspeed
- The speed of an aircraft relative to the surrounding air.
Common Mistakes to Avoid
- Thinking the RAT is a normal engine, which is wrong because it does not burn fuel or produce thrust for flight.
- Assuming the RAT works while the aircraft is parked, which is wrong because it needs strong airflow from forward motion to spin effectively.
- Forgetting that power depends on v^3, which is wrong because doubling airspeed can greatly increase available turbine power.
- Treating RAT power as unlimited, which is wrong because it only supplies essential emergency systems and has efficiency and size limits.
Practice Questions
- 1 A RAT has a swept area of 0.40 m^2 and the aircraft is moving at 90 m/s through air with density 1.0 kg/m^3. Estimate the ideal airflow power using P = 1/2 rho A v^3.
- 2 If the ideal airflow power into a RAT is 120 kW and its overall efficiency is 35 percent, what useful output power does it provide?
- 3 Explain why a Ram Air Turbine is useful after engine failure but cannot replace the full electrical and hydraulic power normally supplied by the aircraft engines.