Ram Air Turbine (RAT)
A ram air turbine (RAT) is installed on many transport-category aircraft to provide an emergency source of hydraulic and/or electrical power if the primary power systems are lost. A RAT is typically used after the loss of all engine-driven generators, hydraulic pumps, or both. Once deployed into the airflow, the RAT uses ram air to rotate a small turbine. The rotating turbine drives a hydraulic pump, an electrical generator, or a combined hydraulic pump and generator depending on the aircraft design.
The RAT is usually stowed inside the fuselage or wing root behind a spring-loaded or hydraulically actuated door. When activated, the door opens and the RAT extends into the slipstream. On some aircraft, the flight crew can deploy the RAT manually by using a cockpit control or emergency handle. On others, the RAT deploys automatically whenever the aircraft loses all normal electrical power, all hydraulic pressure in a critical system, or both.
The amount of power produced by the RAT depends on aircraft speed because the turbine is driven by the airflow. At higher airspeeds, the RAT can supply enough power to operate essential flight controls, selected hydraulic systems, and critical electrical equipment such as flight instruments, radios, and flight control computers. At lower airspeeds, the available output is reduced.
Many modern aircraft use the RAT only to power an emergency hydraulic system, while others use it to power both a hydraulic pump and an emergency electrical generator. For example, some aircraft use the hydraulic pressure produced by the RAT to operate essential flight controls and then use a hydraulic motor-driven generator to create electrical power.
 |
| Ram air turbine |
Hydraulic Motor-Driven Generator (HMDG)
A hydraulic motor-driven generator (HMDG) is a device that converts hydraulic power into electrical power. The HMDG consists of a servo-controlled variable-displacement hydraulic motor connected directly to an AC generator. Hydraulic pressure from the aircraft hydraulic system or from an emergency RAT-driven hydraulic pump turns the motor, which rotates the generator.
The HMDG is designed to maintain a constant generator output frequency of 400 Hz, regardless of changes in hydraulic pressure or motor speed. This is accomplished by the variable-displacement motor and its servo control system, which automatically adjust motor displacement to keep generator speed constant.
In the event of an electrical failure, the HMDG can provide an alternate source of electrical power for essential aircraft systems. Typical loads powered by the HMDG include essential flight instruments, navigation and communication equipment, flight control computers, and selected cockpit displays. Because the HMDG depends on hydraulic pressure, it is often used in combination with a RAT to provide emergency electrical power after total engine or generator failure.