libmatt.com A Ram Air Turbine (RAT) deploys automatically to provide emergency hydraulic and electrical power by converting the airstream into energy during engine failure, while an Auxiliary Power Unit (APU) is a separate engine that supplies power and air conditioning when the main engines are off. Your choice between RAT and APU depends on the need for emergency backup power versus routine ground and flight support.
Table of Comparison
| Feature | Ram Air Turbine (RAT) | Auxiliary Power Unit (APU) |
|---|---|---|
| Function | Emergency power generator during main engine failure | Provides electrical power and bleed air on ground and during engine start |
| Power Source | Airflow-driven turbine from airstream | Small gas turbine engine |
| Power Output | Limited, emergency use only (typically <10 kW) | Substantial, supports aircraft systems (up to several hundred kW) |
| Operation | Automatically deploys in emergency situations | Manually started by flight crew or automatically on ground |
| Primary Use | Backup to maintain flight controls and essential avionics | Ground power and engine start assistant |
| Weight Impact | Lightweight, minimal drag when stowed | Heavier, integrated into aircraft systems |
| Maintenance | Low maintenance, tested periodically | Regular maintenance required due to mechanical complexity |
| Typical Installation Location | Wing or fuselage recessed bay | Rear fuselage or tail cone area |
Introduction to Ram Air Turbines and Auxiliary Power Units
Ram Air Turbines (RATs) are emergency devices deployed in aircraft to provide critical hydraulic or electrical power by harnessing the airstream during main power failures. Auxiliary Power Units (APUs) serve as independent energy sources that supply electrical power and pneumatic pressure for engine starting and onboard systems, especially when the main engines are inactive. Both systems enhance aircraft operational safety and reliability by ensuring continuous power availability under various flight conditions.
Functional Differences: RAT vs APU
Ram Air Turbine (RAT) functions as an emergency power source by deploying a small turbine into the airstream to generate hydraulic or electrical power during engine failure, ensuring critical systems remain operational. Auxiliary Power Unit (APU) serves as a ground or in-flight power source providing electrical power and pneumatic pressure for engine start, air conditioning, and onboard systems without reliance on main engines. The RAT is primarily a backup mechanism activated only during emergencies, whereas the APU is regularly used for power supply when main engines are off or to supplement power demands.
System Design and Components
Ram Air Turbine (RAT) system design includes a small, deployable wind turbine that harnesses airflow to generate emergency electrical and hydraulic power, featuring components like the turbine blades, gearbox, and generator. Auxiliary Power Unit (APU) incorporates a compact gas turbine engine, fuel system, and electrical generator designed for independent power supply during ground operations and as backup in flight. Your aircraft's reliability depends on understanding the distinct mechanical setups and operational roles of RAT's aerodynamic-driven power source versus APU's onboard fuel-powered engine system.
Operational Scenarios for RAT and APU Deployment
Ram Air Turbines (RATs) deploy automatically in emergency situations such as complete engine failure or electrical system malfunctions to provide essential hydraulic and electrical power, ensuring critical flight controls remain operational. Auxiliary Power Units (APUs) are primarily used on the ground for engine start, air conditioning, and electrical power supply before engine start or during engine shutdown, with some aircraft enabling limited in-flight use for backup power in non-emergency scenarios. The RAT plays a vital role in emergency backup during total power loss, while the APU supports auxiliary functions and non-critical power needs during routine ground and limited in-flight operations.
Energy Sources and Power Output Comparison
Ram Air Turbine (RAT) harnesses kinetic energy from the airstream during flight to generate emergency electrical and hydraulic power, producing typically around 5 to 35 kW depending on aircraft size. Auxiliary Power Units (APU) operate on fuel combustion, delivering significantly higher power output, often between 50 to 250 kW, supporting aircraft systems on the ground and during engine start. The RAT serves as a critical backup driven by aerodynamic forces, while the APU provides continuous, robust power from an onboard fuel source.
Role in Aircraft Emergency Procedures
The Ram Air Turbine (RAT) provides essential emergency hydraulic and electrical power by deploying during total engine or generator failure, ensuring critical systems remain operational. The Auxiliary Power Unit (APU) supplies electrical power and pneumatic pressure primarily on the ground and can serve as a backup power source during in-flight emergencies before RAT deployment. Both systems play distinct roles in aircraft emergency procedures, with the RAT acting as a last-resort power source during extreme failures and the APU supporting operational readiness and redundancy.
Maintenance and Reliability Considerations
Ram Air Turbines (RATs) require minimal maintenance due to their simple mechanical design and are highly reliable as emergency power sources activated only during in-flight power loss. Auxiliary Power Units (APUs) demand regular inspections and servicing to ensure operational readiness, including fuel system checks, lubrication, and electrical component testing, impacting overall maintenance schedules. Your choice between RAT and APU should consider these maintenance demands alongside reliability needs for emergency versus continuous ground power support.
Efficiency and Performance Metrics
Ram Air Turbines (RATs) provide emergency power by harnessing aerodynamic forces during flight, delivering limited but reliable energy to essential systems, primarily measured in terms of immediate power availability and deployment speed. Auxiliary Power Units (APUs) offer sustained power independently of aircraft motion, with performance evaluated by fuel consumption rates, power output capacity, and operational endurance on the ground and during flight. Efficiency metrics for RATs focus on aerodynamic drag and energy conversion under varying airspeeds, while APUs are optimized for fuel efficiency, power generation consistency, and reduced emissions during extended use.
Integration with Modern Aircraft Systems
The Ram Air Turbine (RAT) integrates as an emergency power source in modern aircraft, deploying automatically to provide hydraulic and electrical power during main system failures, ensuring critical flight controls remain operational. In contrast, the Auxiliary Power Unit (APU) is a self-contained engine system that supplies electrical power and bleed air for engine start, air conditioning, and other functions while the aircraft is on the ground or during flight when needed. Modern aircraft systems rely on RAT for backup power during emergencies due to its rapid deployment and independence from onboard fuel, whereas the APU supports normal operations and reduces load on main engines.
Future Developments and Technological Innovations
Future developments in Ram Air Turbine (RAT) technology aim to increase aerodynamic efficiency and power output through advanced materials and improved blade design, enhancing emergency power reliability. Auxiliary Power Units (APUs) are evolving with cleaner, more fuel-efficient turbine engines and integration of hybrid-electric systems to reduce emissions and operational costs. Your aircraft's reliability and sustainability will benefit significantly from these technological innovations, ensuring enhanced performance during critical phases of flight and ground operations.
Ram Air Turbine vs Auxiliary Power Unit Infographic
