Secondary Power Systems Auxiliary Power Units/Emergency Power Units (APU/EPU) Ram-Air Turbines (RAT) Batteries

Auxiliary Power Units Useful before flight Enables aircraft to be independent of ground power Generally located in the tail of the aircraft Useful during flight Provides power in case of engine failure Very important in modern fly-by-wire aircraft

APU Manufacturers and Weights Three main manufacturers: –Honeywell International Inc –Pratt & Whitney Canada –Hamilton Sundstrand Boeing 777 uses Honeywell which weighs 730lbs dry Boeing uses PW901 which weighs 835lbs dry MD-90 and A320 use Honeywell models that range in weight from lbs dry

APU Design Approaches Pneumatic Link Location flexibility Decrease in overall efficiency Electrical Link Location flexibility & increased system integration Increased startup requirements Mechanical Link Limited Positioning High Efficiency Hydraulic Link Location flexibility Complex operation and ground startup Hybrid A combination of all these

Ram-Air Turbines Deployed in power system failure An air-driven turbine for power generation Relatively simple design Low power output Does not require stored power for startup

Batteries Power storage medium Batteries provide emergency power between complete main system failure and backup system startup Provides power to startup secondary power systems

References Pallett, EHJ. Aircraft Electrical Systems. John Wiley & Sons, Inc., New York rd Edition. Secondary Power Systems. Mechanical Engineering Publications Limited, London Raymer, Daniel P. Aircraft Design: A Conceptual Approach. AIAA, Reston, VA th Edition Fielding, John P. Introduction to Aircraft Design. Cambridge University Press, New York Moir, Ian and Seabridge, Allan. Aircraft Systems: Mechanical, Electrical, and Avioncs Subsystems Integration. AIAA, Reston, VA. 2001