Plane Sense 3 March_FinanceNov 19/02/2013 12:43 Page 1
Plane Sense on Engines
APUs: The Little Engines That Do...
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WORLD AIRCRAFT SALES MAGAZINE – March 2013
www.AvBuyer.com
many other reasons, operators with the option will choose to equip their business aircraft with an Auxiliary Power Unit (APU). Think of this as the little jet engine that could (and does), roaring along out of sight from the un-pressurized space near the airplane’s tail.
APU BASICS The Auxiliary Power Unit is that jet engine you often hear while crossing a ramp; with no aircraft propulsion powerplants running they stand out for their decibel output – often in excess of idling jet engines. In reality the noise you hear is a combination of airsuction noise from the APU’s inlet passage, combined with the sound of hot, fast-rushing exhaust air making the short trip through a two-stage turbojet engine. Instead of providing motive thrust, however, the jet exhaust turns a turbine wheel from which all other power flows. Consider the following itemized list of APU Aircraft Index see Page 4
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usiness aircraft do not work by powerplants alone. Beyond propulsive power, modern business-turbine aircraft need other forms of motive force: electricity – usually in multiple forms; fresh air flow to keep cool; air pressure to start main powerplants, and stand-by power at altitude (should something befall the main sources of electrical power). Much of the time powerplants fulfill all these needs simultaneous to propelling the airplane forward. Sometimes, however, the powerplants need help – or an alternative of lower costs – in fulfilling those non-propulsive roles. Batteries can make up some of the electrical needs (both aloft and grounded) as well as power fans to circulate air. But batteries are finite power sources - without concomitant replenishing they run out of juice; no engine generator or alternator, no replenishing. For engines with air-starting, ground sources do not always exist. For these and