The mysterious "third engine" of the CRJ-900.
If you look closely at commercial aircraft, you might notice something that looks a bit like an extra engine. Consider the CRJ-900 for example. It clearly has only two engines, but take a look at the tail. It has an additional jetpipe which surely resembles a third engine.
So what’s the deal? Hold onto your butts: it is a third engine. In fact, it’s a very special kind of engine found on airliners and some corporate jets called an Auxiliary Power Unit, or APU.
The APU is an internal and highly automated powerplant that provides backup power to a number of systems and provides pressurized air for main engine starting.
As you aviation-savvy readers already know, redundancy is the name of the game, and an APU provides plenty. A typical APU spins an electrical generator that is capable of providing electrical power to most (or all) onboard systems in the event of a dual engine or primary generator failure.
Furthermore, bleed air is pulled off the APU’s compressor to provide hot pressurized air for use in air conditioning and pressurization systems.
The APU exhaust of an Air Canada ERJ-190
Under normal circumstances, APU’s are used by flight crews to aid in engine starting and to provide air conditioning for passenger (and crew!) comfort on the ground. Typically, the APU will be shut down at some time before takeoff or as part of the climb checklist, effectively transferring all electrical and air conditioning systems to the main engines.
In the interest of safety, APU’s are heavily isolated in modern aircraft. For example, on my aircraft (the CRJ-200), the APU is enclosed in a fireproof titanium box and features its own semi-automated fire detection and fire extinguishing systems, completely separate from other aircraft systems. In theory, the APU could burn itself to a cinder without endangering the passengers or crew, which is a comforting thought.
You can read more about APU’s at Wikipedia.
Patrick Flannigan is a regional airline pilot and aviation blogger. You can read more of Patrick’s articles at AviationChatter.com.