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04/07/2009, 12:41 AM Aviator|Chris Joined on 28/01/2009 EGBO / Birmingham Posts 301 Few commercial questions... Hi All, Just got back from seeing my fiancee's family in Poland and had a few questions on my mind during the flight. 1.) On the tail of most commercial aircraft, at the very aft tip of the tailplane there appears to be some sort of exhaust. A hole surrounded by a metal protected edge? What is this exactly? Aircraft is 737-800 in this case. 2.) Whats the reason behind turning lights off in the pax cabin during night operations? Is this to aid the view outside for the cabin crew should they see something and need to alert the flight crew? Interesting route around several thunderstorms in Europe tonight, got some great views of 'nimbus clouds flashing away in the distance! Cheers.    Report

04/07/2009, 11:38 AM McGoo Joined on 15/11/2005 Posts 225 Re: Few commercial questions... Hi Chris, the exhaust you spotted is for the APU, this small engine can be run and supplies electricity for lights, instruments etc, also provides air for engine start and air conditioning packs etc    Report

04/07/2009, 12:15 PM extenddownind Joined on 22/02/2009 North Hertfordshire Posts 35 Re: Few commercial questions... From 'Ask the Captain' http://www.usatoday.com/travel/columnist/getline/2004-10-11-ask-the-captain_x.htm Question: Why do you dim the cabin lights before takeoff and landing? Answer: This is a safety consideration. In the event it becomes necessary to evacuate the airplane, your eyes are already adjusted. Otherwise, it takes at least several seconds to go from bright light to being able to see in the dark.    Report

04/07/2009, 2:29 PM Aviator|Chris Joined on 28/01/2009 EGBO / Birmingham Posts 301 Re: Few commercial questions... Thanks for the replies both! All makes perfect sense now, especially when I sit further aft and can hear the sound of the little engine spinning up!    Report

04/07/2009, 2:32 PM Aviator|Chris Joined on 28/01/2009 EGBO / Birmingham Posts 301 Re: Few commercial questions...  McGoo wrote: Hi Chris, the exhaust you spotted is for the APU, this small engine can be run and supplies electricity for lights, instruments etc, also provides air for engine start and air conditioning packs etc Regarding the Aux Power Unit, what type of engine is this? Is it a mini jet-turbine of some sort, and where does it draw air from? The front of the vertical fin section? Or does it vary with different aircraft types? Cheers.    Report

04/07/2009, 6:59 PM McGoo Joined on 15/11/2005 Posts 225 Re: Few commercial questions... It varies with type, the 737-800, the APU inlet is on the starboard side between the rear door and elevator.    Report

05/07/2009, 10:04 AM Aviator|Chris Joined on 28/01/2009 EGBO / Birmingham Posts 301 Re: Few commercial questions... Cheers, McGoo!    Report

20/08/2009, 3:40 PM fokkerjet Joined on 18/01/2009 Posts 3 Re: Few commercial questions... "Regarding the Aux Power Unit, what type of engine is this? Is it a mini jet-turbine of some sort, and where does it draw air from? The front of the vertical fin section? Or does it vary with different aircraft types?" It does vary from type to type but essentially the Apu (Auxilliary Power Unit) is a small gas turbine engine situated in a fire-proof box in the tail. You can see the exhaust outlet and some form of air inlet. It works in just the same way as a jet engine except it is constant speed and has no controls other than on and off. If there is a fire it is detected and usually will be self-estinguished by an on-board monitoring system. The fireproof box it resides in should contain any any fire. The APU will supply AC and DC power and bleed air for air conditioning and engine starting etc. On some aircraft (such as the Fokker 100) the APU will only supply bleed air on the ground- not in the air. Hope this helps.    Report

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