Jump to content
Sign in to follow this  
Guest bobsk8

APU Usage

Recommended Posts

Guest Claude troncy

Bonjour,As you I am not clear of the use of APU during TO.It seems that when maximum power is needed for takeoff, only the left pack is used and air is provided by the APU not by the engines.Claude Troncy

Share this post


Link to post
Guest boxjockey99

ClaudeThis is termed a 'bleeds off takeoff' and is only normally used when you are heavily loaded or on a short runway which limits your performance. The APU is used to supply air to one pack and the engine bleed valves are turned off. Once airborne the system is returned to normal ensuring that you never back pressure the APU as this has pretty disastrous affects on the APU. Hope that helpskris

Share this post


Link to post

"When saying which pack to choose I meant on the ground! Ie do we let the pax keep cool or do we keep all the cold air for ourselves in the pointy end!"Sorry, Kris... I'm still confused...On the ground or in the air, if both packs feed a mix manifold on the Classic (presumably they do) and this cool air is distributed evenly throughout the cockpit and cabin, why should turning off one pack affect only one area. Surely with one pack off, you would get 1/2 cooling in all areas, rather than proper cooling in one area (e.g. cockpit) and no cooling in another? I'm not familiar with the shape of the mix manifold on a 737... Perhaps the Classic mix manifold is shaped in such a way that one pack favours the cockpit? (In schematics it is always depicted as a box, rather than a complex shape). The 737 NG mix manifold is sort of (upright) cyclindrical shaped with the cool air going into the manifold from 3 directions (2 packs and a ground connection) at the bottom of the cylinder and cold air going out at the top through pipes to the various areas.Hope this makes sense.Cheers.Ian.

Share this post


Link to post
Guest boxjockey99

IanOn the Classic there is a direct feed from the Left pack to the cockpit via the cockpit supply duct so effectively 50% of the air created by the Left pack goes direct to the pointy end, the other 50% goes to the mix manifold and is distributed accordingly. Thus by tuirning on the Left pack we get nice cold air almost straight from the pack without any warming affect from the mix manifold. If the right pack is used the mix manifold gets the lot and we just get the remnants of what is not used in the cabin as the flight deck outlet is pretty much at the end of the distribution ducting as I understand it. As to its physical shape we leave that to the gingers and live in our own little happy world also known as Bill Bulfers guideKris

Share this post


Link to post
Guest tcable

There is a bit of airline procedure involved, but too certain conditions require the apu- ETOPS in a 737 requires the apu to be running (ask Aloha Airlines- they fly 737's from Hawaii to the mainland- the longest scheduled ETOPS 737 flights)Or on the ERJ the APU is running for every takoff and landing- at least on American Eagle. Gives more margin in case of an engine faliure or goaround.Tim

Share this post


Link to post
Guest boxjockey99

TimAs a non-ETOPS operator I wouldn't know the exact procedures however I would suspect that for ETOPS the APU must be 'servicable' rather than operating for the entire flight. Running the APU for a long range flight would accomplish only one thing... turning valuable fuel into noise as the APU would not be running any services during the normal operation of the aircraft, only in an emergency or malfunction would the APU be required to provide electrical and/or bleed airBear in mind that even the NG APU is limited to the following:For 2 bus electrical power ONLY max alt 41,000ftFor 1 pack OR both transfer buses max alt 17,000ftFor 1 pack AND both transfer buses max alt 10,000ftAPU start is not guaranteed above 25,000ft on the classic (not sure about the NG)So you see running the APU would simply waste fuel for a long cruise.in the case of operating the APU during takeoff and landing, this is a prudent course of action and I suspect it is a protective comapany procedure rather than an aircraft requirement (although I have no experience with the ERJ). We operate the B737 calssic with the APU on for take off and landing in poor weather but there is no need in fair weather as you are less reliant on the instruments and more on the mk.1 Eyeball. In poor weather if you lost a gen you COULD lose a bus if the bus transfer system doesn't work correctly, by having the APU on and running it means you can very rapidly restore the inoperable bus by moving one switch allowing you to concentrate on flying the aircraft.Hope that muddy's the waters some more, you can't beat a good discussion on trivia!Kris

Share this post


Link to post
Guest Murf

Hi Bob,I realize you were probably looking for APU info during takeoff and you have a fair number of good responses in that area on this thread already.One other area where the APU is used in flight is during ETOPS operations. Most airlines (or perhaps it's an ETOPS requirement) must have the APU running during ETOPS flights. The following is an excerpt from an AOM.APU UsageAn operational APU is required for ETOPS flights. Due to the limited time thatthe B737 can operate on standby electrical power it is required that the APU berun continuously in ETOPS airspace. The policy for ETOPS flights is to startthe APU on the ground and leave it running until past the Extended Range ExitPoint (C-EXP). Continued operation of the APU beyond the C-EXP is at thediscretion of the Captain.Operation of the APU on non-ETOPS LRN flights is not required.

Share this post


Link to post
Guest boxjockey99

I stand corrected! This must be due to the lack of a RAT on the B737Kris

Share this post


Link to post
Guest sj3

Geez, I am eating this up!!!! Thanks to everyone for the wonderful detail. Here's one vote for this level of discussion.Sorry to be dense, but what is ETOPS??Thanks so much.

Share this post


Link to post
Guest boxjockey99

ETOPS = Extended Twin OPerationSAlso known as EROPS - Engines Refuse to Operate Please Swim (or rather Extended Range OPerationS)Kris

Share this post


Link to post
Guest Murf

"Also known as EROPS - Engines Refuse to Operate Please Swim (or rather Extended Range OPerationS)":-) Hadn't heard that one before.

Share this post


Link to post

Just to add,Whether to leave the APU running after engine's start or shut it down is usually covered by Company SOP. An airline that fly's high cycles everyday (multiple short flights) may tell it's pilots to leave the APU running for the entire flight to reduce APU maintanence problems usually caused from to many APU starts. This use to be a problem on the -200 and may be better now.The reason to do this is the fuel used by the APU might cost less than the cost to replace an APU. Then, when the price of fuel goes way up, the company may say to turn off the APU's after starting engines to save fuel. The fuel savings is now more than the cost to replace APU's.Anyway, the Captain has the final say in the operation of the APU. Ya see?Floyd

Share this post


Link to post
Guest bobsk8

I would like to thank everyone for the answers on my APU question. I feel that I know alot more about the APU know than I did before.

Share this post


Link to post
Guest moose_VUSN

looking through the PMDG 737NG options ini file there is an optio that does not appear in the FS menu, that is:[FuelFlow]APUConsumeFuel=0wondering if anyone else has tried this. i've not yet done so.any response from PMDG as to why its not in the menu within FS?

Share this post


Link to post
Guest cf_sas

I think APU fuel consumtion was disabled in one of the previous updates in order to deal with some kind of problem. A search on this forum will give you the answer.

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
×
×
  • Create New...