Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

A320 APU Gen loses power with altitude

Tech Log The very best in practical technical discussion on the web

A320 APU Gen loses power with altitude

Old 3rd May 2019, 01:20
  #1 (permalink)  
Thread Starter
 
Join Date: Nov 2004
Location: Here, there, and everywhere
Posts: 716
A320 APU Gen loses power with altitude

"A third AC generator driven directly by the APU is rated at the same power as each engine driven generator and can replace either or both engine generators at any time. However, the maximum power output of the generator decreases with altitude. "

Why?
punkalouver is offline  
Old 3rd May 2019, 02:36
  #2 (permalink)  
 
Join Date: Feb 2019
Location: shiny side up
Posts: 431
Its only a small gas turbine and the reduced air density to maintain a load.

From the FCOM


Last edited by Smythe; 3rd May 2019 at 03:03.
Smythe is offline  
Old 3rd May 2019, 03:18
  #3 (permalink)  
Thread Starter
 
Join Date: Nov 2004
Location: Here, there, and everywhere
Posts: 716
So it is not a generator caused problem but a power unit problem.
punkalouver is offline  
Old 3rd May 2019, 04:14
  #4 (permalink)  
 
Join Date: Feb 2019
Location: shiny side up
Posts: 431
Not a problem, just physics...gas turbine needs air. The gas turbine powers the generator in the APU.
Smythe is offline  
Old 3rd May 2019, 10:35
  #5 (permalink)  
 
Join Date: Jul 2016
Location: Broughton, UK
Posts: 67
Also, am I reading the graph correctly.. You can't restart the APU from the battery, above FL 250.. ?
scifi is offline  
Old 3rd May 2019, 12:16
  #6 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,305
Originally Posted by scifi View Post
Also, am I reading the graph correctly.. You can't restart the APU from the battery, above FL 250.. ?
No you are not reading correctly. This limit is only for APU start in ELECTRICAL EMERGENCY configuration.
vilas is online now  
Old 3rd May 2019, 16:14
  #7 (permalink)  
 
Join Date: Feb 2019
Location: shiny side up
Posts: 431
While this is not an APU turbine, this is an example of the turbine power curve with altitude.

Smythe is offline  
Old 3rd May 2019, 22:10
  #8 (permalink)  
 
Join Date: Sep 2007
Location: New York
Posts: 142
APU Battery Start

Originally Posted by vilas View Post
No you are not reading correctly. This limit is only for APU start in ELECTRICAL EMERGENCY configuration.
Perhaps you misunderstood his question. My Operator lists "Max FL for APU Battery Start" as FL250. -Both CEO & NEO. -note that's a battery start; which is what he was asking I believe. APU Max start envelope is upto the Service Ceiling, though it's no good for Bleed Air until FL225...
neilki is offline  
Old 4th May 2019, 00:48
  #9 (permalink)  
 
Join Date: Feb 2000
Location: 500 miles from Chaikhosi, Yogistan
Posts: 3,522
Originally Posted by scifi View Post
Also, am I reading the graph correctly.. You can't restart the APU from the battery, above FL 250.. ?
Correct.

Normally you can start the APU anywhere in the flight envelope.

But if you’re in cruise and suddenly down both your engine generators, you’d need to descend to FL250 to start the APU.
compressor stall is offline  
Old 4th May 2019, 01:17
  #10 (permalink)  
 
Join Date: Jul 2010
Location: Asia
Posts: 523
The engine driven generators have an integrated constant speed drive to keep them running at the same speed while the engine speed varies, as the APU is either ON or OFF its generator doesn't need this feature. Dispatch with an engine generator U/S is allowable with the APU running for the entire flight. ETOPS requires a diversion in the event of an IDG failing and being unable to get the APU generator online.

The A320 is much more dependent on electrics and hydraulics then previous generations of conventional aircraft and a higher level of redundancy is called for, two generators offline and I would be looking at suitable enroute alternates rather than continuing a long flight. The emergency electrical configuration is a last resort serious emergency requiring a MAYDAY call, not to be thought of as an additional back up.
krismiler is offline  
Old 4th May 2019, 01:51
  #11 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,305
Perhaps you misunderstood his question. My Operator lists "Max FL for APU Battery Start" as FL250. -Both CEO & NEO. -note that's a battery start; which is what he was asking I believe
Does the APU ever start without battery?
vilas is online now  
Old 4th May 2019, 04:05
  #12 (permalink)  
 
Join Date: Nov 2006
Location: Tropics
Posts: 308
The battery is required to start the APU regardless of whether we are in EMER ELEC or not. So why is it that we are limited to FL250 for APU starts in EMER ELEC?
dream747 is offline  
Old 4th May 2019, 06:39
  #13 (permalink)  
 
Join Date: Jun 2004
Location: Just Around The Corner
Posts: 1,138
In EM Electrical Conf each APU start attempt will drain the battery life , so the FL 250 i think is to wait for a level with dense air that will help the successful start .
Nick 1 is offline  
Old 4th May 2019, 09:37
  #14 (permalink)  
 
Join Date: Nov 2017
Location: australia
Posts: 60
I'm flabbergasted at the question on an APU battery start, the mention of battery starts itself means no other generator involved, the start is off the ships battery alone and each attempted start is going to drain the battery.

This is pretty basic stuff and any pilot who has been operating a descent turbo prop or jet should know this.
exfocx is offline  
Old 4th May 2019, 09:43
  #15 (permalink)  
 
Join Date: Nov 2017
Location: australia
Posts: 60
I'm flabbergasted at the question on an APU battery start, the mention of battery starts itself means no other generator involved, the start is off the ships battery alone and each attempted start is going to drain the battery.

This is pretty basic stuff and any pilot who has been operating a descent turbo prop or jet should know this.
exfocx is offline  
Old 4th May 2019, 09:51
  #16 (permalink)  
 
Join Date: Feb 2002
Location: UK
Age: 54
Posts: 2,659
Is the question being asked by a pilot or just an interested party?
TURIN is offline  
Old 4th May 2019, 09:52
  #17 (permalink)  
 
Join Date: Feb 2017
Location: Isla Grande
Posts: 787
Originally Posted by krismiler View Post
The engine driven generators have an integrated constant speed drive to keep them running at the same speed while the engine speed varies, as the APU is either ON or OFF its generator doesn't need this feature.
Yep.
So if the APU runs with constant RPM how does the geny lose power with altitude?
gearlever is online now  
Old 4th May 2019, 09:59
  #18 (permalink)  
 
Join Date: Feb 2002
Location: UK
Age: 54
Posts: 2,659
If the gen is 'on load' then it needs a finite amount of power to drive it at a constant rpm, if you increase altitude you will get to the point where the required demand load on the gen is too much for the reducing available power from the gas turbine. The APU will begin to slow down and once the frequency drops off below its limit, EG 398Hz, then the APU gen breaker will trip leaving you on battery power only until the RAT comes on line.
TURIN is offline  
Old 4th May 2019, 10:16
  #19 (permalink)  
 
Join Date: Mar 2000
Location: Sunrise Senior Living
Posts: 1,298
Folks, the APU starts from the DC BAT Bus which is not powered in EMER ELEC - unless the APU Master is pressed. So, as someone has said above, every APU start in the A320 CEO is from the batteries. I have no knowledge of other variants.
cheers
mcdhu
mcdhu is offline  
Old 4th May 2019, 11:55
  #20 (permalink)  
 
Join Date: Dec 2013
Location: US
Age: 61
Posts: 399
Originally Posted by gearlever View Post
Yep.
So if the APU runs with constant RPM how does the geny lose power with altitude?
I assume you are a non pilot. When you ask a generator to produce electricity it requires power to do so. 500 HP can produce a lot more power than 5 HP. A gas turbine loses about half it’s available power from sea level to 20,000 feet. At some point in a climb the available power usually becomes less than the rated output of the generator. You must reduces electrical output at that point or the gas turbine will either overtemp or be unable to maintain the required RPM resulting in the generator frequency output falling below limits.
Sailvi767 is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.