Jump to content



The AVSIM Forums will be temporarily offline for software upgrades beginning 23FEB17 at 22:00Z. The upgrade could last as long as 7 hours. We apologize for any inconvenience.

In Loving Memory Of

Tom Allensworth,
Founder of AVSIM Online

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.


APU Problem

  • Please log in to reply
6 replies to this topic

#1 iMoveQuick



  • Members
  • 7 posts

Posted 11 January 2017 - 02:46 PM

Hi there,


So I've been figuring out how to get the plane (NGX 737) from cold and dark to engine start, but after a few steps I already run into a problem.

When I want to start the APU the needle(EGT) moves really slow towards 4, and doesn't go any higher or lower and also at that point the "Low oil pressure" light comes on. (Nothing weird about that since that's normal I guess). But the problem is that it won't go out. It only goes out if I turn the APU on and off.


So to make it clear, from the point that it reaches the 4 mark the APU gen off buss also goes on and when I click it the APU start (I think I'm not sure, but it sounds like it.)


But why doesn't the "Low oil pressure" light go off? 


Here's a picture of my overhead panel : http://imgur.com/a/O8uUx





  • 0

Help AVSIM continue to serve you!
Please donate today!

#2 downscc


    Member - 8,000+

  • Members
  • 12,113 posts

Posted 11 January 2017 - 03:46 PM

The APU start sequence should indicate a much higher EGT initially before settling down to about where yours is showing.  The fact that it is not supports your finding that your are not getting a start from the APU.  I notice you have ground power available but are not using it therefore you are relying on batter alone to start APU.  I might be wrong, but try connecting to external power. Maybe your battery is weak, I cannot see the DC panel in your picture.


I've started APU from battery alone, so I am just guessing here.


If nothing else, you can hook up start air and use that instead of APU to start engines but you shouldn't have too.

  • 0

Dan Downs

#3 mvelure



  • Members
  • 6 posts

Posted 11 January 2017 - 04:00 PM

I might be wrong, but try connecting to external power. Maybe your battery is weak, I cannot see the DC panel in your picture.
I've started APU from battery alone, so I am just guessing here.




While I would like to see the DC panel, if ground power is available it should always be connected until AC power is available from the APU.

  • 0


Regional Mgr, Check Pilot

#4 Stearmandriver



  • Members
  • 202 posts

Posted 11 January 2017 - 05:04 PM

If the APU gen off bus light illuminates, the APU is running. It's off in your pic. Had you already connected the APU gen to the busses at that point?

Shouldn't still have the low oil pressure light though.
  • 0
Andrew Crowley

#5 flightdeck2sim



  • Members
  • 89 posts

Posted 12 January 2017 - 02:07 PM

Hi Casper,


Firstly you've definitely got a source of AC power connected because the low pressure light for your AFT Fuel Boost Pump 1 is extinguished with the switch on. I can't tell from the picture if you're connected to the GPU (The GND PWR light stays illuminated even if you're using it as a source of AC power) or the APU (the APU GEN OFF BUS will extinguish if it's providing power to a transfer bus)


Just make sure you're starting the APU correctly.


1. Hold the switch to Start momentarily (you don't need to be connected to Ground Power, it will start using DC power from the battery but the battery must be on) The switch will return to the on position and doesn't need to me moved until you turn the APU off.


2. The start process takes 40-120 seconds, you will see the LOW OIL PRESSURE light illuminate during the process and EGT peak to a high temperature and then stabilise at approximately 400C


3. When the APU has completed it's start cycle the APU GEN OFF BUS light will illuminate telling you that you can use it as a source of power but it currently isn't providing any power


4. Move both APU Gen switches to ON, this will extinguish the APU GEN OFF BUS and you're now using the APU as a source of AC power. 



If you did this correctly the only reason the LOW OIL PRESSURE light is still illuminated is that you genuinely had low oil pressure! 


Good luck! 

  • 0

Sam B

#6 Spin737


    Member - 2,000+

  • Members
  • 2,255 posts

Posted 12 January 2017 - 06:57 PM

I bet you're simply selecting the APU ON without actually starting it.


I should have looked at the pic, first. You've got EGT, so it's doing something. How long did you wait? RW, it takes ~50 seconds for the APU GEN to come on.

  • 0

Matt Cee

#7 ALF1



  • Members
  • 291 posts

Posted 14 January 2017 - 05:39 PM

How long have u been using the Battery ? Like Dan says maybe not enough juice in it for APU start. Try connecting the GPU and then start the APU. The battery should be used only briefly for the fire check and three green ldg gear and a few checks.

Also, if you have a host of things running and did not do a proper shutdown before you will drain the battery very quickly. You should only draw about 9 amps DC (check electrical) on bat alone.
  • 0

Alberto Ferracuti