Forum Navigation
Please or Register to create posts and topics.

APU Operation and Discussion

(Original thread started on 08-12-13 by Alan Norris)

I did some work with the APU switches. By using the start function for Engine #3, I managed to get the START light on the APU AML to come on when it was pressed and go off after 8 seconds (similar to the offsets for Engine 1 and 2 start switches) The AVAIL light on the Electrical panel comes on when the START switch times out. It goes off when I set the APU MSTR switch to OFF. What I cannot do is get the APU RUN light to come on after the START light goes out. I guess I'll have to wait for an InterfaceIT update or find out what the Internal Variable options such as LESS THAN or EQUAL TO do.


I did a short video showing how I got the APU to operate correctly (by some smoke and mirror programming). The start switch sends an offset to start the APU but I don't think that FSX does a good job of modeling the APU. In fact if I go into the APU panel in FSX and turn it on, there is no indication that APU power is available on the EICAS.


I had to be very creative to get those START/RUN lights to work using Engines 3&4 generator switches (as you did for engines 1 & 2 start switches). I used 8000 time to sleep for engine 3 to get the START light to go out after 8 seconds and used -8000 for engine 4 to get the RUN light to come on after 8 seconds. Trial and error really and I have lots of time on my hands.


(Posted by Ron Rollo on 08-14-13)

Here are a couple photos of my APU for reference:

Ron 628


Ron 629


Mine works the same way, thanks for working on this Alan!  However, I was trying to see if we could take it one step further, (if it is possible), by being able to select the RUN/STOP button after the APU is up and running to stop the APU and from applying power to the electrical panel. In other words, turning it off properly by the book. Although the way you turned it off is also in the book as a secondary way to kill it.


Also, another thing that I was reading about the APU is when you initially hit the MASTER switch, it applies power to the APU and does an internal test of the APU system. I have not seen anywhere what this test looks like. As an example, do the other lights on the APU panel momentary light up? If so, we could create an APU light test variable.


(Posted by Alan Norris on 08-14-13)

The START/RUN switch is not supposed to work until the MSTR is in the down/on position. There is no way at present to do this in InterfaceIT so I created two virtual joystick buttons that FSUIPC can detect and then edit the .ini file to add the button conditional. I have done that but for some unknown reason FSUIPC has decided not to recognize the START/RUN switch. According to the flight training manual the start up test does not illuminate any lights except if the test fails in which case the APU FAIL light comes on. You are right about the stop sequence -- pressing the START/RUN switch. That can easily be done by using the Up/Off action from the MSTR switch for the Up/Off action of the START/RUN switch. I'll change mine today.


(Posted by Ron Rollo on 08-14-13)

I got mine working. But there was two other things that I had to do to make it work. As you pointed out, the first thing was to create an Action Up/Off for offset 0A5A. But that did not kill the lights on the electrical panel depending which one was on at the moment.


So I created two additional Action Up/Off Internal Variables for the START/RUN switch. One to turn off the "APU AVIL LIGHT" and the other to turn off the "APU PWR ON" LEDs on the electrical panel.


Now it works perfectly, as long as I do it by the book. I tried it every way possible by the book and it works. If I start hitting buttons out of sequence, then there are problems. As an example, don't hit the START/RUN switch prior to hitting the MASTER switch.


Just to be clear, the PROPER WAY to turn it off would be to select the APU power switch on the electrical panel so that it says "AVIL". Then hit the "START/STOP" switch to kill the APU and to remove the "AVIL" LED light from the electrical panel. Last but not least, hit the "MASTER" switch on the APU panel to kill the APU electrical system.


The alternative way is just hitting the "MASTER" switch which kills everything at once, which is the only option that you currently have.  There is a reason why they have labeled the START/STOP switch on the APU panel START/STOP.  You can select this switch to start the APU and you can select it to stop the APU.


All in all, I am very happy with it and I can now show visitors exactly how the APU should work! If you have any APU questions please ask. There is plenty of room for improvement!


(Posted by Gerry Vermaelen on 10-30-13)

I was wondering how do you manage the APU with FDS SYS3 card & FSX. Only a few offsets available in FSUIPC 0B51-0B53 and 3 functions in FSX.


Found some info on AVSIM from English rebel, but after spending several hours or maybe days with the APU, I think I have to replace the AML21 switch " START/RUN" which is actually momentary with a latching one. I let you know if any progress.


(Posted by Ron Rollo on 11-01-13)

Hey Gerry, are you using FDS cards? If so, momentary or latching will work, you would just need to tell the EnterfaceIT software what your using.


I have pretty much what Alan Norris (AKA English Rebel) has working in my sim. At this point, all we really have is some light logic working properly. The actual APU simulation does not work. It's beyond my pay grade. Hopefully someone will make that happen for us.


(Posted by Gerry V. on 11-02-13)

Hi Ron, yes I use FDS boards,what else would we use!   About APU,my son which is an electrical engineer made a small device ( picture below) to simulate the APU startup. The first version was displaying the Amps but alternating. The actual version displays 0 to 28v.


Should work like this:

Push AML MASTER switch on APU panel.

Check if plane is on ground ( with FSUIPC offset )

LED ON comes on and display indicates 00.0.

LED ON comes on, on electrical panel.

LED START comes on also.

Push START (will flash for 1o seconds which simulates internal check of the APU)

RUN LED will come on and display will increment slowly, then more rapidly and finally very slowly till 28.0v.

AVAIL on electrical panel also comes on.


This is my first test version which is partially still in my mind. Let me know if I'm wrong in my logic somewhere.


(Posted by Alan Norris on 11-04-13)

Don't forget that the START/RUN function is not supposed to work until the APU MSTR latching switch is in the DOWN (ON) position and cannot run for 10 seconds after the APU MSTR is pushed so it can go through its start up test. The START/RUN is a momentary switch (not latching) and the START light is constant for a period of time until the APU comes up to 95% operating RPM (in my program I just used 10 seconds as the APU in FSX is not really functional) when it extinguishes and the RUN light comes on.


Hope that helps. BTW I like your seats and the APU generator amps display is cool (another item for someone to make).


(Posted by Gerry V. on 11-07-13)

Hi Alan, thanks for giving those details, I will try to program this according to your info. If it works well, module could be made for other members if interested, at a reasonable price of course.


All you need is a 5v input and InterfaceIT programing. When I'm ready I'll put a video on the forum. About the APU, could you send me the xml.files you have for the APU so I can do mine the same way as you have? You can use the contact link I have under, and where to send it also.


(Posted by Genoroof on 12-12-13)

When the APU is fired up, what would the “GEN AMPS” reading be?


(Posted by Alan Norris on 12-12-13)

According to the manual:

"During start, it indicates zero. When the APU generator output is commanded, the display shows the actual load being supplied. Maximum load indication is 500 amps and is frozen at 500 amps until the load drops below this value. APU generator output (amps) is also displayed on the EICAS electrical schematic page."


This last item is not currently programmed in JET45.


Elsewhere in the APU section of the manual:

"The APU generator can supply up to 300 amps of normal power and can be operated in conjunction with the aircraft engine generators. APU amps above 300 will cause an APU AMPS HIGH amber CAS message........"


"The amour of power extracted from the generator is displayed GEN AMPS indicator on the APU control panel.  Any load higher than 500 amps will not be displayed."


I hope that helps. For the APU panel to display the correct amperage it will have to be programmed with the actual load which is not available in FSX as all the panel displays is a static value. I'm not sure if JET45 will be able to provide the varying value or not.


If you are building an actual LED display then for now maybe just show 300.


(Posted by Eric Williams on 12-12-13)

I had been doing some thinking on this myself. Looks like Alan got you set up with the specs, so here's my thoughts.


I had planned on using Arduino to run the display so I looked into the Simconnect variables that I could work with. I found there is one for "Battery Amps" that is directly affected by starting the APU in FSX. It actually shows zero anytime the APU is off but once you start it- the values ramp up and stabilize around 467. The "Main Bus Amps" also ramps up when APU is started and sits about 25 amps with aircraft fully running with no APU.


Using the above a person could actually use these (I would add some level shift in the code personally) and could also add some change depending on whether or not some loads are on (that we have data for) such as Avionics, heating, lighting etc.


(Posted by Alan Norris on 12-12-13)

Eric, if I open the APU panel in FSX and click on the START button, the display starts to ramp up to a maximum value (I forget what it is). This is not the way it's supposed to work -- it's supposed to start at zero because there is no draw (well maybe a few amps to power the DUs etc) which is why I said that FSX doesn't model the APU at all. Any thoughts?


(Posted by Eric Williams on 12-12-13)

Yep it's definitely not accurate and has a few issues- but the fact that it has a simconnect signal can allow us to use it throughout our sims regardless of what system/hardware. Whenever possible you want to be using an actual SIM output (FSUIPC or Simconnect) for any items you can. The cockpit display is way out to lunch on the APU but the simconnect values are as I posted above.


So what you can do is use this by developing the code driving anything (such as what I did in the Arduino) and adapt it, scale it, delay it etc for any output. For the APU- you would just use the trigger to say go- but can scale the digits any way you like. This would allow any part of your sim to be using the same source "trigger" regardless of what hardware you are using. Right now- a lot of the things we do with internal logic in Interface-IT is confined to affect items on that board alone.


Another way of thinking- if you are not using the actual trigger from the sim itself- Jet45 can never display properly as per your custom variables because they don't share the same trigger (not sure how many more items will be added in Jet45 for CAS etc. just food for thought).


(Posted by Eric Williams on 12-13-13)

Well I just couldn't resist giving this a try. I found one of my 7 segment displays and hooked it up to a sub $10 Arduino board. Unfortunately, I don't have a 3 digit version currently so I had to prototype it on my 4 digit one. I had never used any of these LED displays before (only LCD) but I was able to get my head around it pretty quick thanks to Google.


There is indeed a Simconnect output from FSX that becomes active when the APU is started. It ramps up from zero to around 467 AMPS with the APU running and the Lear running on both engines normally. It is definitely not the best- but this can be fixed. Using the FSX data we can scale it and affect it any way we desire in the Arduino code. Jim's Link2fs program makes a tonne of data available so we can affect any internal variables however we wish.


Also of note for future builds if you try to use the data "as-is" - The variable actually drops in amp reading down to approximately 442 if you turn off the aircraft engine generators with the APU running and drops even further as you add electrical loads such as pitot heat, Nav Lights, or turn Avionics off (so sad Aces..) Looks like they got the loading backwards. We can fix this though.


Took me about 2 hours to get the code working as I had never worked with these displays at all. I had a bit of trouble parsing the data from the serial data. These simple displays are actually a fair bit trickier code than the full microprocessor LCD's I'm used to- odd huh?  I'll post the code and any relevant information on Thingiverse so others can use it and make it much better. I'll add the link here when I get that done.


Materials are:

1 Arduino Nano

1 seven segment display

4 resistors



Here's a video of it on the breadboard with the raw FSX data on APU start. Click the image for the video:


Anyone seen or have a pic of the actual APU display by chance? The ones common for prototyping are far too large for the panel and I'd like to source the correct one so I can mod the code for it. I have some old I/O cards with 3 digit ones but they are seemingly a fair bit smaller than our APU panels. Any info would be great. Hopefully this will help someone someday.


(Posted by Randy Buchanan on 12-16-13)

I think Alan has a good idea and so I will work though the showing the picture here which I think you can use:

Randy 2


So by knowing the size of the AML 21 button size you can get a good idea of how big the display is.  Here is the illustration a little bigger:

Randy 3


If the button is 3/4" square then the display is 1 and 3/16" wide by 9/16" tall. I hope this helps.  Now if I could do what you do in this thread I would be all set.

(Posted by Ron Rollo on 12-16-13)

Here are a couple reference photos of the real Lear45 APU:

Ron 630


This is a light test photo that Peter Cos Captured:

Ron 631

I don't have any of the APU AMP window lit up.


(Posted by Eric Williams on 12-16-13)

That's great Ron. The one you sent me I can clearly see the display does seem to take up the full space in the panel- which means none of the displays I have could even be close to correct. For some reason it seems to be an uncommon size- unless I'm just missing an obvious source. I need to go on the hunt now.


On a plus note- I spoke with the developer of the Link2FS program and it may be possible in the future to add FSUIPC offsets to the options we can directly read and control via Arduino. That may open up basically all the doors left between us and being able to link all the the interface cards we currently use with Arduino, FSX, Jet45 and all others. Will see what happens though.


I'll see if I can find one that fits the same as our rendered photos. The code for the Arduino needs to be set up for the pinout of the actual display and I've found they almost all differ. Once I get it sorted I'll post it up here so others can duplicate if they desire.


(Posted by Ron Rollo on 09-29-14)

If your like me and have an APU fetish, I did a little work to try to add a little more realism to my sim. First off, I am tagging in on Gerry's thread in an effort to keep all of our APU information and discussion in one area. (Thanks Gerry!) And my second thing that I want to say before I get into what I have done is that we are all limited to what we can do with the APU until some software is developed that helps address the electrical system in a little more detail. (The problem is it is not a very high priority compared to other things like the CDU and FGC software.) If you guys are like me, we just want something that looks like it is working and sounds like it is working. After all, it is just another toy and eye candy. You might fire it up before a flight to help start your engines but it would never be running in flight. It is just another neat thing to have.


In the meantime, I did some more reading on how the real APU works and especially the AMP window. At first, I was under the impression that the voltage was displayed there in the AMP window. (When I say "at first" I mean five or six years ago!) The AMPs normal operating range is under 400 AMPs. Between 400 and 440 is the CAUTION range. Above 450 is the WARNING area. (This is by the book which makes me ask what about the range between 440 and 450??) Of course this value is variable depending on the electrical load on the system.


With all that said, I opted to make a simple AMP window that reflects three things: Nothing at all, 0 and 100. I decided to go this route until a software solution is implemented into the Jet45 AAS. Once that happens, a 3 pack seven segment AMP display would be needed to take advantage of it's new capabilities.


According to the Flight Safety Pilot Training Manual, when the APU is on and START is selected, the AMPs window displays a 0 while the APU is spooling up. Once it is actually running and available, it will then begin to display the load value which will range anywhere from 0 to 500. (500 is the MAX value indicated and the actual value may be even higher which of course anything over 450 is in the WARNING area anyway.)


So what I have done is created an AMP window with two light chambers in it. One with just the 0 and the other with 10. During start up, the 0 is displayed in the AMP window:

Ron 632


Once start up is complete and the APU is running, the 10 illuminates giving me a figure of 100 in the AMP window:

Ron 633


This is great! Now it looks like the APU is doing something. And at the same time, it is reflecting what the electrical system in FSX is doing which is nothing! My point is that the electrical system within FSX is so sub par that anything would be an improvement.


The one thing that I would like to add to my APU is a couple sound files making it sound like something is spooling up and running and then spooling down. Anyone have any information or thoughts on that? Alan, I know you have played around with some files.


In the meantime, I know Gerry and his son are working on a seven segment display with variable figures. I am looking forward to seeing this Gerry!


(Posted by Eric Williams on 09-29-14)

If you would like you can do this with an Arduino and Link2FS. Here is the video I made a while back (real FSX display at the end):


I used the FSUIPC variable so it is indeed a bit high and I'm not sure it varies much due to the modelling being poor- but at least it spools up and down. You could easily put an offset in the code to drop the values down by whatever quantity you like.  Not sure if this helps but another option for some.


Code is here so I don't have a huge long post:

It is also back up HERE


(Posted by Gerry Vermaelen on 03-03-15)

An APU seven segment display is now available. It fits perfectly the APU panel that Eric supplies. One has been sent to Alan Norris for trial/comments.  The seven segment displays are directly connected to an ATmega (328) Micro controller, which is the only chip on board.


(Posted by Alan Norris on 03-14-15)

I received the display from Gerry today and it is very well made. I will be documenting it's installation and start up and will post here as soon as it's done. I need to change my single pole AMLs to double poles for this and Ron kindly sent me two for this purpose.


I finally got the display that Gerry so kindly sent me installed in my APU panel. What with being sick for over a month in December and doing some freelance design work this year it has taken me far too long to evaluate the display, sorry Gerry. Then we had back and forth emails about wiring issues and that I couldn't get it to to work. I finally pulled it back out this morning and it turns out I had the wiring to the APU panel switches backwards!


I must say it works like a charm. Great job Gerry and son!  One thing I do not know and that is when does the START light extinguish and the RUN light come on. Is it at a certain percentage of the APU motor power. Maybe Eric knows. It will be easy to go into InterfaceIT and change the time delay between one light going out and the other coming on.\

Here is the way I wired it. I had to change my two switches to double pole as I still needed to interface with the FDS board to initiate the LEDs. Sorry it's a photo as the scanner on my all in one doesn't work since I update to the latest version of Yosemite:

Alan 46


(Posted by Shane Barnes on 03-15-15)

Just a reminder to anyone who may have pulled the connectors out of the AML's used for the landing gear indicators and saved them. You can slide those connectors into a single pole AML and make it a double pole. Just thought I would share, it might save you a few dollars. Thanks for the testing Alan!


(Posted by Mark Speechely on 03-15-15)

Well done to Gerry & your son and of course to you Alan for the testing. Another feather in the cap for our group. Just stand back and look how far this Lear jet dream has taken us thus far.  Awesome! Thanks again!


(Posted by Dave Simmons on 08-21-16)
I am having difficulty finding sample code to write the APU voltage to a 3 number LCD alphanumeric display that I purchased from There is plenty of help to connect the wiring but little code examples to send the digits to the Pokeys card from FSUIPC via LUA.  Anyone have any code examples to write to the LCD Alphanumeric display?


(Posted by Alan Norris on 08-22-16)
I don't do complex LUAs, just the real simple ones like the sounds.  However, I received a completed board and code from a Gerry over in Europe.  Now his code may not be what you want but he can probably do one for you.  His email address is verdrug05@gmail(dot)com  His son does the boards.  Look at my post where I show a video of the APU starting up to see if that's what you had in mind.


(Posted by Dave Simmons on 09-05-16)
Further research has found a great package (free) that interfaces an Ethernet Pokeys card (Pokeys56E or 57E) with FSUIPC including alphanumeric support thru the card.  His name is Ruediger Ebert and the product is FSSymphony.  I may break down and buy a Pokeys57E. I pretty much have a Pokeys card at the major panels. Have to finish building and program.


I ended up purchasing a Pokeys57E board fro use for the APU and Trim panels. (I have decided to use Pokeys and group panels around the card and send the USB/Ethernet cable from the Panel/MIP site to the PC).


I plan to use FSSymphony with the Ethernet Pokeys card to display Amps. Through some research, I have found that the Lear APU is supposed to generate 28 volts at 302 Amps. Giving that, one can compute the Watts (8400) and resistance (0.0933 ohms). Given that and an APU_Volts from FSUIPC (Offset: 0B5C Size: 4 APU generator voltage level, 32-bit float) one can compute the Amps at APU_Volts / 0.0933.
Thanks for your help.


NOTE: If you have any APU questions or answers, please post here. There is plenty of room for improvement and learning on this topic!


Uploaded files:
  • You need to login to have access to uploads.

Up to this point we still do not have a fully functional and viable solution to the APU AMP display.  But that might be changing soon!  I have been working with an interface developer towards a solution to the problem.  To date, we might have the interface hardware part of the problem resolved.  See photo:

The next step in the process is for me to build a couple APU Panels to insure that the APU AMP window fits perfectly into the APU backer.  (I should have this complete in about eight weeks.)

At that point, the physical hardware part of the APU will be complete and it is just a matter of feeding the APU AMP window the correct data.  This got several of us thinking including DonnyRay who suggested creating a complete AMP load calculator that simulates EVERYTHING in the Lear45 that draws current and AMPs.  This is something that is lacking in the simulation and in theory, should be fairly easy to create.

The idea is to create a AMP Load Calculator using an Excel spread sheet that monitors everything that draws power and also everything that creates power like engine generators,  the APU, Left and Right batteries, EMER battery, etc..  Once the AMP Load Calculator has been developed, we would then simply tap into the software via the APU interface to monitor the APU AMP load.  We would also use this AMP Load Calculator to generate accurate AMP data to be displayed on the EICAS / DU2 and the Electrical page of the Jet45 AAS software.  So as you switch things on, like Taxi Lights as an example, you will see AMP values change in various AMP windows, depending on what is generating power.

This is the idea and so far so good!  The interface cards and AMP window displays have been resolved and  DonnyRay has started the AMP Load Calculator spreadsheet and it is a strong solid start!  One last thought to throw out there, the new MSFS 2020 in development has reported that the aircraft they have developed so far have a modeled electrical system.  We will wait and see how detailed it is.

More information on the APU AMP Display as it develops!


Update on the APU design and solution.

Jason and I have solved all the issues with the APU display over a year ago.  It was actually an APU seven segment display pcb "price quote" from a well known sim developer that encouraged us to look into our own pcb development for not just the APU AMP display but for ALL the panels and modules in our Lear45 build!

This is something  I wished we looked into a lot sooner than now because what I have discovered is that pcb is fairly easy to design, way faster to manufacture, way more affordable than clad boards and last but not least, neater and more professional looking over clad.  Thanks to Will Sasse for pointing me to a great pcb manufacture and some helpful hints getting me up to full pcb speed!

Back to the APU, what you see below is my first attempt at pcb which was a complete success.  Three seven segment displays and a double row pin header, that's it.

The new v2.0 APU panel was specifically designed around the APU seven segment AMP display.

And here is the rear side wired up and ready to go.

What you see here is just the physical aspects of the APU panel and display.  The real magic is in the Jet45 Systems Software that Jason is developing.  I have seen a short video of the APU AMP display in action and it is awesome!  (Can't find the video or photo atm)

The goal for the APU display and everything else related to the Jet45 Systems Software is to make things as easy as possible for the end user.  In other words,  just make sure you have all the pins wired up exactly as they are outlined in the Jet45 AAS / Systems Modules document.  The software takes care of everything else!

You can find the latest version of the Jet45 AAS / Systems Modules document HERE

See pages 37 and 43 for the APU AMP display pin assignments.

More info, photos and videos coming soon!