APU Max Mode Logic Correction Required

[Joshua757_YT]

"The Max mode position is a momentary switch position"

[IniJamir]

Hi, for this one I am gonna need a reference of some sort as I don't fully understand what you mean, but I know none of what those are and how to phrase it to our team 😄
Thank you and I appreciate all your contributions 

[zbrainlezz]

What he's saying is that, in the APU section of the flight engineer's panel, the Max position on the APU mode switch knob should be momentary/spring-loaded, i.e. it should turn itself back to Norm when the user "lets go" of it / stops clicking it.

I personally think this is a secondary concern though, as that knob doesn't seem to actually have any effect at the moment.

Edited April 6 by zbrainlezz

[CaptainCAVU]

On 4/6/2026 at 3:41 PM, zbrainlezz said:

...the Max position on the APU mode switch knob should be momentary/spring-loaded...

I personally think this is a secondary concern though, as that knob doesn't seem to actually have any effect at the moment.

You're really not going to see much effect in MAX MODE...that's why they have a light to tell you the APU is in MAX MODE. The APU shifts to this mode anytime the ground start switch for an engine is pressed. It opens the APU inlet guide vanes fully to provide max air to the compressor.

Edited 17 hours ago by CaptainCAVU
Clarity

[zbrainlezz]

57 minutes ago, CaptainCAVU said:

You're really not going to see much effect in MAX MODE

Good to know. What I meant by that second sentence in my original reply, though, is that turning the APU mode knob appears to have absolutely no effect at all. Yes, the knob can be moved, but it doesn't seem to actually trigger any change in the APU's operation. Therefore, I think iniBuilds should add the necessary code to make the knob actually change whatever APU parameters it's supposed to change before worrying about how the knob itself behaves, if that makes any sense.

[CaptainCAVU]

11 hours ago, zbrainlezz said:

...Yes, the knob can be moved, but it doesn't seem to actually trigger any change in the APU's operation. Therefore, I think iniBuilds should add the necessary code to make the knob actually change whatever APU parameters it's supposed to change ...

As I pointed out, you will not see any change in APU parameters. Going to MAX MODE fully opens the APU inlet guide vanes...you can't see that. It may raise the APU TGT as more air goes through the open inlet guide vanes to the load compressor but not necessarily. When you're starting engines, the packs are turned off, galley power is turned off, ATMs are off...the load on the APU is greatly reduced so you may actually see a decrease in TGT.

The only time we ever manually used the MAX MODE spring-loaded position was if we got an INLET FLOW light (probably due to ice forming in the inlet duct) that caused an automatic APU shutdown. After 3-5 minutes and the INLET FLOW light going out, we would then start the APU in MAX MODE.

 

[zbrainlezz]

3 hours ago, CaptainCAVU said:

As I pointed out, you will not see any change in APU parameters. Going to MAX MODE fully opens the APU inlet guide vanes...you can't see that. It may raise the APU TGT as more air goes through the open inlet guide vanes to the load compressor but not necessarily. When you're starting engines, the packs are turned off, galley power is turned off, ATMs are off...the load on the APU is greatly reduced so you may actually see a decrease in TGT.

Alright, but what about when switching from MIN to NORM or vice versa? Would there be no difference in APU gauge readings between those two modes?