APU disable time, could someone explain?

[sirrah]

I noticed the APU disables at various times. Sometimes fairly quickly while I'm still on the ground and sometimes a few minutes after take off.

 

I was wondering why that is. I asume the hydraulic accumulators get re-pressurized by the engine driven main hydraulic pumps. So why is the APU on for such a long time? And why isn't this allways the same?

(I know the F-16 APU, or JFS as it's called there, disables pretty much directly after engine start)

[AG-51_Sabot]

I think after the 2nd engine starts it's no longer need after a certain time and goes into a "Self Protect" mode and shuts down automatically.

[mvsgas]

What do you mean by disable? To me means that is broken and not usable. I ask because the F-16 JFS should not be disable after start. It turns off or shut off at around 54% engine RPM. Is that what you mean?

[sirrah]

I indeed meant "shut off" (couldn't find the correct English word for it :P )

 

I know it shuts off automatically, but I was wondering why this seems to happen at random times (There must be some kind of sequence for it to run though I'd expect)

[mvsgas]

The APU should turn off 1 minute after second generator comes on line AFAIK, is it not doing that for you?

I don't have the DCS F/A-18, talking based on the available manuals online.

Edited November 13, 2018 by mvsgas

[sirrah]

The APU should turn off 1 minute after second generator comes on line AFAIK, is it not doing that for you?

I don't have the DCS F/A-18, talking based on the available manuals online.

I know it often tirns off while I'm allready up in the air. Considering ins allignment takes a few minutes (it counts to about 3:03, but I'm not sure those are seconds), it certainly stays on for more than 1 minute

[erautour]

Why don't you time it and provide some actual useful information

[sirrah]

Why don't you time it and provide some actual useful information

Sorry erautour, I obviously disturbed you with my question, which wasn't my intention.

 

My question was purely out of interest and I just thought maybe someone here would know this.

 

As soon as I have time (which isn't exactly easy with a one year old daughter :P), I'll use a timer on it and post the results here (unless anyone else here can provide the answer in the meanwhile)

[Woodford]

If both engines are up and running correctly, the APU should have shut off long before you get airborne. On a hot start, for example, the APU is off on initial load.

 

If it's happening *extremely* quickly (say, before you can even get the right engine spooled up), you're out of fuel. Some servers start aircraft at bone dry.

[sirrah]

Hmm.. so am I the only one here that has the APU still running while up in the air?

 

In that case I'd better go through the entire startup procedure again to see if I missed something.

 

I never have any cautions on when taking off, so I never thought about maybe forgetting something..

[javelina1]

Hmm.. so am I the only one here that has the APU still running while up in the air?

 

In that case I'd better go through the entire startup procedure again to see if I missed something.

 

I never have any cautions on when taking off, so I never thought about maybe forgetting something..

 

 

I encounter this from time to time as well. I follow a start sequence, which I bring up via my kneeboard.

 

 

At times the APU will shutoff while on the boat. Other times, it's still running after I launch and start my 1st turn. Then I notice the APU shutting off, because you can hear it.

 

 

Not sure why it does this. As noted, I follow the startup sequence in Chuck's manual, which I added into my kneeboard.

[erautour]

 

Not sure why it does this. As noted, I follow the startup sequence in Chuck's manual, which I added into my kneeboard.

 

The Chuck's guide is kinda more like a strategy guide than a manual. Make your kneeboard from the Natops manual and your APU won't be left on again. Step 17 is to verify the APU is off.

[sirrah]

The Chuck's guide is kinda more like a strategy guide than a manual. Make your kneeboard from the Natops manual and your APU won't be left on again. Step 17 is to verify the APU is off.

 

Is this the correct Natops manual?

 

Am I correct that the F/A-18C startup procedure can be found in chapter 10.2.1.2?

 

If so, I indeed see the APU must be switched off at step 20

 

20. (ABC) APU -OFF

 

So does this mean it has to be switched off manually (officially then)?

[xavnl]

The APU should turn off 1 minute after second generator comes on line AFAIK, is it not doing that for you?

I don't have the DCS F/A-18, talking based on the available manuals online.

 

MVSGAS is totally right. APU will come offline 1 minute after second engine, left one, is online, ie. L Gen caption is OFF.

It's in the NATOPS and implemented in DCS (I just checked it).

[xavnl]

Is this the correct Natops manual?

 

Am I correct that the F/A-18C startup procedure can be found in chapter 10.2.1.2?

 

If so, I indeed see the APU must be switched off at step 20

 

 

So does this mean it has to be switched off manually (officially then)?

 

It means you need to check it's OFF, nothing more, you have to check it in case the auto cut-off didn't work.

[sirrah]

It means you need to check it's OFF, nothing more, you have to check it in case the auto cut-off didn't work.

So if the auto cut-off doesn't work, which seems to be the case not only for me (refering to javalina's post), we'll have to switch it off manually?

[Frederf]

Chapter 10 is functional checkflight procedures and atypical. I think it's ensuring or setting APU off to check crossbleed start. Check section 7.1.5 which is normal flight.

 

APU is normally triggered to shut down 1 minute after both generators are online but can remain on if the bleed air augmentation system is in use. Yes you can always turn off the APU by setting the switch off.

[Charly_Owl]

From the NATOPS (source page I-2-28)

 

2.4.2.1 APU Switch. The APU switch is a two-position switch with positions of ON and OFF

 

APU Switch - ON

 

Starts the start cycle of the APU. Switch is electrically held in the ON position

and automatically returns to OFF 1 minute after the second generator comes on

the line provided the bleed air knob is not in AUG PULL

 

 

***

 

The Chuck's guide is kinda more like a strategy guide than a manual. Make your kneeboard from the Natops manual and your APU won't be left on again. Step 17 is to verify the APU is off.

 

"Make your own kneeboard from the Natops" is much easier said than done.

 

Also... I never pretended my introductory guide was anything close to a flight manual or a NATOPS-style manual. It's just to get the ball rolling. Think of it more like hand-written notes and reminders. Some people don't seem to be able to grasp this simple fact and get angry when they expect to see in this document something it's clearly not meant to be.

Edited November 14, 2018 by Charly_Owl

[SC86]

I have always had it turn off before takeoff, I've been following the start-up procedure I have seen in a few of the videos online about starting the F 18. Even if I perform the fire check, then turn ground power on and start aligning my INS early, turn on my lights and displays, change my ordinance and whatever else, it still turns off a short time after my engines are both on, and long before taxiing to the runway.

[MrDieing]

I do not want to be that guy, but I'd rather say it before some moderator finds out and gives consequences to it.

 

 

Rule 1.16 Posting images, file links, and file sharing links of military aircraft documents newer than 1980 is strictly prohibited on our forums. Such posts will be removed.

 

 

And yes, its a rather new rule.

[Eldur]

The APU should turn off 1 minute after second generator comes on line AFAIK, is it not doing that for you?

 

That's exactly what happens in DCS. I never had it still running even when I start taxiing. It's always shut down by itself before the INS alignment is done.

 

Try providing a track, so we could see what you're doing and help solving that issue

[sirrah]

Ok, point taken.

 

Apparently I'm one of the very few who experiences this late APU deactivating. I'll revise my startup procedure and try to follow the Natops steps next time I get the chance. Maybe then my APU will turn off as it should.

 

Until now I also used Chuck's manual (which I'm very grateful for, as unfortunately I don't have time to struggle through the entire Natops manual)

[Mule]

Why don't you time it and provide some actual useful information

 

Yep, you should provide a few graphs that display trend data. If you could at least submit a comprehensive scientific report with notes and references and some outsourced analysis from a National Accredited Laboratory at the very least then your question might be answered.................in due course after the committee has had several meetings and a vote has been passed.

 

These are the usual channels you should go through before even considering a response.

 

Thank you.

Edited November 14, 2018 by Mule

[sirrah]

Yep, you should provide a few graphs that display trend data. If you could at lest submit a comprehensive scientific report with notes and references and some outsourced analysis from a National Accredited Laboratory at the very least then your question might be answered.................in due course after the committee has had several meetings and a vote has been passed.

 

These are the usual channels you should go through before even considering a response.

 

Thank you.

:megalol: :thumbup:

[Eaglewings]

I once experienced the APU staying longer than it normally would and found out that my hotas throttle was not in sync in idle position.

Just a one time experience. Ensure your hotas throttle is all the way back when starting the two engines