APU disable time, could someone explain?

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?

Yes, you will.

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.

Looking forward to read it. :megalol::doh:

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.

emh dude, that file is not clasified , its a public file so what are you talking about??

Hello Sirrah, in more than 50 flights I never had the APU running during taxi.

Unless you start second engine (left) really just before starting taxing, you shouldn't have APU running.

My only suggstion is to start your chrono as soon as you have both engines running and check.

Let us know!

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)

You might want to check bleed air and generator settings. There's no good reason for the APU to run once both engines are turning unless there's a misconfigured panel. As for how the panel got misconfigured (it won't be when the aircraft is initialized)...that's a mystery.

If you're following Chuck's guide make sure you're not accidentally pulling the bleed air control when you turn it after the fire tests. Mice, even trained mice, are not approved by NavAir for pilot assistance, and you never know what the little rodent is doing.

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.

Sirrah it's curious this is happening to just you...understand I am NOT throwing stones at you or questioning your procedures. NONE OF US are Stan Eval or actually qualified in the F-18C

The APU description states

"Operation of the APU is automatic after the APU switch, on the left console, is placed to ON. The APU may be shut down at any time by placing the APU switch to OFF. After the APU has completed its start cycle a green READY light comes on. After the second generator is on line, the APU runs approximately 1 minute then the APU switch returns to OFF."

If you are seeing such a dramatic variation in how long the APU is operating following engine start I'm curious if you are doing something that causes it...and if you are what that might be.

Obviously following the NATOPS should yield the same results every time. Try running an auto start...and time how long from the time the Second engine stabilizes to APU cutout and then do a Manual start using the NATOPS checklist...if ya don't have it in your library PM me and I will point ya to an open source for it.

Sirrah it's curious this is happening to just you...understand I am NOT throwing stones at you or questioning your procedures.

It's not just me :smilewink:

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.

But sure enough, I must be doing something wrong in the start sequence. Will try and figure out what it is

It's not just me :smilewink:

 

But sure enough, I must be doing something wrong in the start sequence. Will try and figure out what it is

There's also the possibility that this is all due to a bug. The best way to test that is to change your startup procedure.

Battery on

APU start

..wait for green

Right engine start

..at 15% right throttle to idle

..at 65% Left engine start

..at 15% left throttle to idle

That's it. See if the APU turns off about a minute after the left engine gets to 65%. Its always helpful if you can get the long-running APU to repeat at will, but we don't always get that luxury in code.

If you can get the APU to shut down with the abbreviated start every time start adding the normal start steps and see what happens. QA testing is nothing if not tedious if you can't automate the process.

Turns off 1 minute after 2nd engine started for me, as per manual.

If both generators are online, the other possible cause would be that the AUG PULL is on. This is the bit that's in the center of the bleed air knob. Pull it up and the APU won't shut off until you're airborne. When weight is off the wheels the switch snaps back down (probably, as the maual says it can only be used on the ground) and the APU should turn off one minute after that as usual. Could it be you inadvertently pulls this switch during the cycle bleed air knob phase?

PS: In case you're wondering what it does, the AUG PULL makes the APU inject air into the ECS in case the bleed air from the compressor(s) is insufficient (for air conditioning).

If both generators are online, the other possible cause would be that the AUG PULL is on. This is the bit that's in the center of the bleed air knob. Pull it up and the APU won't shut off until you're airborne. When weight is off the wheels the switch snaps back down (probably, as the maual says it can only be used on the ground) and the APU should turn off one minute after that as usual. Could it be you inadvertently pulls this switch during the cycle bleed air knob phase?

 

PS: In case you're wondering what it does, the AUG PULL makes the APU inject air into the ECS in case the bleed air from the compressor(s) is insufficient (for air conditioning).

Robin, this has got to be it! I'll check later tonight, and do a couple cold starts with it up and down. :) I'll report back later.

Jesus this thread has blown up. I've been leaving it running the last few days and it always turns off as it should.

That was it! thanks Robin! I tried with both the bleed air knob up & down. Down, and the APU shuts off within a min after the left engine is at speed. When up, the APU won't shutoff until after I launch and airborne.

more than likely as you suggest, from time to time, I inadvertently pull that knob.

many thanks for solving the mystery.

can remain on if the bleed air augmentation system is in use.

I am invisible.

I am invisible.

Didn't you know? :D:D:music_whistling:

I am invisible.

sorry, but I didn't read all of the thread.

thanks man. :thumbup:

If anything, I'm just another data point.

The manual says that the key to APU automatic shutdown is the generator line. In AUG PULL, the APU is only manually shut down (if one engine works and the other is starting, the APU will be temporarily shut down and restarted after the second engine starts). At the same time, the throttle in the Military Thrust will also withdraw from the APU AUG.

Just to get back on this topic to try and find where the "glitch" is:

I might have found what was causing my APU to stay on for way to long (several minutes).

In my startup steps I forgot to turn the bleed air knob to off and back to norm just before starting the left engine. As stated in Natops step 10 of the engine start sequence:

10. BLEED AIR knob - CYCLE THRU OFF TO NORM

 

The bleed air shutoff valves close during the fire warning test and the BLEED AIR knob must be

cycled thru OFF to NORM with ac power on to reset the valves.

After doing this you'll actually hear the air start flowing, which I didn't notice before. Then, after starting the left engine I noticed the APU turned off as it should

I also tried it by:

- starting both engines while not touching the bleed air knob

- wait for several minutes while the APU kept running

- turn bleed air to off and back to norm

- after less then a minute APU automatically turned off

I'm not fully sure that this bleed air switching is causing the APU to run way too long (very hard to test as there are so many variables that could cause this), but maybe someone here can verify this.

Are you running the fire tests or not? The cycle bled air knob is only needed if the fire tests have been run.

Are you running the fire tests or not? The cycle bled air knob is only needed if the fire tests have been run.

You're right, I forgot to mention that :thumbup:

(I did run the test indeed)