Lemmy
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CRJ Loud Noise When Parked: What Is It?

Tue Feb 15, 2005 3:49 am

I flew from SBA to PHX the other day on a CRJ-200 and, as I was waiting at the bottom of the stairs, I heard an extremely loud noise that sounded like high-pressure air being released. This has happened the last few times I've flown on a CRJ. In all cases, the engines weren't running, but the APU was.

Any ideas what this noise is? It's not a big deal, but my ears are pretty sensitive from years of unprotected rock and roll, so I've taken to walking up to the airplane with my fingers in my ears, which looks kinda funny (about par for the course with me, though).
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troubleshooter
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RE: CRJ Loud Noise When Parked: What Is It?

Tue Feb 15, 2005 4:16 am

This sound comes from the APU. The APU electronic control unit (ECU) operates the load control valve (LCV) to close, if bleed air is selected and the EGT reaches a predetermined level. As the EGT drops, the ECU commands the LCV to open again. This pulse action from the ECU is done by the LCV for modulating purposes.

In other words: this sound is normal when the APU runs on ground and the air conditioning packs are selected to "on". By the way, I noticed that noise on CRJ´s only. Never heard it on other aircrafts.

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Lemmy
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RE: CRJ Loud Noise When Parked: What Is It?

Tue Feb 15, 2005 11:08 am

Any idea what pressure the bleed air is at when the LCV opens? Sounds pretty high to me.
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HAWK21M
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RE: CRJ Loud Noise When Parked: What Is It?

Tue Feb 15, 2005 4:34 pm

Where is the outlet of the LCV located.
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MEL
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troubleshooter
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RE: CRJ Loud Noise When Parked: What Is It?

Tue Feb 15, 2005 6:09 pm

Lemmy:
If I remember right, the APU bleed air pressure is somewhere around 41 PSI. My training manuals are at work, maybe someone here may correct me if I´m wrong with this.

Hawk:
The LCV is installed on top of the APU. It opens the APU bleed port to the bleed duct to supply the aircraft system.
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Woodreau
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RE: CRJ Loud Noise When Parked: What Is It?

Wed Feb 16, 2005 12:39 am

Before the LCV opens and the engines aren't running the 10th stage bleed air manifold pressure should be 0 psi. But after the LCV is open it's should be at least 40psi. That's the minimum pressure for an engine start utilizing the APU as the starting air source.
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HAWK21M
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RE: CRJ Loud Noise When Parked: What Is It?

Wed Feb 16, 2005 4:57 pm

Thanks Troubleshooter.
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MEL
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CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Fri Feb 18, 2005 8:13 pm

Quoting Troubleshooter (reply 4):
If I remember right, the APU bleed air pressure is somewhere around 41 PSI.


I've always seen it around 43 to 46psi

You should hear it when its cold outside and the apu intake has sucked in some ice!
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troubleshooter
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 6:10 am

CRJ200Mechanic:

I stand corrected. The 41 PSI were just a guess out of my memory. I´ve seen no CRJ from inside for about 6 months now and I have actually no access to my training manuals or the AMM.
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CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 11:12 am

Troubleshooter

My applause for such a close guess. I thought about it later and do remember some of the older CRJ's showing about 41

I have access to the manuals if you are ever interested
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troubleshooter
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 12:00 pm

I´ve worked on CRJ100/200. They are "very" old, because they are operated by the CRJ launch customer. Maybe this explains it.
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CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 2:57 pm

wow, you were working on some of the very first. Did they still have the tail skids installed?
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tito
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 3:29 pm

The sound is definitely a result of the LCV closing, however it appears as if the LCV closes in response to the EGT rising momentarily FIRST (you can observe the EGT rise followed by the corresponding noise and resulting EGT decrease). My guess is that the EGT rise is a result of the variable inlet guide vanes being modulated (intentionally?), the LCV is just doing what it's supposed to do to keep the combustion cooler. I haven't heard this happening with any other aircraft so perhaps this guidevane cycling is unique to this APU. Is this a result of system logic exercising the guide vanes or just modulating the guide vanes as a way to induce modulation of the LCV?
 
troubleshooter
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 4:41 pm

CRJ200Mechanic:
Yes, the tail skids are still installed. Some of the first CRJ100 will be returned to Bombardier. Retired from service with Lufthansa CityLine.

Tito:
The variable guide vanes are commanded to open when bleed air is extracted. This allows a higher mass flow through the compressor caused by the higher air demand. As more air is available, the EGT will not rise above the given limits. The only modulating part in this circuit is the LCV. The variable guide vanes are simply commanded open or close.
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HAWK21M
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 5:05 pm

Quoting Troubleshooter (reply 13):
Yes, the tail skids are still installed.


Any Pics of these Tail Skids.
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MEL
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CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 5:37 pm

Quoting Troubleshooter (reply 13):
The variable guide vanes are commanded to open when bleed air is extracted. This allows a higher mass flow through the compressor caused by the higher air demand. As more air is available, the EGT will not rise above the given limits. The only modulating part in this circuit is the LCV. The variable guide vanes are simply commanded open or close


right on the money troubleshooter
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CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Sat Feb 19, 2005 6:31 pm

HAWK21M

I found some pictures for the tail skid. I accidently posted them under the topic lightening diverters. Sorry about the mistake
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tito
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RE: CRJ Loud Noise When Parked: What Is It?

Sun Feb 20, 2005 2:27 am

Quoting Troubleshooter (reply 13):
The variable guide vanes are commanded to open when bleed air is extracted. This allows a higher mass flow through the compressor caused by the higher air demand. As more air is available, the EGT will not rise above the given limits. The only modulating part in this circuit is the LCV. The variable guide vanes are simply commanded open or close.


Thanks troubleshooter. What puzzles me is that the EGT spikes (to the top of the green arc) momentarily which sets this whole thing into motion, and every CRJ i've seen does this constantly. What causes this spike? This is just sitting there with a constant AC and air load. I've tried timing it but there seems to be no discernable pattern to its frequency, and some airplanes are worse than others, but they all do it.
 
CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Sun Feb 20, 2005 7:53 am

TITO

the older airplanes tend to do it more often because the APu's are getting tired and worn out
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undehoulli
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RE: CRJ Loud Noise When Parked: What Is It?

Sun Feb 20, 2005 8:23 am

What about the packs??? Those can get awufully noisy with the bleed air running through them, those compressors spinning and all! On the ERJ one pack is located on each side underneath the fuselage just ahead of the wings. Pack 1 is on the boarding side. It's so noisy that a lot of times it isn't even run during passenger boarding. That's why it gets so damn hot in the flight deck - pack 1 supplies 80% of the air for the flight deck and only 20% for the cabin. Pack 2 does the opposite. I am not a CRJ expert but am in the aircraft systems groundschool right now, so hopefully soon I'll know more before long. I do know that on the ERJ the APU bleed valve is either open or closed. I'll have to ask if the LCV has variable settings in the CRJ.
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troubleshooter
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RE: CRJ Loud Noise When Parked: What Is It?

Sun Feb 20, 2005 10:05 am

Undehoulli:

The packs of the CRJ are not as loud as the ERJ packs.

Air distribution on the ERJ is as follows:
Pack 1 56% cockpit - 44% cabin
Pack 2 100% cabin
A total of 28% air from both packs is routed to the cockpit and the remaining 72% of both packs supply the cabin.
The operation of the APU LCV on the ERJ depends on the APU type installed. On the "-C11" APU the valve is simply commanded open or close. The air flow is limited by a flow limiter venturi. On the "-C14" APU the LCV modulates bleed pressure and air flow.

I know this answer is not related to the original topic, but I think it is worth to be discussed here.
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HAWK21M
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RE: CRJ Loud Noise When Parked: What Is It?

Sun Feb 20, 2005 2:06 pm

Quoting CRJ200Mechanic (reply 16):
I found some pictures for the tail skid. I accidently posted them under the topic lightening diverters. Sorry about the mistake


No prob,I've seen them.Thanks.
Have you got any close views Available.
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troubleshooter
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RE: CRJ Loud Noise When Parked: What Is It?

Sun Feb 20, 2005 3:31 pm

Hi Hawk,

this is the closest view I could find. You can see the tail skid on the aft fuselage below the "A" of the tail registration. Hope this helps.

View Large View Medium
Click here for bigger photo!

Photo © Charles Falk

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CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Sun Feb 20, 2005 10:42 pm

Quoting HAWK21M (reply 21):
Have you got any close views Available



those were the closest views I could find. the A/C I work on are too new and don't have them. Next time I get a comair callout I will take my digital camera with me and see if it has one. Comair is the only operator that flys into Tulsa with them. I will see what I can do.
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CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Sun Feb 20, 2005 10:44 pm

Quoting Undehoulli (reply 19):
I'll have to ask if the LCV has variable settings in the CRJ



The CRJ's is either open or closed. The exact ratio's for the packs I don't know. All I do know is that the left pack supplies the flight deck and right pack is for the cabin
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RJ
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RE: CRJ Loud Noise When Parked: What Is It?

Fri Feb 25, 2005 2:14 am

CRJ:

Left pack supplies 70% to the flight deck and 30% to the cabin.

Right pack is the opposite, 70% to the cabin and 30% to the flight deck.

Thanks for keeping the birds in the air!

RJ
 
CRJ200Mechanic
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RE: CRJ Loud Noise When Parked: What Is It?

Fri Feb 25, 2005 7:16 am

Quoting RJ (reply 25):
Left pack supplies 70% to the flight deck and 30% to the cabin.

Right pack is the opposite, 70% to the cabin and 30% to the flight deck.



Thanks for giving me some info I should already know, since I work on the things.

Thanks for the info. I have often wondered
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HAWK21M
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RE: CRJ Loud Noise When Parked: What Is It?

Fri Feb 25, 2005 1:01 pm

How Vunerable are the CRJ MLGs to collapse.
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CHQIAH
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RE: CRJ Loud Noise When Parked: What Is It?

Mon Jun 30, 2008 10:00 pm

I'm leaning about 99% to the APU, however, just for the sake of boredom, it had about a 1% chance (if the a/p was a line base for the airline's mx) of being mlg tire air pressure being let out close to the plane. And I only say that b/c you stated the "high-pressure air being let out". We do it all the time down here, and it's about 20-25ft away from the a/c and it is very loud!
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josekmlb
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RE: CRJ Loud Noise When Parked: What Is It?

Tue Jul 01, 2008 5:11 am

I love that APU in the winter its like a small fire place boy does it keep you warm...

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