B727LVR
Topic Author
Posts: 233
Joined: Fri Jul 11, 2008 7:54 am

JT8D-15A EGT Boost Circuit

Sat Feb 02, 2013 8:14 pm

Hello all,

Working an engine change on a DC9-33F, and having issues with the EGT system. We are replacing a -15 with a -15A, the new engine has the Boost Circuit System installed, where as the old one did not. When running the engine, after start we got no roll back, the engine stabilized at idle at approx 500*c then slowly continued to creep up, we shut it down at about 625*c. We checked the probes and got 6 probes reading 2.5 ohms, 1 at 4.0 ohms, and the last one at 1.0 ohm (possible bad probes). Unfortunately our manuals do not discuss the boost circuit, and they way it is hooked up electrically to the junction box (JB) is slightly confusing to myself and my counter part avionics buddy. The way it is hooked up, probe #6 has its chromel wire hooked up separately to spot 6 on the JB, which is then hooked directly in line with the chromel wire to the indicator. Probe #1 chromel wire is hooked to spot 1 on the JB, and is not connected to anything else. The alumel wires for probes 1 and 6 connect to spot J on the JB, which is bussed together with all chromel wires from all the other probes. The alumel wire from the the other probes (not 1 & 6) all connect together at spot AL on the JB.

While doing some research we have noted that in some installations there is are additional jumpers on the JB which connect 3-5 and 1-6, which then all connect to the chromel side of the system, going to the indicator. Our installation does not have this set-up. Also talking with a friend that has about 30 years on 9's he mentioned placarding that was required for the boost system installation, but again our manuals don't show the system.

My questions are:

1. What could cause the EGT to not roll back, and continue to rise after start?
2. Given the description above is there a possible probe/harness fault? (The lack of extra buss bars on our harness leads us to believe there is a problem here)
3. Can a non-boost circuit system be used on a -15A (All research done by us shows the boost circuit as one of the -15A differences among many others, so far)?
4. Is there any placarding needed?
5. Finally does anyone have a good description of how this system works?

I hate playing behind the power curve, and not having all the information at hand. Thanks in advance for all of your guy's help!!!
I'm like a kid in a candy store when it comes to planes!
 
KC135Hydraulics
Posts: 441
Joined: Sat Nov 03, 2012 2:05 am

JT8D-15A EGT Boost Circuit

Sat Feb 02, 2013 8:54 pm

Surely PW/Boeing would be a better point of contact than an aviation enthusiast forum! I would be very cautious of using any info from this site for anything more than reference.
MSgt, USAF
KC-135R / C-17A Pneudraulic Systems Mechanic Supervisor
 
B727LVR
Topic Author
Posts: 233
Joined: Fri Jul 11, 2008 7:54 am

JT8D-15A EGT Boost Circuit

Sat Feb 02, 2013 10:13 pm

Oh I know, and agree. I know there are a few on here that can offer places to go and look for sources of info, plus I like hearing from those who have had similar problems in th past and filing it away in the ol' roll-a-dex. I could have worded my questions better (as always), but am looking more for personal experiences than out right maintenance references.
I'm like a kid in a candy store when it comes to planes!
 
KC135Hydraulics
Posts: 441
Joined: Sat Nov 03, 2012 2:05 am

RE: JT8D-15A EGT Boost Circuit

Sun Feb 03, 2013 1:59 am

In that case you're in the right place! I am surprised that your organization's QA section doesn't have the capability to send for an engineering disposition from Boeing. This sounds like a highly technical question that must be answered for safe operation of the aircraft. Surely you're not being left to your own devices to figure out an unsatisfactory engine condition without the appropriate tech data.
MSgt, USAF
KC-135R / C-17A Pneudraulic Systems Mechanic Supervisor
 
B727LVR
Topic Author
Posts: 233
Joined: Fri Jul 11, 2008 7:54 am

RE: JT8D-15A EGT Boost Circuit

Sun Feb 03, 2013 2:58 am

We do, however they had the day off, and are already working on getting that information, I was hoping someone else may have had a similar experience in the past and I could pick their brain. Never hurts to try right?  
I'm like a kid in a candy store when it comes to planes!
 
AA737-823
Posts: 4955
Joined: Wed Mar 01, 2000 11:10 am

RE: JT8D-15A EGT Boost Circuit

Mon Feb 11, 2013 9:04 am

Quoting KC135Hydraulics (Reply 3):
QA section doesn't have the capability to send for an engineering disposition from Boeing.

Do you know how expensive a proposition that is? It's frightful.
And if this is not for an air carrier of any sort, but a restoration project, then funds are limited at best!
 
twincommander
Posts: 185
Joined: Thu Apr 14, 2005 11:54 pm

RE: JT8D-15A EGT Boost Circuit

Wed Feb 13, 2013 5:58 am

why are you using a different part number engine?

STC maybe? or is it for restoration?
 
ak907
Posts: 39
Joined: Wed Mar 07, 2012 10:56 pm

RE: JT8D-15A EGT Boost Circuit

Wed Feb 13, 2013 10:41 pm

On TCDS E2EA Rev.31, Note 18 mentions which P/N to use if you have complied with Service Bulletin 5455. I don't have access to the S/B, might be something to look into.

I have never worked on this engine, but from your description above:

- Probe 1 CR is not hooked to anything, it shouldn't work at all.
- Probe 1 and 6 AL is connected to 2, 3, 4, 5 CR? Wouldn't that cancel out the voltage?
- I'm not sure what you mean about 1-6 and 3-5 jumpers. A schematic might help.

Does you manual say anything about the ohms and voltages you should be reading?

If you find the solution, could you please post it? I find this interesting and great to learn off.

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