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smartt1982
Topic Author
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Trigger Of Take Off Config Warning 737 NG

Tue Feb 14, 2012 1:57 pm

Hello,

With regards to the take off confg warning on the 737 NG. On our checklist item I have seen some crew members check it by individually checking each thrust lever separately. Is this necessary? it is possible for the TO config to be triggered by one thrust lever and not the other?

Thanks
 
tdscanuck
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RE: Trigger Of Take Off Config Warning 737 NG

Tue Feb 14, 2012 2:08 pm

Quoting smartt1982 (Thread starter):
Is this necessary?

Not exactly necessary but it does work and is the more conservative way to do it.

Quoting smartt1982 (Thread starter):
it is possible for the TO config to be triggered by one thrust lever and not the other?

Yes. TO warning is triggered by thrust lever angle via a series of microswitches in a switchpack. There is a separate switchpack for each thrust lever. It's possible for one switchpack to be bad and one not (this is the usual failure...both failing at once independently would be rare).

Tom.
 
737tdi
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Joined: Sun Sep 02, 2007 10:05 am

RE: Trigger Of Take Off Config Warning 737 NG

Wed Feb 15, 2012 2:37 am

Tom is right on the money. You really should check them separately just for safeties sake because the switch packs do go bad.
 
N243NW
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RE: Trigger Of Take Off Config Warning 737 NG

Wed Feb 15, 2012 5:53 am

I'll add to what's already been said...

The switch packs each contain three microswitches, each one corresponding to a different thrust lever angle. These switches interface with different systems and provide information for various things such as takeoff config warning, landing gear warning, spoiler auto-stow, weather radar, EEC mode and thrust reverser lockout. These switches do indeed go bad - I'm currently dealing with an issue where worn wiring to one of the switches can cause a REVERSER fault. The switch packs can indeed fail independently, so I would recommend checking them both individually. Boeing has improved the switch packs several times on the 737NG in order to improve reliability, so they are (fortunately) getting better.
B-52s don't take off. They scare the ground away.
 
aviopic
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RE: Trigger Of Take Off Config Warning 737 NG

Thu Feb 16, 2012 9:28 pm

Quoting N243NW (Reply 3):
The switch packs each contain three microswitches, each one corresponding to a different thrust lever angle.

I am not entirely familiar with 737 but 3 sounds a bit minimal to my ears or maybe it is the reason why the system is less reliable.
Something like 12 per Throttle lever sounds better  http://www.honders.net/tmp/D43126-443_DSF4308.jpg
The truth lives in one’s mind, it doesn’t really exist
 
BoeingGuy
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RE: Trigger Of Take Off Config Warning 737 NG

Thu Feb 16, 2012 11:24 pm

Quoting tdscanuck (Reply 1):
Yes. TO warning is triggered by thrust lever angle via a series of microswitches in a switchpack. There is a separate switchpack for each thrust lever. It's possible for one switchpack to be bad and one not (this is the usual failure...both failing at once independently would be rare).

A side note: the 737, 747, 757 and 767 look at thrust lever resolver angle to determine takeoff thrust as Tom indicates. The 777 and 787 looks instead at the actual engine N1, not thrust lever position. IIRC, it's 60% N1.
 
737tdi
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RE: Trigger Of Take Off Config Warning 737 NG

Fri Feb 17, 2012 5:51 am

Double post, sorry. It wouldn't let me just delete it.

[Edited 2012-02-16 22:03:23]
 
737tdi
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RE: Trigger Of Take Off Config Warning 737 NG

Fri Feb 17, 2012 5:59 am

Quoting aviopic (Reply 4):
I am not entirely familiar with 737 but 3 sounds a bit minimal to my ears or maybe it is the reason why the system is less reliable.
Something like 12 per Throttle lever sounds better

There is 1 switchpack per throttle on the NG, each pack has 9 switches (S1 through S9).
 
tdscanuck
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RE: Trigger Of Take Off Config Warning 737 NG

Fri Feb 17, 2012 6:04 am

Quoting BoeingGuy (Reply 5):
A side note: the 737, 747, 757 and 767 look at thrust lever resolver angle to determine takeoff thrust as Tom indicates. The 777 and 787 looks instead at the actual engine N1, not thrust lever position.

It's a little more nuanced than that...the 777 and 787 don't use switchpacks but they are still looking at thrust lever angle rather than actual N1. The difference is that the 777 and 787 have control RVDT's in the thrust levers that provide a very robust redundant position signal and the 777 and 787 have a lot more computing power than the other planes. The computers just look at the thrust lever angle directly, rather than relying on microswitches for a binary position signal.

You can tell they're not looking at actual N1 because the takeoff config warning will go off as soon as you advance the thrust levers, regardless of how far behind the thrust lever position the engine N1 is. This gives the pilots a lot more time to correct the situation.

Tom.
 
ODwyerPW
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RE: Trigger Of Take Off Config Warning 737 NG

Sat Feb 18, 2012 12:15 am

Quoting tdscanuck (Reply 8):
The difference is that the 777 and 787 have control RVDT's in the thrust levers that provide a very robust redundant position signal

Tom, regarding the RVDTs... are the using an optical encoder to feed digital pulses to a counter to indicate the thrust lever angle, or are they relying on some sort of analog potentiometer or lvdt (0-5v,-0-10v or 4-20ma signaling) to indicate the thrust lever angle? Do they do routine Homing (pulse counter) or Zero-Span (analog) calibrations of these? Do they have redundant instrumentation?
learning never stops.
 
N243NW
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RE: Trigger Of Take Off Config Warning 737 NG

Sat Feb 18, 2012 1:59 am

Quoting 737tdi (Reply 7):
There is 1 switchpack per throttle on the NG, each pack has 9 switches (S1 through S9).

My mistake - you're right. I think I was thinking 3 instead of 9 because of the three wire bundles I'm used to seeing.

Here are some pictures (first the old, then the new). The new ones are much smaller and less complicated looking.

http://img.photobucket.com/albums/v293/N243NW/old.jpg
http://img.photobucket.com/albums/v293/N243NW/new.jpg
B-52s don't take off. They scare the ground away.
 
tdscanuck
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RE: Trigger Of Take Off Config Warning 737 NG

Sat Feb 18, 2012 5:36 am

Quoting odwyerpw (Reply 9):
Tom, regarding the RVDTs...

The technical details are outside my sphere of knowledge, sorry. A CMM for the RVDT would tell you everything you need to know if you can find one.

Quoting N243NW (Reply 10):
Here are some pictures (first the old, then the new). The new ones are much smaller and less complicated looking.

The "new" one is really the 767-400ER switchpack design, repurposed for the 737NG.

Tom.
 
737tdi
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Joined: Sun Sep 02, 2007 10:05 am

RE: Trigger Of Take Off Config Warning 737 NG

Sun Feb 19, 2012 5:45 am

Quoting tdscanuck (Reply 11):
The "new" one is really the 767-400ER switchpack design, repurposed for the 737NG

Yes, and it takes away the rebuilding of the switchpack like we used to do. My last bid location was electric shop and we rebuild the switch packs plus a bunch of other stuff plus OTS aircraft. Now with the new packs we are losing another little bit of the bid location.

Oh well, things advance. I've bid back to line maint. now so there ya go.

737tdi
 
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HAWK21M
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RE: Trigger Of Take Off Config Warning 737 NG

Tue Feb 21, 2012 7:22 pm

Quoting N243NW (Reply 10):
The new ones are much smaller and less complicated looking.

Huge difference in size & weight too.amazing....
I may not win often, but I damn well never lose!!! ;)

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