Can anyone explain me the theory of light test in B737NG?

[sky-738]

During rescent flights , when doing the light test , I got a question, it looks like when put the light test switch in test position , it not just test the bulb of each light, because when some CB pop out , the related system light are extinguished but the bulb of that light is OK. so may i take a guess that the light test is not only test the bulb , but also the status of each system work ? can any system expert explain me how the light test work? i think i will learn more form you reply. THX

[ampclamp]

It tests the lamps, not a functional test of any systems.

[de facto]

Theory of the B737 ng LIGHT TEST :

It TESTS the LIGHTS of the B737 ng.

[sky-738]

amp,
when you pull out a CB, say the speedbrake.the related light(speed brake arm) will not illuminate when doing the test. but the lamps issssssss OK. when you reset the CB, everything returns nomal and good.WHY?

[mathy]

If you pull any circuit breaker it interrupts the flow of current from source to destination. Normally a lamp is intended to show if such a flow is passing through such a breaker. So as you have pulled a breaker the lamp goes out. However the TEST circuit is situated ahead of the breaker and if the lamp lights then it means the bulb is okay. My brain hurts. I hope you never have a fire in the APU because a fire can be raging in the tail and suddenly the warning lights go out. That does not mean that the fire is out - the wires to the warning system have burned through... so every lamp has to be isolated in such a way as to test the intended function. My brain doubly hurts. So tired....

[172_driver]

Speaking about it, I am curious, anyone knows why the WHEEL WELL FIRE WARNING lamp is not hooked up to the light test? :O

[ampclamp]

172 driver, to give pilots engineers and instructors something to ponder.I think they made it the same as the classic.why ? :confused: Cant remember.

sky-738 , thats the way it works,pulling the spd brk c/b =no light , pilot thinks hmm something wrong, use NNC or contact engineering. sometimes a light not being on is as much a warning as one being illuminated.
sometimes tripping a breaker will illum a light rather than stop it, ie windshield heat. You have to know your systems and understand what and why you maybe seeing something.
But original point stands, lights test s/w is just that.

[aveng]

The light test circuit is from the 'master dim and test' system - the actual annunciator side is (usually) powered by its own system c/b.

[de facto]

amp,
when you pull out a CB, say the speedbrake.the related light(speed brake arm) will not illuminate when doing the test. but the lamps issssssss OK. when you reset the CB, everything returns nomal and good.WHY?

Sky738,

If you do that with me in flight or on the ground, Ill smash your head into the CB panel and then make a light test,see if you illuminate,finally:E

[aviatorhi]

de facto,

I hope you're kidding about reacting that way to a CB being pulled.

Otherwise... :rolleyes:.

Regarding the subject at hand, I believe the fire handles will remove power from the annunciators for the systems they shut-off / disconnect when pulled, hence lights do not illuminate.

[FlyingStone]

Systems schematic manual shows (here (http://imageshack.us/photo/my-images/443/737cb.jpg/) - not up to date version) that the speedbrake armed light is tested with the use of auto speedbrake CB, so the auto speedbrake CB should not be tripped in order to show when the lights test switch is set to TEST.

[de facto]

de facto,

I hope you're kidding about reacting that way to a CB being pulled.

Hmmm let me think...of course and I would disembark him just after.:E

CB are not to be touched on ground,Maintenance only is to reset CBs for other reason than a NNC action, especially in such unecessary case.

Otherwise

Otherwise what? You re gonna pull another CB on me?:suspect:

Peace:E

[aviatorhi]

Sounds like you've got a few CBs tripped in your braincase.

[de facto]

On a serious note and after you get your morning chill pill, do you think it is OK for someone to play around with CBs?

[InSoMnIaC]

De facto I think you are the one in need of that chill pill.

It was a hypothetical question. The poor fellow is trying to get a more in depth understanding of the system. he doesn't need your abuse.

btw on some aircraft there are procedures in place to pull and reset a CB on the ground by flight crew. Resetting a tripped CB is another story.

[de facto]

I was noooooooot serious in banging his head into the cbs...:cool:

[LeftHeadingNorth]

I believe de facto was attempting to be humorous. I believe it was somewhat entertaining at least :}

[aviatorhi]

It's a poor attempt at humor, particularly after I asked if it was humor or not.

I see no problem with (as a crew) pulling / resetting CBs (apart from fuel related ones) in flight or on the ground, very useful in troubleshooting and even more useful in disabling systems which aren't performing up to par or are being an annoyance.

One particular airport I flew into regularly a few years ago was not recorded in the TAWS database correctly (database showed it 5 miles away), company responsible for maintenance of said database would not fix it, hence pulling the CB for the warning horns blaring in my ear was the best solution given the circumstance.

Same goes for when I had a stall warning system get stuck in on, scared the **** out of the other guy.

"Recycling" CBs is a non issue for me whatsoever.

As far as resetting is concerned, have no problems with it so long as the system is necessary and consideration is given to the matter.

[de facto]

It's a poor attempt at humor, particularly after I asked if it was humor or not.

Reset all you want humour patrol aviator:E

I have yet to reset a CB in the last 9 years....but if i have to,sure I will.
However if maintenance is around, then they will do so and the appropriate work and log entries related to such action.

Please give us a favour,those nuisance cbs you like to pull out.(and those for your trouble shootings and so forth).make sure you push them back in for the next crew.:hmm:

[Kuchan]

I see no problem with (as a crew) pulling / resetting CBs (apart from fuel related ones) in flight or on the ground, very useful in troubleshooting and even more useful in disabling systems which aren't performing up to par or are being an annoyance

I didn't see the humour side of pulling /resetting CBs in flight ...very useful in troubleshooting........

Do crew members pulling CBs in flight??? for troubleshooting???

[zzuf]

These guys also had no problems experimentally pulling circuit breakers in flight.
The results were not so humorous.
TWA 727 (http://fromtheflightdeck.com/Reviews/727/TWA727/)
www.airdisaster.com/reports/ntsb/AAR75-02.pdf
I hope that aviatorhi has full knowledge of every circuit which shares any circuit breaker he may pull and the effect of pulling CB's on these systems.

[sky-738]

looks like it will be very violence , bloody........to touch a CB, but how about when it pops up by itself ? now i can guess CBs cut off the power to systems including the system lights. thanks all above.

[ampclamp]

My thoughts exactly. few if any engineers would know in enough detail what you would lose by pulling any breaker nor would many flt crew. A lot of common ones sure , but where I work flt crew must not reset or cycle breakers without explicate direction from engineering or maint watch. So many ccts and boxes talk to each other you just presume to know what will happen. Must check the cct schematics and WDM first.

[ampclamp]

now i can guess CBs cut off the power to systems including the system lights. thanks all above

No! don't guess , don't assume. If it happens in one cct it is not the rule ok? If you care enough consult the schematics and WDM or at least speak to your tech people.

[aviatorhi]

Yeah, that's what I do, get on the airplane and immediately pull half the CBs for fun. :}

What about my statement leads you to believe that actions are performed without crew coordination?

I don't subscribe to the idea that it's only safe to operate CBs on the ground, nor do I suggest that only ground crew are knowledgeable enough to do so.

Furthermore, (particularly) in the age of high technology and computers I am able to check the relevant ATA chapters prior to taking any action. In my view, the head in the sand ("follow the book into the mountainside") approach that many have regarding aircraft operations is precisely what leads to events like LO16.