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LGIntoxx
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Boeing 747 Landing Light Question

Mon Feb 12, 2018 10:55 pm

While I was watching videos (and real ones, for that matter) of the Boeing 747, I've noticed that the nosewheel light doesn't come on until the main landing gear hits the ground. There should be a logistical reason for it, but wasn't able to find any. I thought that this would be the place to ask.

This may be the wrong subcategory to post this in, but I've only been on this forum for <week. Thanks for any help.
 
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Starlionblue
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Re: Boeing 747 Landing Light Question

Tue Feb 13, 2018 12:36 am

I don't know the 747 specifically, but the nose gear mounted units tend to be taxi and turnoff lights, not landing lights. Landing lights are normally mounted in the wing roots. So ground sensing logic makes some sense.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
RetiredWeasel
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Re: Boeing 747 Landing Light Question

Tue Feb 13, 2018 12:55 am

Stationblue is correct. Although I think the 747-400 had some slight variations between the airlines, we had two taxi lights that pointed straight ahead above the nosewheels and two runway turnoff lights pointed in opposite angles. Even if all the switches were on, the circuitry was not activated until the air/ground sensing mode was in ground. The 200 however only had runway turnoff lights as I recall. You used the landing lights to taxi if you needed them.
 
Max Q
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Re: Boeing 747 Landing Light Question

Wed Feb 14, 2018 9:57 am

I think nosegear taxi lights on the Classic were an available option as I’ve seen a few equipped with them
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nuke
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Re: Boeing 747 Landing Light Question

Wed Feb 14, 2018 10:02 am

Runway turnoff/taxi lights become active once the main landing gear compresses on landing.
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fr8mech
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Re: Boeing 747 Landing Light Question

Wed Feb 14, 2018 10:07 am

I just looked at our WDM for the -400, and found that out taxi and turn-off lights are wired through the gear handle, not the air-ground system.

On our -8F's, the taxi and turn-off lights are wired to air-ground (main gear tilt), but the landing lights are dimmed (4 volts worth) when the gear handle is down.

I believe the turn-off lights on the -100/200's were wired through the gear handle.
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zeke
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Re: Boeing 747 Landing Light Question

Wed Feb 14, 2018 11:52 am

I thought I remember being taught the lights on the 747 always had power through them to keep the filament warm to extend life.
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fr8mech
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Re: Boeing 747 Landing Light Question

Wed Feb 14, 2018 8:07 pm

zeke wrote:
I thought I remember being taught the lights on the 747 always had power through them to keep the filament warm to extend life.


Think about that. I seem to recall, from way back in the stone ages when I was a simple a&p student, that a circuit required 3 things: method of control, method of protection and a load.

When I get to a computer, I’ll see if I can find the relevant FAR.
When seconds count...the police are minutes away.
 
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fr8mech
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 1:41 am

fr8mech wrote:
method of control, method of protection and a load.

When I get to a computer, I’ll see if I can find the relevant FAR.


Hmmm, a quick skimming of Part 25 didn't reveal a requirement for control devices, and thinking about it, there a plenty of things on an aircraft that don't have a dedicated switch, e.g. instruments.

But, the landing lights on a B747 are not "hot" all the time.
When seconds count...the police are minutes away.
 
aeropix
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 3:19 am

fr8mech wrote:
fr8mech wrote:

But, the landing lights on a B747 are not "hot" all the time.


Then why do they glow softly at all times during flight? Believe me you can plainly see this on night flights, especially from opposite direction aircraft. I don't think it's because of friction...
 
RetiredWeasel
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 3:43 am

aeropix wrote:
fr8mech wrote:
fr8mech wrote:

But, the landing lights on a B747 are not "hot" all the time.


Then why do they glow softly at all times during flight? Believe me you can plainly see this on night flights, especially from opposite direction aircraft. I don't think it's because of friction...


This topic is centered on 747's so can't speak for other aircraft but:

I just went through my old NWA Airplane Operating Manual for both the 400 and the 200 and there is no mention of any trickle current that flows through the landing light circuit to keep the bulbs warm when the switches are off. However, just a disclaimer, maybe that's something the powers decided that pilots didn't need to know...but I doubt it. And I was also an FE on the 200 for quite a while and don't recall any mention of it.
 
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fr8mech
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 3:50 am

I don't know, maybe it's a customer option. The classics certainly didn't have it, nor do I think the -400. Will need to look closer at the AMM D&O and the WDM. I'll look at the -8 also.
When seconds count...the police are minutes away.
 
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zeke
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 4:00 am

I seem to recall in air mode a heater voltage was applied, something like a drop from 28V down to 24V when the light switch was in the off position and gear up, this reduced thermal shock and extended their life. Might have been a customer option, pretty sure the 777 is the same.

Been a while for me.
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fr8mech
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 4:05 am

Well, well, well. We learn something new everyday. The -8 does have a "preheat circuit". Just the -8 though, that circuit does not exist on our -400's and most certainly did not exist on our -100/200's.
When seconds count...the police are minutes away.
 
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zeke
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 4:15 am

RetiredWeasel wrote:
I just went through my old NWA Airplane Operating Manual for both the 400 and the 200 and there is no mention of any trickle current that flows through the landing light circuit to keep the bulbs warm when the switches are off. However, just a disclaimer, maybe that's something the powers decided that pilots didn't need to know...but I doubt it. And I was also an FE on the 200 for quite a while and don't recall any mention of it.


Yep I think that is far too detailed that Boeing thinks pilots don’t need to know. The little amount of information they put in the manuals is disappointing.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
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zeke
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 4:19 am

fr8mech wrote:
Well, well, well. We learn something new everyday. The -8 does have a "preheat circuit". Just the -8 though, that circuit does not exist on our -400's and most certainly did not exist on our -100/200's.


How far off was I with the logic and voltages ?
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
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fr8mech
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 4:51 am

zeke wrote:
How far off was I with the logic and voltages ?


Spot on. I'm kinda surprised that 24V produces a "soft glow" and 28V is full bright. I'd of thought the voltage difference would be larger.
When seconds count...the police are minutes away.
 
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zeke
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 7:51 pm

Thanks my 74 ground school instructor was a 74 classic flight engineer and “AME” equivalent. He had so much knowledge and so many good stories one couldn’t help remain engaged. A real teacher and mentor.

Larger than life chap, always super happy, may he continue to RIP.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
chimborazo
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Re: Boeing 747 Landing Light Question

Thu Feb 15, 2018 9:40 pm

Threads like this are why I can put up with the sometimes hate and vitriol and keep me loving this site.

Not disputing the 24vdc to avoid thermal shock - and what a great solution - but is there not also some kind of current limiter? On a dc halogen dropping from 28 to 24v would in theory just dim the bulb to around 85% as bright. Or is it a special filament material that doesn't react to fully brighten until the full 28v is applied?
 
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fr8mech
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Re: Boeing 747 Landing Light Question

Fri Feb 16, 2018 1:22 am

chimborazo wrote:
but is there not also some kind of current limiter?


Nope, no resistor in the circuit at all.
When seconds count...the police are minutes away.
 
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fr8mech
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Re: Boeing 747 Landing Light Question

Fri Feb 16, 2018 3:52 am

chimborazo wrote:
On a dc halogen


Just saw this...really doesn't make a difference, but I'm pedantic enough to point out that the lighting is AC not DC.
When seconds count...the police are minutes away.
 
MatthewDB
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Re: Boeing 747 Landing Light Question

Fri Feb 16, 2018 4:30 am

fr8mech wrote:
chimborazo wrote:
On a dc halogen


Just saw this...really doesn't make a difference, but I'm pedantic enough to point out that the lighting is AC not DC.


I was noodling on how exactly 1970's technology was going to deliver a low cost way to drop from 28V to 24V without a resistor. Then your post makes it rather clear.

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