737NG Overhead switch rubbers

[MrHorgy]

This has been bugging me for a little while...

On the 737NG overhead, some of the toggle switches have red rubber rings inside the switch mechanism, that you can see. A great example is the L an R RECIRC fan switches on the AIR CON panel, one has a red seat in the switch, and one does not. The Landing Light switches don't have them, yet the strobes do. So does the CVR toggle switch.

Why do some have them, and some don't? I shall provide a picture on wednesday if people don't follow what I mean - appreciate any feedback!

Horgy

[ampclamp]

I'd have to look to make sure Mr H but some s/w's are a sealed unit others operate from a mech toggle to operate individual micro s/w's.
Some s/w's are a single piece but not fully sealed.Just depends on the spec / area of use.

The red ones as described I would say are a sealed toggle s/w the red stuff being the sealant around the toggle lever.

In any case you've given me a reason to get interested and have a sticky beak

[NSEU]

I've also seen them on the 747-400. There appears to be no rhyme or reason why some have them and some don't (even on panels adjacent to each other). I guess it helps prevent fluid (spilled coffee) ingress, but then why would the overhead panels have them? :}

Rgds.
NSEU

[ampclamp]

My guess is the circuit designers/ engineers look at the circuit and what it carries and controls and fit a switch capable of the loads and other design parameters required.
Most switches are "off the shelf" items so if it happens to be a switch that can be used in harsh envronments but fulfills the design spec's required, even if its in a cosy cockpit upside down , so be it.

[rmm]

Different vendors perhaps?

[MrHorgy]

Hmm, well some of them do exactly the same thing - in my example of the RECIRC fan switches, they are identical and do the same job! It just seems hit and miss.

Amp - you might be onto something, I originally thought the red rubbered ones were ones with relays to work the switch automatically, but that was scuppered when I took a closer look.

Horgy

[Mike744]

I know that Honeywell supply hermetically sealed toggle switches suitable for aviation, which quote 'Reduced sensitivity to changes in altitude or pressure' .
Can I suggest if silver coated contacts are employed then possibly less likely to produce an oxide coating if hermetically sealed?
(I note they also supply toggle swiches for aviation that cannot be accidentally switched - i.e. pull toggle before actuating switch - neat)

[clivewatson]

load shed items?

[ChristiaanJ]

load shed items?If so, you would have expected that to be made VERY clear in the flight manual.
A doubtful functional clue like that would be perfectly useless in an emergency.

CJ

[allendc]

Horgy
You are seeing the toggle switch seal a example part number would be Micro Switch 1TL1-3. From the 1TL1-3 Spec sheet "Sealed to prevent entrance of liquid or dust"

Some switches control more circuits and they use multiple micro-switches, a good example is Micro Switch part number 64AT34-3 In Part Number 64AT34-3 the individual micro-switches are sealed, so there is no need for the shaft to be sealed.

The landing lights are a special type of toggle switch which are made by Cutler Hammer part number A3-1092 for the two position and 8906K1592 for the three position switch

David
Boeing 737-700 flight simulator construction (http://www.737FlightSim.com)

[ampclamp]

thanks allen, exactly what I was saying.

[clivewatson]

ChristiaanJ (http://www.pprune.org/members/105267-christiaanj) - it was only a guess. The inclusion of the eroteme should have made that clear. Having thought it through though, you are of course correct - how silly of me.

[ChristiaanJ]

... how silly of me.Not silly at all, it occurred to me for a moment. But after re-reading the original post, and then the other posts, it did not really make sense it being a "colour code", if you see what I mean.
CJ

[MrHorgy]

I think that must be it, some are environmentally sealed. Which leads me to my main annoyance/query with them - why is the L RECIRC FAN switch sealed, and the other is not?

This is probably something only Mr Boeing might be able to answer, but ask it I shall.

Horgy

[CaptainSandL]

Just a guess but the R recirc fan is standard on all series of 737 whereas the L is a kind of "add on" for the longer -4/8/900's. So maybe the spec is not as high because it is not as important. Or maybe the spec was written at a different time and nobody has noticed that they are different.

The only operational difference that I can think of is that the R recirc fan has a link to the overboard exhaust valve. If you switch the R fan off (not L on the -8/900) the cabin RoC will increase as the overboard exhaust valve opens.

[allendc]

The R recirc fan switch is a 3 pole switch and the L recirc fan switch is a single pole switch. The R recirc fan switch also controls "Smoke Control" so it controls more circuits. L recirc fan switch controls 1 circuit

David
Boeing 737-700 flight simulator construction (http://www.737FlightSim.com)