I'm a Standard EE and I've never heard of it...

I'm in avionics primarily TCAS/ATC Transponder systems, with a year or so of shop support work, all commercial airline work.  I never heard of it either.

I guess it's possible that it is a requirement based upon a MIL-Standard??  Anyone with further insight?  I'll try to get a close up picture of a couple of the components and see if I can post those...

I have heard from my colleque that it's the ground point of the unit .  The ground point is a chassis ground that build inside the component .

That's along the lines of what I thought.  Been too busy (lazy?) to take the picture.  I'll try to get a couple of shots on my next mission.  What is the significance of indicating externally where the internal chassis ground is?

We've NEVER designed such a feature in any of our airborne systems.  Since the entire chassis is supposed to be electrically bonded, there is no preferentially location.

Moreover, internal circuitry are not allowed to be grounded to chassis; only shields, which are carried through the bulkhead connectors and grounded to the chassis via the backshells.  Any other connection to chassis is usually not allowed, per EMI control plan.

TTFN

Well, I have pictures now.  I also feel quite confident in a solid explanation I received from a former avionics tech.  He said that that is the point where the face plates get their ground so that the lights in that plate can backlight the surface.  Seeing how we fly some unusually old technology and equipment, I tried it out on my last trip.  One small panel was completely unlit.  When I pressed on the "+" it lit up!  Nowhere else on the panel could produce the same results when pressed.  To drive it home, however, I tried it on a large (approx. 1.5'x1.5') panel which was only partially lit.  When I pressed the "+" it would move a good 1/8" and you could feel some type of male/female connection being made which made the panel light up.  If anyone knows how to post pictures, let me know and I'll put examples up.  Thanks for the help.

Cool thanks for the "get back"!

Here's you picture posting info.

I'd like to see a pic.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

Sorry itsmoked, but I don't see any pic posting instructions.  Let me know if you have instructions for posting on Eng-Tips Forum.

Click on "Process TGML" link at bottom of dialog box on this page.  Scroll down to "Images"

TTFN

You have to look much, much, closer!!
FAQ238-1161

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

Ok, here are the pictures.  The first three show components with the "+" of which I spoke:
---this one is below the turn wheel---

---this one's easy to see---

---there are actually 3 in this one...top, "ENGA +"; middle, COMM/NAV panel; and left between "H" and "N"---


This next one is the large overhead panel with the dark center section:



And finally, this one (with the shadow of my arm from the left) is where I'm pressing the "+" and lighting the center section:



Enjoy!
Chuck

An interesting explanation, but not consistent with the AH-1S cockpit behavior.  The EL panel backlights are all connected together and controlled with a dimmer switch, as one might expect.  You'd certainly not want your display backlight to accidentally turn off during operation because you bumped the panel.

A more likely possibility is that it indicates where the connector to the backlight is, and when you press on it, it makes the connection better.  This would make more sense, i.e., if the panel blacked out, you'd push on the plus sign to force the connector back into contact.

TTFN

Is it possible that the "+" sign is just a reference point for the AutoCAD Layout Panel Engineer.

Thank you so much for your help.
Your cooperation is highly appreciated.

I would suppose that the many of these panels predate the usage of Autocad for any layouts.

TTFN

This seems to be on older panels with discrete controls/displays only. I used to work on a Nav CDU and the "+" symbol was located directly over the connector for the panel backlighting. In this case, it looked like a mini-RCA plug embedded in the panel which slotted into the receptacle on the chassis. In might just be an aid for aligning the panel during assembly.

Based on the comments and photos, I'll hazard a guess that the installation instructions required the installer to:

1 - Put the light plate in place and start the screws
2 - Press on the X while tightening the screws.

Any body have access to old light plate specs or the AMM Maintenance Practices?

Great discussion and inputs!  I agree with IRstuff and jmcaero's assessments that there is a connection for backlighting and was attempting to make that point in my last post.  I also agree that you wouldn't want a panel to lose its light during operation, but it happens in old, tired Air Force equipment!  This jet probably has between 20,000 and 25,000 hours and the screw next to the "+", I could tell, was stripped out when I tried to tighten it.  I've heard several people mention the AutoCad theory but don't buy it for a couple of reasons--1.  I agree many of these components pre-date its use, and 2.  I doubt the designs (lettering & symbols) are applied to the components until after the panels are machined.  I never would've guessed I'd get this many responses--GREAT website!!