Case grounding requirement of electronic equipment in composite aircraft. 5

[DHambley]

Every circuit I have ever designed for aircraft requires the case to be grounded to aircraft frame, or a braided cable tied to a nearby structure, or a grounding pin on the connector. EMI test labs I have worked with routinely check for a 2.5 mill Ohm resistance from case to the test table. In short, the industry as a whole has a culture expecting these boxes to be grounded. The installation and maintenance manuals specify how to ground the equipment and these manuals must be followed.

Recently I have a client who wants the circuits/case to be designed so that it can be bolted anywhere, grounded or not. The example he gives is "those composite aircraft" but he can give no specific reference. The only composite aircraft I worked on was the 787 and it has a electrical grounding/return structure to which electronic equipment is meant to be tied to.

I would like to know if anyone on this forum is aware of any aircraft for which the avionics cases are not tied to chassis, just floating and if that was the situation, how these were tested in an EMI lab.

Darrell Hambley P.E.
SENTEK Engineering, LLC

 

[pattheengineer]

That is going to be very difficult if not impossible. Are we talking commercial, military or general aviation here?

If it is something that has to pass rigorous EMI testing I just don't see that as possible. Once EMI gets onto something it has to bleed off somewhere or it will couple onto whatever it can.

An aircraft without a ground plane would not be a safe aircraft when it came to things such as lightning strikes, static buildup.

My guess is this customer thinks you can just design away the work and requirements.

 

[Sparweb]

I've worked with a few of "those composite aircraft" too. They all had bonding straps for electrical/electronic equipment. What's wrong with adding a braided cable off the device chassis to one of these provisions?

I suppose it's possible for a "black box" to exist in an aircraft without being bonded... but it would be carried like cargo, or used like personal laptop computers.

If the "box" :
[ul]
[li]Operates on a self-contained power supply,[/li]
[li]Can be secured against all flight and emergency loads,[/li]
[li]Emits negligible RF,[/li]
[li]Has no connections to aircraft systems (electrical or otherwise),[/li]
[li]Performs no function related to the mission of the aircraft,[/li]
[li]The enclosure is sealed enough and fire-proof enough to maintain integrity if the contents should catch fire,[/li]
[li]And you have evidence to prove all this...[/li]
[/ul]
...then you could make the case. I may have overlooked something.
But by the time you're done, the black box might be completely inert and useless.

Any time I've dealt with users' electronic equipment on aircraft, I've treated them as "non-critical" equipment, then limited my testing to showing that the devices were not harmful to the aircraft. Tests were necessary before approval, usually in flight, and the same test plan would be repeated for every follow-on serial number of that aircraft type by the installer. The first test would be witnessed by a delegated engineer, the following tests by a licensed mechanic or avionic technician.

I don't see you circumventing the test requirements with this device, and needless to say, there are more ways for it to fail the test if it's not bonded somehow.

STF

 

[berkshire]

As a person who has worked on General aviation Composite aircraft for the last 43 years I can tell you bonding is a must.
Older fiberglass sailplanes had no conductive pathways in the wings and fuselages except for the control rods which were all bonded together with braided copper wire bonding straps then to a ground plane on the aircraft. Later general aviation powered aircraft used a conductive layer in the layup of the structure ( Thorstrand or carbon fibre) to carry lightning strikes or static away from the aircraft through static wicks .
This material had bonding straps connected for attachment to other parts of the structure and the control systems.
So unless your device has no need of an outside power source you are going to have to bond it.
B.E.

You are judged not by what you know, but by what you can do.

 

[VEBill]

This document (pdf link below) explains some of the fundamental purposes related to these requirements.

I'd suspect it would be better, faster, cheaper to include a bonding scheme in a new all-composite aircraft than it would be to try to leave it out and get it through Approvals. Sailplanes might be an exception due to weight and not being used for commercial passenger service.

Beware relying on simple bonding for EMI purposes. Being RF, there's a bit more to it than just miliohms.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980201283.pdf

 

[IRstuff]

I would imagine that testing would be done in accordance with the installation, so the bonding strap and grounding structure that would be used for installation is part of the testing.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[DHambley]

Berkshire, Good info about the conductive layer in composites. I went to hit the "Star it" button but hit the "Red Flag" by mistake. Then it went off the screen so I don't know what you'll see in your in-box. Sorry.

Darrell Hambley P.E.
SENTEK Engineering, LLC

 

[berkshire]

DHambley,
You can talk to the powers that be and they will rectify your mistake.
B.E.

You are judged not by what you know, but by what you can do.

 

[DHambley]

answering some of the questions: This is for commercial and general aviation for avionics in the cockpit as well as power for other LRUs. The goal is to save a few dollars by not supplying a grounding strap or spending additional money to remove anodizing from the case. This is very important to them and they are adamant about it. Client is saying that DO-160 does not require a grounded case. That surprised me, so I dove into it a bit. OK, so the only part which addresses grounding that I could find was in section 20.3.a.1. " . . . ground plane . . . shall be bonded to the shielded enclosure . . . it is recommended that the DC bonding resistance should be 2.5 milliohms or less."
Their key word is "recommended" not, "shall". OK, so he's correct on a technicality; great, until you try to explain that to the DER. Bottom line though, this box has to meet EMI as well as HIRF, Lightning, pin injection, etc, etc. In spite of what I've told them about the need for a grounded case they continue with counter-arguments to that. I can't agree that the design will meet all specs without a grounded case so we're at an awkward disagreement.

Darrell Hambley P.E.
SENTEK Engineering, LLC

 

[IRstuff]

" it is recommended that the DC bonding resistance should be 2.5 milliohms or less."

Your guys are misinterpreting that requirement. It does not imply that bonding optional; it recommends a VALUE for the bonding resistance, which implies that the bonding resistance could be allowed to be higher, not that it be infinite. This is because 2.5 milliohm is often very difficult to achieve, so if it were 5 milliohms, that would be much better than nothing.

Bonding of the chassis and the ground is a must. If the plane gets a lightning strike, the ungrounded chassis could get some arcing, which may damage the equipment unnecessarily, or potentially spark some sort of fire.

The full wording of the requirement states: "When a shielded enclosure is employed, the ground plane shall be bonded to the shielded enclosure at intervals no greater than one meter and at both ends of the ground plane. It is recommended that the DC bonding resistance should be 2.5 milliohms or less."

The first statement is a "shall" The enclosure is required to be bonded to the ground plane. The second sentence is the recommended value of that bonding resistance. There is absolutely no ambiguity about the requirement for bonding. Your customer is pipe dreaming.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[VEBill]

The goal is to save a few dollars by not supplying a grounding strap...

It would be unusual to supply a ground strap with an LRU. For one thing, the avionics OEM doesn't know how long it needs to be. The bonding strap would be determined at installation or installation design, and typically supplied by the installer.

Sometimes the LRU can be bonded by fayed surfaces at installation. A lazy OEM can even let the installer deal with scraping through any anodization, provided they don't mind getting boxes back in such condition.

The bonding resistance can be spec'ed higher, ohms not milliohms. But you have to justify that in terms of what's inside the box. For example, if the LRU contains aircraft power, then you need a low enough bonding resistance to properly pop the breaker in the event of a failure, while keeping the LRU ground grounded.

I don't buy the EMI justification for extreme bonding because in normal operation the current in the bonding strap should be essentially zero. If it's not essentially zero, then there's something strange going on. And if one is assuming significant currents in the bonding strap during normal operation, then the strap should be coaxially shielded to keep it from radiating EMI (LOL).

If the box is metal, just include a grounding stud, or a hardware stackup of a screw, washers, and nuts to the same function. Or provide a grounding tab and let the installer connect to it.

The LRU and its manufacturer plate will outlive the memory of the price, so it might be better to do it right.