NOT Grounding Airframe

[Karetaker]

Howdy everyone. I plan to run all electrical returns back to a home location and not have any local returns. (grounds) Should this single point of return also be isolated from the airframe?

Thanks.

[vlittle]

No. You must ground the airframe as well to prevent electrostatic buildup and to shunt lightning strikes. It's likely that many of the electronics items have a protective ground connected to their metal case that can benefit by being attached to the airframe (which is grounded). Of course, most antennae have a shield that must be grounded to the airframe.

There are two or three schools of thought about using the airframe in an RV as the ground return for electrical circuits.

Firstly is your method to provide individual grounds for all circuits. Lowest potential for electrical noise, but more than doubles the weight of power wiring (larger wire gauges x twice the wires). Also, more costly.

Secondly is using local grounds to the airframe. Lightest weight, lowest cost, but can lead to more electrical interference and audio noise.

Thirdly is a hybrid system. All audio connections use a single point audio ground and shielded cables. Noise generators (strobes, some LED lights) also used shielded cables and individual ground wires in the cable. Benign loads such as pumps, incandescent lamps, heated pitots and so on can use airframe grounds. So can the main battery. Keep avionics on a single ground point whenever possible.

Some will object to the potential corrosion effects of passing ground current through an airframe. Use the method in AC43.13 for the stack-up of washers to mitigate this at the points of connection. As for rivets passing large currents... I will defer to another authority, but I've never seen same-metal rivets corrode due to electrical currents in an aircraft.

V

p.s. the term 'Ground' when used in an aircraft can be confusing. The term 'Common' is more correct, but, uh, less common. Back in the day when I used to receive updates to my AIM/AIP that had to be inserted manually into a binder, some overzealous bureaucrat in Transport Canada decided to replace the term 'ground' with 'common' in the regulations pertaining to electrical systems. While technically correct, it required hundreds of pages of regulations to be replaced, often to replace a single word. Aside from TC having to print and mail this information to tens of thousands of pilots, we had to spend hours replacing pages in our binders.

[bjdecker]

Maybe a better response is "why would you NOT want to use the airframe as ground or common." Is there a problem, perceived or imagined, that you're attempting to work around?

Fiberglass aircraft comes immediately to mind, and that is standard practice for wiring Glasairs, Lancairs, Wheeler, et al.

[PilotjohnS]

If you dont use the aircraft structure as ground, some units may not operate as well. Some electronic boxes want to have super low impedance return so that the internal electrical noise filters work properly. Other units may provide a return wire, but may also have an internal connection from the return wire to the box?s chassis, defeating what you are trying to accomplish anyways.

[Karetaker]

Thanks. I didn't think about avionics chassis needing return paths through airframe.

[1001001]

Originally Posted by vlittle

No. You must ground the airframe as well to prevent electrostatic buildup and to shunt lightning strikes. It's likely that many of the electronics items have a protective ground connected to their metal case that can benefit by being attached to the airframe (which is grounded). Of course, most antennae have a shield that must be grounded to the airframe.

There are two or three schools of thought about using the airframe in an RV as the ground return for electrical circuits.

Firstly is your method to provide individual grounds for all circuits. Lowest potential for electrical noise, but more than doubles the weight of power wiring (larger wire gauges x twice the wires). Also, more costly.

Secondly is using local grounds to the airframe. Lightest weight, lowest cost, but can lead to more electrical interference and audio noise.

Thirdly is a hybrid system. All audio connections use a single point audio ground and shielded cables. Noise generators (strobes, some LED lights) also used shielded cables and individual ground wires in the cable. Benign loads such as pumps, incandescent lamps, heated pitots and so on can use airframe grounds. So can the main battery. Keep avionics on a single ground point whenever possible.

Some will object to the potential corrosion effects of passing ground current through an airframe. Use the method in AC43.13 for the stack-up of washers to mitigate this at the points of connection. As for rivets passing large currents... I will defer to another authority, but I've never seen same-metal rivets corrode due to electrical currents in an aircraft.

V

p.s. the term 'Ground' when used in an aircraft can be confusing. The term 'Common' is more correct, but, uh, less common. Back in the day when I used to receive updates to my AIM/AIP that had to be inserted manually into a binder, some overzealous bureaucrat in Transport Canada decided to replace the term 'ground' with 'common' in the regulations pertaining to electrical systems. While technically correct, it required hundreds of pages of regulations to be replaced, often to replace a single word. Aside from TC having to print and mail this information to tens of thousands of pilots, we had to spend hours replacing pages in our binders.

Is it OK/recommended to have a "ground bus" that everything is connected to, or have everything make home runs back to a single or small number of connected ground points?

By "ground bus" I mean essentially a heavy ground wire that snakes through the airplane and is connected to local ground terminals and bonds everything together.

[rv8ch]

Originally Posted by 1001001

Is it OK/recommended to have a "ground bus" that everything is connected to, or have everything make home runs back to a single or small number of connected ground points?

By "ground bus" I mean essentially a heavy ground wire that snakes through the airplane and is connected to local ground terminals and bonds everything together.

From what I've read, this would not be needed in a metal aircraft. If you do local ground to a very clean place, and put a dab of dielectric grease or some ACF-50 to protect it from corrosion, you'll have a pretty good path for your electrons.

http://www.aeroelectric.com/articles...y_Grounds.html

Bob Nuckolls devotes a lot of time to discussing grounding in his book, which will help you a lot on that topic and many others. Takes away the magic from electricity, at least at the voltages and currents we're using.