Airframe as Return Leg of Electrical Circuits

Discussion in 'Instruments / Avionics / Electrical System' started by GESchwarz, Sep 9, 2019.

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  1. Sep 12, 2019 #41

    Winginitt

    Winginitt

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    Why not run individual ground wires AND also make short ground wire connections to the airframe. Then if an individual wire should develop a problem, there still is a path to a ground.
     
  2. Sep 12, 2019 #42

    Dan Thomas

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    Way too much overthinking going on here. Some of us think we need dedicated ground wiring for everything. Some figure it's necessary for just the stuff that makes noise. Some figure it's a waste of time and money and weight. And then the story about the Australian NDB/T-18 ADF comes up, pointing out that there are numerous and various ways for things to fail, so we'd better consider all that. And I exhort mechanics to check contactor and connection voltage drops through the starting circuit before replacing a supposedly weak battery or starter, so there's that, too. Worn stuff.

    Cessna and Piper and Beechcraft and Mooney and Champion and deHavilland and Avro and Aerospatiale/Socata and Rockwell and about a hundred other manufacturers have been using airframe grounds for about, oh, 90 years or so, and crimp-style connectors for more than 60 years, and they still do it, indicating that the risks and warranty comebacks are insignificant. They did it and do it because it's the least expensive and the least weight and the least build time, all factors important to the average homebuilder. Why do we have to reinvent the wheel, adding so much misery to a process that adds so little return?

    All mechanical things degrade with time and use. An airplane rots whether it flies or not; no one knows that better than a mechanic like me that has seen low-time 40-year-old airplanes that need extensive work to get flyable again. It won't matter how many extra ground paths you add, or how much weatherproofing of connectors you do; the elements will do their thing and destroy them, and flight hours will wear them out. Which is the whole reason the feds demand annual and/or periodic inspections and testing. Inspections are supposed to catch corroding and deteriorating wires and connections. I get under the panel of the airplane and see that the wires on the breakers and ignition switch and so on are tight and that there's no evidence of heating (discoloration) that indicates resistance forming, or green copper oxide from brass and copper terminals and wires corroding. Make sure all the lights and radios are working properly, without alternator or strobe noise. And yet, when I saw some airplanes for the first time, I'd find all this stuff and it hadn't just happened in the year since the last inspection. Previous mechanics were either lazy or just keeping the bill small so the customer would come back every year.

    You know the stuff I check most closely? The exhaust system. If a cracked pipe fails in flight you have a huge inflight fire risk and plenty of carbon monoxide to kill everyone quickly. If a pipe or muffler used for cabin heat develops even a small crack, it will slowly incapacitate the occupants and they will die. Aircraft exhaust systems run red-hot most of the time and suffer corrosion and stress and can't be expected to last forever. In Canada we have an AD that demands an inspection of that heating system, on every airplane that uses exhaust heat, every year or 150 hours to find cracks that could kill, and yet I've found multiple cracks that surely didn't all develop in the 10 or 15 hours since the annual inspection.

    The propeller is another good-look item. Nicks in the blade can create stress risers that cause cracks, and a blade crack will progress until a big chunk leaves and imbalances the prop so much that it might tear the engine off the airplane. What happens to the CG when 300 pounds of engine and prop leave the firewall? It won't even glide. All from a little nick that wasn't cleaned up and inspected. Rare, but it happens. The propeller is probably the most highly loaded part of the whole airplane, with huge centrifugal forces on it (among others) and is more likely to fail than, say, a wing spar.

    Build it so it works safely, and maintain it well.
     
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  3. Sep 12, 2019 #43

    PW_Plack

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    Because every redundant ground path creates another ground loop, which is an antenna for noise pickup.

    If the resistance of the connection between equipment and ground changes in resistance even slightly,it can be enough to put instruments out of calibration, introduce feedback squealing on com radios, and cause other gremlins that make this all look like a black art.

    Using the airframe as a ground return means any relative movement between panels, no matter how small, creates a varying resistance. I don't know, but strongly suspect, that reported radio issues of noise which occurs when the aircraft is in motion but can't be recreated during run-up may be caused by this.

    In audio and broadcast studios, the standard 19-inch equipment racks in which much of the equipment is mounted have optional insulated screws and/or shoulder washers, to keep equipment from being directly grounded through the rack to avoid ground loops. One studio I worked in had separate ground wires on each electrical outlet, all routed back to a common ground point at the breaker panel, specifically for this reason.
     
  4. Sep 12, 2019 #44

    BJC

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    It’s not?


    BJC
     
  5. Sep 12, 2019 #45

    Derswede

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    PFM, pure F'ing magic. It was one of the last courses in my university education. Also, a major class was "Confusing Managers, how to preserve Engineering Leadership in a Marketing World."

    Derswede
     
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  6. Sep 12, 2019 #46

    12notes

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    That's a good argument if you're only going to own the aircraft for two years, which is the length of the warranty on Cessnas and Pipers. It doesn't matter to them if the electronics go bad after that.

    An extra ground is not "so much misery" or build time, it's simple and usually takes about the same time. You still have to terminate the ground wire if you're using the frame ground, it doesn't make a bit of difference in time which end of the cable you're terminating. If you're using a ground block or strip, you don't have to drill/debur and tap all the extra holes for grounds at the equipment end, which takes about the same amount of time as needing to splice the ground wire. All the crimp connectors are installed in one spot. Running a 2 conductor wire takes the same amount of time and effort as running a single conductor.

    Flying around with constant buzzing or crackling in your ears can make an otherwise normal flight a miserable one. Flaky readings from sensors can add a bunch of stress to a flight. Is it the only cause of these problems? No, but it is one of the most common and spending an extremely small amount or zero time now can save you hours of diagnosis later and misery later.

    The reason some of us use separate grounds is similar to the reason we use alclad aluminum, or paint steel parts. Not because these assure that these parts will never fail, not because these eliminate the only ways these parts can fail, but because it moves the most common failure point of these parts years further into the future.

    If you're fine with using a frame ground, great, go ahead. Those of us who don't find it more important to us not to have an annoying problem crop up from the most common cause than to save the minimal amount of weight it costs. If you don't agree, fine, don't build your plane that way. But don't accuse those who run separate grounds of reinventing the wheel and adding "misery" to a project, because that's clearly not the case.
     
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  7. Sep 12, 2019 #47

    Dan Thomas

    Dan Thomas

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  8. Sep 12, 2019 #48

    Dan Thomas

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    Anyone wanting to run separate ground leads is welcome to it, but if they think they're circumventing a common problem, they're not. I have worked on many airplanes, some more than 70 years old, and ground loops creating noise in headsets and flaky radio and instrument readings was very rare. Maybe two percent of the total I've ever dealt with, if that, and almost all of it was repairable with simple fixes like isolating the mike and headset jacks from ground, or running separate ground wires for instrument senders, or just cleaning dirty ground connections. Didn't need to run ground wires for every load in the airplane.
     
  9. Sep 12, 2019 #49

    Dan Thomas

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    One should be aware that noise in the audio, or misbehaving avionics, has more than one cause (bad grounding somewhere). Alternator noise can be fed back into the bus from the alternator's field (back EMF) via the regulator/alternator output wiring. The alternator gets its field feed from the regulator, which takes it from the sense wire, which is connected to the alternator's output). The bus then transmits this wavy DC to other stuff that might not appreciate it. The alternator's output is also wavy, a characteristic of rectified AC, and much of this should be cleaned up by a filter between the alternator and bus or loadmeter. If that filter's ground is bad, the filter's performance is hurt.

    Strobe noise can also be fed back via the power wire. The old rotating beacons, with their motors, can make noise and feed that back to the bus.

    Bad grounding of the antenna cable shielding at either end can cause mysterious noises as well as some baffling issues with the alternator going offline or the ELT activating. If that shielding has a bad ground, RF escapes when you transmit, and the aircraft's wiring picks it up and turns it back into electricity. An electronic voltage regulator needs only microamps from the alternator's output for its voltage sensing function, and that RF makes the voltage go all snaky and the regulator interprets that as an overvoltage condition and shuts the alternator down. Cleaning the antenna cable bayonet connectors usually stops it. A separate ground would not help in that case, since the sheet metal is the antenna's ground plane and the shielding has to be solidly grounded to it.
     
  10. Sep 13, 2019 #50

    C Michael Hoover

    C Michael Hoover

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    First, a screw terminal can certainly be gas tight, especially if treated with an anti-oxidant when assembled.

    I started repairing hi-fi audio while still in high school and not too much later was a field service tech on facsimile receivers and transmitters. (All well over 50 years ago) So I am constantly concerned with the maintainability of anything that I work on. While I don't relish the idea of laying on my back (with a fuel select lever poking a hole in me), I will know that I have taken everything that I could into account to minimize that pain.
     
  11. Sep 13, 2019 #51

    Winginitt

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    Lots of well written opinions here with both sides adamant about what they believe works best. Simplistic answer seems to be that a builder needs to try what he thinks is best for himself and see how it works. My personal opinion is that I would rather have too many grounds than not enough. I'd rather have something that has two chances to work.....than one, even if it added a whole lb to the airplane. Then....,if I had a radio interference problem, I'd trouble shoot it. If I didn't have an interference problem, then I would be happy and move on. I would prefer a radio problem over the failure in flight of any electrical unit. That's just my opinion though........
     
  12. Sep 13, 2019 #52

    pictsidhe

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    Running seperate grounds for everything is a waste of time. It's also unlikely to improve reliability. I suspect that the contrary may be true. In the electrical trouble shooting that I do, half the problems are the ground wire... If I happen to have a hefty section of aircraft to use as a conductor, that will be more reliable and have lower voltage drop than a seperate wire. there are exceptions, certain things, like audio and radio, have grounding requirements that should be followed.
     
  13. Sep 13, 2019 #53

    rv7charlie

    rv7charlie

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    Thanks for admitting to my ignorance. :) I couldn't figure out EE-EL, either, but didn't want to admit it since I'm supposed to know something about herding electrons.

    WKT, your dad is one of my heroes. It was a crying shame when EAA took down the exhibit detailing his round the world flights in that T-18. I owned a T-18 for a short time, followed by various RV-x's. It's amazing how many in exp aviation think that Jon Johanson's RV-4 was the 1st homebuilt to fly around the world, and I blame EAA, at least in part, for not keeping your dad's achievements in our memory.

    Charlie
     
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  14. Sep 13, 2019 #54

    rv7charlie

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    Unfortunately, multiple ground paths often cause exactly the kind of problems you're describing. There are situations where multiple paths make sense, for redundancy. Most obvious is engine to airframe; two 'fat' ground wires are worth their weight (connected to different points on the engine and on the airframe), if you have an electrically dependent engine (electronic ignition and/or fuel injection). But intentionally adding multiple ground paths to most 'appliances' can add the potential for introducing noise, and make troubleshooting noise problems a real nightmare.
     
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  15. Sep 13, 2019 #55

    pictsidhe

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    Of those advocating multiple grounds, does anyone have any - electronics education? Being a ham counts.
     
  16. Sep 13, 2019 #56

    BJC

    BJC

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    526608BB-CB9B-41A9-B509-6F9D1C5DA1B4.jpeg
    I didn’t know that they took it down; it should be displayed. Their selection of displays is not what I would display.


    BJC
     
  17. Sep 13, 2019 #57

    rv7charlie

    rv7charlie

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    Oh, good; they've put it on display again. The display I used to see was much larger, IIRC there was a huge map covering the wall behind the plane. The last time I toured the museum (admittedly, at least a decade ago), the entire exhibit had disappeared.
     
  18. Sep 13, 2019 #58

    Winginitt

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    Charlie, to me the operative issue here is the continued use of the word "can". Lots of speculation that individual ground wires "can" or "may" or "might"......but no one saying that they "will" definitely/always cause a problem.
    No matter what configuretion someone chooses, they "may" still have a radio problem.
    Its up to everyone to "experiment" with what they think is the best/safest/most reliabile/ etc. and then test it.
    If they don't have a problem or do have a problem, the only way they will ever know for sure is to test it. It doesn't matter about all the speculation and what ifs on HBA. Even using only a fuselage ground is no guarantee that someone won't have a radio interference problem. Interference has always been a problem, yet most airplanes don't have individual ground wires. Single point failure is one of the biggest concerns in any type of airplane system. Possible interference, or possible component failure......which is most important ?:)
     
  19. Sep 13, 2019 #59

    rv7charlie

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    Perhaps I should have chosen my phrasing more carefully. Let's try it another way. Choosing individual ground runs for each device adds weight, but will (variably, depending on installer qualifications) improve the odds of a noise-free installation. For a complete electrical/electronic novice, who understands and is willing to give up the weight to improve his odds of success, then individual runs can make sense. Someone with experience in electrics/electronics (like me, for instance) might know enough about antagonists/victims to choose grounding non-noise-critical stuff locally and run separate grounds for antagonists/victims.

    Those of us who have a background in electrics/electronics realize that running multiple grounds, especially along different paths, for non-flight-critical devices will *increase the odds* of having noise problems, and *complicate the troubleshooting* of any noise problems that do occur.

    On 'single point of failure': Doubling ground runs addresses only one way (out of uncountable possibilities, most obvious of which is supply side) that any particular appliance can fail. It will certainly not double the likelihood of that appliance being available; the availability increase would be so small as to be un-quantifiable, unless we're working on the assumption that grounding practices are already beyond horrible. Properly executed ground installations will be no less reliable than anything else in the a/c. Now, if an appliance is flight-critical, then failure should be assumed, and the obvious answer is: a separate, independent, backup device. Period. With the obvious exceptions of things we can't carry backups for, like wings. But any flight-critical electronic item will be small/light enough that a backup device is feasible.

    Your homebuilt, your choices. But if we're discussing general guiding principles, my opinion is that doubling ground runs for the vast majority of appliances has more downsides than upsides. Mine will have redundant grounds for the electrically dependent engine and related components and 'best practices' for the rest of the a/c.

    Charlie
     
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  20. Sep 14, 2019 #60

    Winginitt

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    Well said. I respect your opinion......and the manner you expressed it.
     

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