# 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 10, 2019

### wktaylor

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In most aircraft with significant electrical equipment and volatiles [liquid/vapor fuel and other flammables], the airframe is for 'bonding/grounding' of tramp/stray electrical charges/discharges away-from/around systems/equipment/people/volatiles/etc. The term tramp/stray electrical charges includes power-shorts-to ground, lightning-strike/diversion, static/corona build-up/dissipation, etc. Circuit return wires provide clean/contained signals and rarely add significant weight.
-----------------------

Rockiedog2, BJC...

Runaway lithium-ion battery fires, and their high flame intensity, are not a statistically insignificant issue.

At least [2] 747-cargo aircraft have been brought-down by runaway LI battery fires in the cargo hold.

Early LI battery installs played hell in the Boeing 787-100s with serious fire-outbreaks and structural damage.

LI battery-pack fires have burned-up many computers and cell phones during charging.

Even a small LI battery pack fire between/around your feet will be a lot more serious situation that You can currently visualize... especially with unprotected fuel tubing/hose nearby... unless the batteries are isolated by a fluid/fire resistant box and flames/hot gas can be vented overboard safely.

Lesson Learned.
In the 1980s I worked on a small USAF attack jet. The aluminum oxygen tube [for supplying the crew] passed-over the [RH] engine 200-Amp starter-generator-relay box made of polyester-resin and fiberglass-matt [~9-inches]. Yep, a corroded conductor pin caused increased resistance that over-heated/ignited the SGR box which then led to the softening/failure of the O2 tube... which dumped pure O2-onto the low-grade fire. Ejection saved the pilots [LH seat]... not the copilots [RH seat].

Might want a 'peek' at the following...

FAA AC20-184 GUIDANCE ON TESTING AND INSTALLATION OF RECHARGEABLE LITHIUM BATTERY AND BATTERY SYSTEMS ON AIRCRAFT

DOT/FAA/TC-TN15/17 FIRE HAZARDS OF LITHIUM BATTERIES

FAA SAFO 16001 SAFETY ALERT FOR OPERATORS - RISKS OF FIRE OR EXPLOSION WHEN TRANSPORTING LITHIUM ION OR LITHIUM METAL BATTERIES AS CARGO ON PASSENGER AND CARGO AIRCRAFT

FAA TSO-C179 PERMANENTLY INSTALLED RECHARGEABLE LITHIUM CELLS, BATTERIES AND BATTERY SYSTEMS

LITHIUM BATTERIES http://www.iata.org/whatwedo/cargo/dgr/Pages/lithium-batteries.aspx

UL 1642 STANDARD FOR SAFETY – LITHIUM BATTERIES

etc...

2. Sep 10, 2019

### Rockiedog2

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Thanks for that. I'm aware of most of it. Considered it in my decisions.

No worries here but I advise others to worry, don't pay any attention to me.

edit: This might be of interest to some. The Earthx batts aren't LI, it's in the link. I don't know the science of the difference but am ok with it til have good reason not to be.

https://earthxbatteries.com/our-batteries/lithium-battery-technology

Last edited: Sep 10, 2019
3. Sep 10, 2019

### BJC

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Everyone that I know who is using a “Lithium battery” in an airplane is using the EarthX, which is a LiFePO4 battery.

BJC

4. Sep 10, 2019

### Rockiedog2

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Yeah same here.I think not near the fire hazard as the ones Boeing was burning up airplanes with. Once I did the research I haven't given it another thought other than to say I probably won't ever have anything else.

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5. Sep 10, 2019

### Dan Thomas

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But ground return wiring has its issues, too. Like the positive wiring, oxidation develops between the wire itself and the crimp terminals, and resistance starts to build. When you have one return wire and it get compromised this way, you have no alternate path like an airframe ground does. There are normally multiple paths through an airfame, and when trouble starts to show up it usually gives lots of warning. A failing ground wire will just heat up or fail altogether. A chafing ground wire might not short out to the airframe, of course, but its cross-section suffers and eventually it burns out. It's just more stuff to build, install, and maintain. You can solder all the terminals if you like, preventing the corrosion, but now you're increasing the electrical system build time by a factor of four or five. Cessna and Piper and the others have used crimp terminals since the 1950s and they're mostly still working. Circuits carrying heavy current loads will usually be the ones that make trouble: the battery and starter cables, the landing light and pitot heat wiring, and so on. An airplane that lets rainwater in is asking for lots of corroded connections.

A fuse in that return wire, as you stated, adds more connections, weight, cost, and more failure points. I have had fuses fail due to age and oxidation, and if a fuse blows or other wise fails like that in flight, how do you replace it? Do you install more fuseholders or breakers in the panel and run all the ground wires to them?

Better wire is a good idea. The usual PVC-insulated automobile wiring works and is cheap but it has some drawbacks. Its insulation chafes easily, it burns and smokes plenty, its wire is not tin-coated to resist corrosion and aid soldering. Aircraft wire such as M22759/16 has aTefzel (teflon) insulation that is tough, thin (less weight), doesn't burn, and the wire is tinned. But it's expensive. Aircraft Spruce wants $.67 per foot for 16-gauge M22759/16 wire, or$67.00 per 100 feet, where auto wire can be had for maybe $10 or$12 per hundred. You choose. The auto wire would be fine for low-current applications like audio systems, the expensive stuff for power. Firewall forward should have the good stuff.

As you say, you choose. But remember that many people have been building many airplanes for many decades and haven't bothered with return wiring unless the airplane was of wood or composite, and they fly safely. As I've said before, there are many other things that will kill you sooner; those are the things that should have time and money spent on them.

6. Sep 10, 2019

### BJC

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16 gauge is way too big for most wiring in E-AB, especially if LED lighting is used.

BJC

7. Sep 10, 2019

### rv7charlie

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To the OP:

Suggest getting a copy of Aeroelectric Connection, and read all of it, then study the section on grounds. (You can purchase a print copy, or download it in pdf form for free.)
edit: There's also an Aeroelectric forum/email list, where you can get answers to specific questions, often from the book's author.

The author (with many decades of experience in the certified and military aviation worlds, and a motivation to help us do 'better' in homebuilts) discusses noise issues in terms of 'antagonists' and 'victims'. Most electrical stuff will be neither, and using the airframe works fine for those.

Corrosion is corrosion, and on a properly built airframe there's not much (if any) more risk of using the airframe than running a bunch of ground wires. You can run them, but remember that the wings and motor have to lift them, on every flight.

Old style incandescent lights generate no noise. On the other hand, traditional strobe lights, and any of the newer tech lights (ex: LED systems) tend to have 'switcher' type power supplies that do generate noise ('antagonists'), and how much gets back into the DC supply and ground returns is highly variable by device.

Stuff that has extremely low level signals, like audio-related items, tend to fall in the 'victim' category and will likely benefit from dedicated grounds (ex: the recommendation to use headphone & mic jacks that are isolated from the airframe). But even in that case, *almost* all radios, audio panels, and intercom boxes have their grounds tied to their metal chassis.

Most (not all, but most) RF antennas just about demand a local ground to the airframe to work properly, and many noise problems can be traced to poor grounding of the antenna.

So it's really about using (un?)common sense in what you ground locally or with a dedicated ground run.

Charlie

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8. Sep 10, 2019

### Dan Thomas

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Yes, it's big. Typicaly 18 thru 22 gauge in most aircraft, with 16 to 12 gauge for incandescent landing lights, which shouldn't be bothered with anymore. LEDs can be noisy, but the certifed (STC/PMA) landing, nav and beacon lights lamps I installed I found no noise whatever when conducting the required tests after installation. Aircraft-grade LEDs aren't supposed to make noise.

I used the 16-gauge as a price point comparison between automotive and aircraft airframe wire. 18 gauge has a similar differential.

9. Sep 10, 2019

### BJC

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LED alone are not noise generators, but their power supply's are, because LEDs like constant current.

BJC

10. Sep 10, 2019

### C Michael Hoover

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I also strongly advocate that returns use a dedicated wire and not the airframe. Twisted pairs are by far the best way to go, especially for sensitive circuits, but also for ones that can cause noise, like strobes and motors. Further, a single point ground buss is the best way to wire the aircraft. A piece of solid copper mounted to the firewall with the battery negative connected to it with the same size conductor as used for the positive wire to the starter solenoid.

My beef with Mr. Nuckols of Aeroelectric Connection fame is his use of Amp Faston connectors. The name tells you what their purpose is. The crimp connection is gas tight if properly done with the correct tool, but the connection to the flag terminal is not gas tight and they do loosen with age and thereby cause high resistance which can cause all kinds of havoc. I am re-wiring a Bellanca Viking, every single wire. My IA has approved the use of two copper bus bars, mirrored on either side of the firewall and connected to each other with copper bronze bolts, effectively making a single point ground for each side of the firewall. All ground returns will connect to one or the other, including individual wires for the low side of the alternator and starter. Overkill maybe, but I can guarantee no noise in my headset! I have also degaussed the airframe so my compass works again.

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11. Sep 10, 2019

### C Michael Hoover

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You beat me to it! Automotive USB chargers for iPads, etc are another source of terrible noise.

12. Sep 10, 2019

### Dan Thomas

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Bellanca Viking. Fun to work behind the panel on that one, isn't it? Ugh. What's its age? They've been around for over 40 years now.

Big return cables for the starter can really improve performance by bypassing the typically dirty/oily/loose engine grounding stuff, but check the master and starter contactors, too, for any voltage drop across their big terminals while the starter is cranking. I have fixed numerous weak starters just by taking voltage drop readings and correcting the resistances. Contactor contacts oxidize with age and burn when they open, especially that starter contactor (inductive loads create voltage spikes when disconnected). The big cables oxidize or get oily between the cable and terminals, too, and sometimes cleaning and soldering will clear that up. Takes a torch to do that. On the bench, of course.

Bad engine grounding can create insrument reading problems. The senders for oil pressure and temperature and for some CHT systems use the engine ground, and if it's not perfect the alternator will try to pump electrons though the sender to the gauge, making it over-read. One solution for that is a small ground wire (20 gauge) from the engine case next to the sender, to the instrument's case ground, to eliminate the differential, but if the engine ground gets bad enough the starter or alternator current will cook it.

13. Sep 10, 2019

### 12notes

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If you're worried about fire, use LiFePO4 batteries instead of lithium ion or lithium polymer batteries. LiFePO4 batteries are much more stable, aren't susceptible to thermal runaway, don't catch on fire when punctured, hold their voltage until nearly completely discharged, and are much harder to ignite with overcharging. The fire risk is much reduced at a 14% penalty of energy density, which is about a half of a pound on a 12V 9Ah battery.

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14. Sep 11, 2019

### C Michael Hoover

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Fortunately, I have the windshield and panel out to make work a bit, (bit as in a microscopic amount) easier! Yes, all of your points are very valid, and I am doing just that. Hence the reason for the 6 inch long ground bus on the firewall. I am also going to the extreme of using a copper based anti-oxidant compound on all connections to the engine and to the ground bus.

15. Sep 11, 2019

### Charles_says

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I'm with Rockie here, I have an" Earthx (ETX 680)" battery. It's a LiFePo, if anyone is interested.
And the Humidity here in South Texas rivals that anywhere.

16. Sep 11, 2019

### gtae07

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The big airplanes at work use the airframe for current return. But the ground points are well-prepared and checked, and then get overcoated with sealant to help deal with corrosion issues.

I’m using dedicated return wires on my airplane, to deal with noise issues among other reasons.

17. Sep 12, 2019

### rv7charlie

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No question that there's lower risk of noise with pure single point ground (though it's impossible to achieve with most equipment having grounded cases). But the compromise is added weight, often with zero improvement in noise reduction.

re: Faston tabs. Do what you like, but everything's a compromise. Screw terminals, as used in 'traditional' a/c wiring, have the same non-gas-tight issue as Fastons, and can be loosened by vibration plus tension on the wire pulling the terminal. While there are cheap 'hardware store' tab style terminals that can lose their tension, high quality Faston style terminals from reputable mfgrs don't loosen over time.

Five years after the completion of the Viking restoration, when you're back under that panel for an avionics upgrade, with a flat blade screwdriver trying to re-insert one of those 1/4" long screws sideways into a terminal strip, get back with us on how you feel about your choice.

18. Sep 12, 2019

### wktaylor

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Does everyone know how to check electrical resistance [resistivity] across an EE-EL connection... and specifically across connector-to-connector and grounds-to-structure???

IF any of You have access... USAF T.O. 1-1A-14 [USN NAVAIR 01-1A-505-1] ORGANIZATIONAL, INTERMEDIATE, AND DEPOT LEVEL MAINTENANCE - INSTALLATION AND REPAIR PRACTICES - VOLUME 1 - AIRCRAFT ELECTRIC AND ELECTRONIC WIRING have great info, this subject.

And, Yes, grounding point... and CONNECTOR... maintenance on military, commercial and GA aircraft maintenance [resistivity checks, corrosion protection, fluid-tightness, FOD, etc] is a VERRRRRY big deal... ESPECIALLY for aircraft in high humidity and oceanic climates. I cannot tell you how many EE-EL CND** investigations I participated-in on USAF flightlines ultimately involved loose/corroded/intermittent grounding or dirty/corroded/wet/FOD-contaminated [shavings, etc] connectors... OH YEAH... and aged/over-loaded silently-failed circuit breakers. CND** = can not duplicate

19. Sep 12, 2019

### BJC

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20. Sep 12, 2019

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