July 2014 Issue
Modern electrically-intensive airplanes demand excellent grounding.
In the flight to electronic ignitions and glass panels, we're glossing over an underlying fundamental says Robert Paisley of engine management specialist EFii. That fundamental is a robust electrical system befitting low-voltage electronic circuits, rather than the still somewhat common, haphazard wiring dating from the tube-and-rag days of Experimental building.
In short, as Experimental aircraft have become increasingly electrically intensive, the bones of the electrical system haven't kept pace.
We didn't have to go farther than our veteran Starduster Too when searching for an example of old-school homebuilt wiring. This rat's nest lives behind the front cockpit seat and exemplifies what happens when basic knowledge, expediency, and years of add-on electrical gadgets come together. Planning ahead with busses able to accommodate future additions will avoid this.
To be more specific, most builders have a workable understanding of the electrical system's supply side, but the ground system is less well understood or considered. This is natural, as it's obvious a wire needs to be run from the electrical source to the electrical consumer, but assembling a cohesive ground system is not so front-and-center. Often it's assumed the airframe is the ground circuit and therein lies the fundamental of Paisley's number one dictum, "The airframe is not a ground."
Instead, to achieve trouble-free operation of sensitive electronics, the airplane must have a ground system, or, if you want to think of it that way, an extended ground bus. In simple terms, all grounds in the airplane run to the ground bus, and the ground bus is in turn grounded to the battery. This means running more ground wires, with a corresponding minimal weight penalty, but the advantage is more robust, reliable ground circuits. Overall, such "star" circuits are best able to handle the more delicate, low current needs of electronics and go far in reducing glitchy and noisy electrical and electronics operation. If you're constructing a dual electronic ignition or electronically fuel-injected kit aircraft with an EFIS instrument panel, Paisley is talking to you. Even if you're not, his recommendations will still result in a reliable, capable, adaptable, and easily serviced ground system.
Now this is better: a recent Rans S-7 build. The ground bus at right is grounded to a firewall pass-through bolt out of sight in this photo. Labeled wires and good organization ease troubleshooting; open slots on the busses are ready to accept more electrical equipment.
The heart of Paisley's recommendations is to provide a single airframe ground stud mounted on the firewall. As shown in the diagram, this single ground stud provides both a convenient place to ground all electrical circuits and is a firewall pass-through for the ground circuit.
Additionally, two separate ground busses should be provided, one for the avionics and another for other high-current loads. Segregating the avionics from starting, lighting, and other heavier-load circuits helps reduce noise in the avionics. Both ground busses are then grounded to the single firewall grounding stud.
Co-locating the grounds on a bus has the practical advantage of making for easier troubleshooting during maintenance. If there is a ground fault, you'll know where to look, instead of having to play "Where's Waldo" around the airframe, and that's a dividend you'll enjoy long after the airplane is flying.
With the Paisley Ground Bus Architecture, the airframe isn't used as a ground connection for any systems. The airframe is grounded only at the firewall pass-through. Engine grounds connect to a case bolt, never to any motor mount bolts. (Drawing courtesy of EFii.)
While we could paraphrase Paisley's specific ground system nut-and-bolt recommendations, the following summation by Paisley himself sums it up best:
Electrical System Grounding
By Robert Paisley
The number one source of electrical problems in Experimental aircraft is a poor ground system layout. Modern aircraft typically incorporate multiple systems that have microprocessors and other sensitive electronic devices. On any modern vehicle, it is imperative to follow good grounding practices. If all of the recommendations below are followed, you should never have an electrical problem due to a grounding issue.
1. The airframe is not a ground. Electrical systems should be grounded to the ground bus system, never to the airframe or engine. The airframe should be grounded to the battery ground system. Electrical current should never have to run through the airframe to return to the battery.
Paisley's personal RV-7 illustrates the single firewall pass-through ground. In this case it's the bolt with ground wire at 4 o'clock to the solenoids. Robert is holding the two ground wires that attach to the engine's accessory case.
2. The ground bus system should center around a firewall pass-through bolt. The easiest way to set this up is to use a piece of 5/16- or 3/8-inch threaded rod, which passes through the firewall and has a nut on each side of the firewall to secure it. The firewall pass-through bolt should be electrically connected to the firewall with direct contact between the nuts and the metal firewall. The threaded rod should have enough remaining length such that after installation in the firewall, there is enough thread remaining on each side to attach two or three ring terminals and an additional nut and lock washer. Everything else in the aircraft should have its ground wiring run to this pass-through.
3. Use a single ground point to feed all systems and battery. This is referred to as a "star" ground layout because if you draw a sketch of the ground system, the ground wire runs radiate out from the central ground point. All modern vehicles use a "star" type ground layout.
4. Below is a list of items that are typically connected to the firewall pass-through bolt, with suggested wire gauges.
Battery—Battery grounds should run directly to the firewall pass-through with a 4 ga. wire. If there are two batteries, their grounds can be connected at the batteries with a 4 ga. wire, and a single 4 ga. wire can run up to the firewall pass-through.
Although common, grounding through an engine mount at the firewall is not ideal. Mechanically, stacking soft electrical ring tabs under the bolt head makes for a squishy installation that can reduce mounting bolt torque. Electrically this bolt moves more than you think and is prone to corrosion. Better to provide a dedicated ground lug through the firewall.
Engine—Two separate 4 ga. ground wires should run from the engine case directly to the firewall pass-through bolt. These wires should connect to engine case bolts, never to motor mount bolts. The engine sump bolts or accessory case bolts are generally good spots. Make sure to remove enough paint from the contact area to ensure a good electrical connection. Starter motor currents and spark plug currents return through this path. This is a very important connection.
Avionics Ground Bus—All other aircraft systems should be grounded through the avionics ground bus. This can be a "field of tabs" bus or simply a copper strip to accept ground wires from each of the systems. The avionics ground bus should not be grounded directly to the airframe. It should be locally isolated from the airframe with appropriate mounting. An 8 ga. wire should connect the avionics bus to the firewall pass-through bolt to complete the ground path.
Other High-Current Grounds—If you have systems with high momentary or continuous currents that require a ground path, these should be grounded directly to the firewall pass-through and not through the avionics ground bus. This can help keep electrical noise out of radios or other sensitive devices. For example, the EFii systems typically have one or two ring terminals in the middle of the harness; these are the ignition coil and fuel injector ground returns. These should be connected directly to the firewall pass-through bolt.
Noisy, high-voltage and high-amperage loads should be grounded separately from avionics. EFii provides such segregation in their EFI wiring looms with ignition coil grounds run to the single ring terminal and the other two wires for the ECU grounds.
Finally, we'll close with some quick advice of Paisley's on the positive side of the electrical diagram. Says Paisley, "Today dual electronic ignition is the norm for the electronically savvy EAA guys. This has good market penetration, but with two electronic ignitions we now have a problem with the electrical system. The [traditional] aviation 12-volt system isn't robust enough for an electrically dependent engine."
EFii sells this ground bus bar. It can be mounted via only the outer two bolt holes, thus isolating it from the airframe as in an avionics bus, or with the center bolt, which picks up the copper strip to provide a ground when attached to the firewall pass-through bolt.
"So, the notion of an essential bus is the thing for dual ignition and glass panels." The essential bus is a protected 12-volt circuit powering only mission-critical items such as the electronic fuel injection and ignition systems. It's in addition to the main bus, which powers everything else.
This is exactly what to avoid, three unidentified grounds run to a rusty tab somewhere on the airframe. Old, simple homebuilts got away with this sort of quick-and-dirty work, back when electrical systems were basic and 12 volts were everywhere. But in the electronic age such workmanship is for the airport manager's utility trailer.
For contemporary builders, EFii offers the Bus Manager at $575, plus a $145 accessory kit with all switches, relays, and solenoids needed to fully implement Bus Manager functions. This little black box automatically delivers the needed main and essential bus architecture and works well with the popular VPX box, which gives electronic circuit breaker protection, but not an essential power supply bus.
When mounted in the aft fuselage, it's tempting to ground the battery to the airframe (white wire), and in a clean, steel-tube installation such as this, a simple electrical system often works fine. But it's still better to ground the battery to the firewall through-bolt along with everything else. And, of course, composite aircraft can't ground through the airframe, so they must run a ground wire to the main grounding stud. In this Rotax-powered airplane, the solenoid acts like a master switch to keep the long positive wire de-energized when not flying.
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