Electrical Currents and Aircraft Hose

 Bonding and return currents:

Aircraft electrical systems use the structure (skin and/or airframe) as a current-carrying conductor. There is no "neutral" wire in aircraft. Take a look at that fat battery cable going to the starter. All of the current flowing from the battery to the starter must also flow back through the airframe to the battery to complete the circuit. Current takes the path of least resistance and travels through the hose if the engine isn't properly bonded to the airframe. Good electrical bonds help to keep the current out of your hoses where it doesn't belong. 

The classic example of poor electrical bonding is the pilot who complained that every time he started his airplane the mixture control knob got warm. It turns out that his engine mount wasn't bonded "grounded" to the airframe. The only conductive path for the battery current was back through the mixture control cable.  I've also seen this happen with a metal braid oil pressure hose going from the engine to the gauge. All the starter current flowing through the hose braid lit it up like a heater element and cooked the hose.

Checking electrical wire connections from the battery to the starter is checking only half the circuit - good electrical bonding completes the electrical circuit while avoiding current flow through hoses.

AN818 fitting 

Anodized aluminum hose fitting can be an electrical insulator

Aircraft can be hit by lightening. Bonded components help the lightening current flow through the airframe without arching.  A bonded component is where a electrical conductive path exists between two aircraft parts. A common example is the installation of a bonding strap between the engine mount and the airframe. A metal braid fuel hose or metal tube that is not bonded may have the potential to create arching or sparking during a lightening strike.

Anodized hose coupling is also an electrical insulator

Aluminum fittings are anodized. Anodizing colors the aluminum and protects it from corrosion; but, it's also an insulator that prevents electrical bonding.  If your application requires bonding, then be careful with anodized aluminum aircraft parts; they may be insulators. Current commercial and military aircraft standard for electrical bonding is a resistance under 2.5 milliohms (0.0025) ohm!

Static Electricity:

Aircraft can develop high static electrical charges as is evidenced by the need for static dischargers. Arching can occur between aircraft parts that are at different electrical potentials. In some aircraft, hose is routed through the fuel tanks. Arching within the fuel tanks can occur if a bonded hose is within spark distance of an un-bonded hose ( Augusta Bell 206B G-AWMK had an in-flight fuel tank explosion. )

Teflon hose construction showing carbon black liner

Teflon hose inner liner showing carbon black layer on inside.

Controlling static discharge in Teflon hoses is controlled through the use of a carbon black lining in the hose inner tube. Notice how the Teflon liner is black on the inside. There is a thin layer of carbon impregnated into the Teflon for control of static electricity. Commercial (non-aerospace) hose does not have this static control.

Military Specification MIL-H-25579E requires that Teflon hoses (through -8)  be capable of conducting a direct current equal to or greater than 6 micro-amperes with a test potential of 1,000 volts dc between the hose inner liner and one end fitting. This prevents the build-up of static charge and arc pin hole leaks to the wire braid.

When I first started selling Teflon aircraft hose in the 1980's many of the old-time mechanics would not use Teflon and called it "garden sprinkler hose" due to its tendency to develop pin hole leaks. Static electricity would build-up in the Teflon liner and then arc to the braid causing a pin hole. The use of carbon black solved the pin-hole leak problem.

Interesting, as I see some home-built aircraft using industrial and automotive Teflon that does not use carbon black. They could be re-creating the static induced pin hole problem that the aircraft industry solved 40 years ago!

The Mechanic's Toolbox Software program that I developed has extensive information on aircraft hoses, fittings, and a whole lot more.

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