Wednesday, December 21, 2011

Aircraft Hose Bonding and Lightening Protection

Lessons from Agusta Bell 206B JetRanger II, G-AWMK

AN919 anodized aluminum fitting is an insulator
Current commercial and military-aircraft standard for electrical bonding: 0.0025 ohm for lightening protection and RF potentials.

Unlike other electrical systems, aircraft systems use the structure (skin and/or airframe) as a current-carrying-conductor. There is no "neutral" wire in aircraft. The aircraft skin and components (and hoses) carry the return current back to the battery.

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 unbonded hose (Augusta Bell 206B had an in-flight fuel-tank-explosion).

Aircraft can be hit by lightening. Bonded components help the lightening current to 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.

Most aluminum fittings have an insulation layer on their outer surface that prevents electrical bonding. This insulation layer is called "anodizing". Anodizing colors the aluminum (as the picture to the left shows) and protects it from corrosion. But, it is also an insulator. Removing and installing the hose or tube several times will wear through the insulation on the threads and sealing surfaces.
Aircraft Hose Assembly with anodized aluminum fittings. Anodized aluminum is an insulator.

In the case of the Bell 206B fue- tank-explosion, the hose was not bonded because of the anodized coating and this led to a static discharge between the unbonded hose and a nearby bonded hose. If your application requires bonding, then be careful with anodized aluminum aircraft parts. They are insulators.

Aircraft Teflon hoses (those meeting mil-specifications) have a conductive layer of carbon black to provide provide electrical conductivity and prevent static charges. Commercial (non-aerospace) hose does not have this static control.

Aircraft Teflon hose assembly showing carbon black inner-liner

Military Specification MIL-H-25579E requires that hoses (through -8)  be capable of conducting a direct current equal to or greater than 6 microamperes 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 teflon hose was first used on aircraft it developed pin-hole leaks. The plastic Teflon develops a static charge so great that it arcs to the grounded steel braid causing a small hole in the Teflon.

--story time--

The classic example of 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 grounded to the airframe. The only conductive path for the battery current was back through the mixture control cable.  I have also seen this happen with a metal braid oil pressure hose going from the engine to the gauge. All the starter current flowed through the hose braid. The braid lit up like a heater element and cooked the hose. Fortunately it was oil-pressure and not fuel-pressure.

Take a look at that fat #2 or #4 battery cable going to the starter. All of the current flowing from the battery to the starter must also flow back to the battery. It does this through the airframe. The current will take the path of least resistance. Good electrical bonds help deliver current to the starter for quick starts. They also help to keep the current out of your instruments and hoses where it doesn't belong.

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