AMC 25.899 Electrical bonding and protection against static electricity

ED Decision 2020/024/R

1       Protection against Lightning Discharges.

Refer to CS 25.581; 25.954; 25.1316 and associated Acceptable Means of Compliance.

2       Characteristics of Lightning Discharges.

Industry standards.

Refer to EUROCAE document ED-84 (including Amendment N°1 dated 06/09/99) titled: Aircraft

Lightning Environment and Related Test Waveforms; or the equivalent SAE ARP5412 document.

The following documents may be used when showing compliance with CS 25.899:

—          EUROCAE document ED-84A dated July 2013 (Aircraft Lightning Environment and Related Test Waveforms) or the equivalent SAE ARP5412B.

—          EUROCAE document ED-91A (Aircraft Lightning Zoning) or the equivalent SAE ARP5414B.

—          EUROCAE document ED-105A (Aircraft Lightning Test Methods) or the equivalent SAE ARP 5416A.

—          EUROCAE document ED-113 (Aircraft Lightning Direct Effects Certification) or the equivalent SAE ARP 5577.

3       Protection against the Accumulation of Static Charges

3.1     General. All items, which by the accumulation and discharge of static charges may cause a danger of electrical shock, ignition of flammable vapours or interference with essential equipment (e.g. radio communications and navigational aids) should be adequately bonded to the main earth systems.

3.2     Intermittent Contact. The design should be such as to ensure that no fortuitous intermittent contact can occur between metallic and/or metallized parts.

3.3     High Pressure Refuelling and Fuel Transfer. Where provision is made for high pressure refuelling and/or for high rates of fuel transfer it should be established, by test, or by consultation with the appropriate fuel manufacturers, that dangerously high voltages will not be induced within the fuel system. If compliance with this requirement involves any restriction on the types of fuel to be used or on the use of additives, this should be established.

3.3.1  With standard refuelling equipment and standard aircraft turbine fuels, voltages high enough to cause sparking may be induced between the surface of the fuel and the metal parts of the tank at refuelling flow velocities above approximately 7 meters/second (23 feet/second). These induced voltages may be increased by the presence of additives and contaminants (e.g. anti-corrosion inhibitors, lubricating oil, free water), and by splashing or spraying of the fuel in the tank.

3.3.2  The static charge can be reduced as follows:

a.       By means taken in the refuelling equipment such as increasing the diameter of refuelling lines and designing filters to give the minimum of electrostatic charging, or

b.      By changing the electrical properties of the fuel by the use of anti-static additives and thus reducing the accumulation of static charge in the tank to negligible amount.

3.3.3 The critical refuelling rates are related to the aeroplane refuelling installations, and the designer should seek the advice of fuel suppliers on this problem.

4       Primary and Secondary Bonding Paths.

(Reference : CS 25.581; 25.899, 25.954; 25.1316; 25.1353; 25.1360.)

4.1     Primary bonding paths are those paths which are required to carry lightning discharge currents. These paths should be of as low an electrical impedance as is practicable. Secondary bonding paths are those paths provided for other forms of bonding.

4.2     Where additional conductors are required to provide or supplement the inherent primary bonding paths provided by the structure or equipment, then the cross-sectional area of such primary conductors made from copper should be not less than 3 mm² except that, where a single conductor is likely to carry the whole discharge from an isolated section, the cross-sectional area would be not less than 6 mm². Aluminium primary conductors should have a cross-sectional area giving an equivalent surge carrying capacity.

4.3     Primary bonding paths should be used for –

a.       Connecting together the main earths of separable major components which may carry lightning discharges,

b.       Connecting engines to the main earth,

c.       Connecting to the main earth all metal parts presenting a surface on or outside of the external surface of the aeroplane, and

d.       Conductors on external non-metallic parts.

4.4     Where additional conductors are required to provide or supplement the inherent secondary bonding paths provided by the structure or equipment then the cross-sectional area of such secondary conductors made from copper should be not less than 1 mm². Where a single wire is used its size should be not less than 1·2 mm diameter.

5       Resistance and Continuity Measurements. Measurements should be made to determine the efficacy of the bonding and connection between at least the following:

5.1     Primary Bonding Paths

5.1.1  The extremities of the fixed portions of the aeroplane and such fixed external panels and components where the method of construction and/or assembly leads to doubt as to the repeatability of the bond, e.g. removable panels.

5.1.2  The engines and the main aeroplane earth.

5.1.3  External movable metal surfaces or components and the main aeroplane earth.

5.1.4  The bonding conductors of external non-metallic parts and the main aeroplane earth.

5.1.5  Internal components for which a primary bond is specified and the main aeroplane earth.

5.2     Secondary Bonding Paths

5.2.1  Metallic parts, normally in contact with flammable fluids, and the main aeroplane earth.

5.2.2  Isolated conducting parts subject to appreciable electrostatic charging and the main aeroplane earth.

5.2.3  Electrical panels and other equipment accessible to the occupants of the aeroplane and the main aeroplane earth.

5.2.4  Earth connections, which normally carry the main electrical supply and the main aeroplane earth. The test on these connections should be such as to ensure that the connections can carry, without risk of fire or damage to the bond, or excessive volt drop, such continuous normal currents and intermittent fault currents as are applicable.

5.2.5  Electrical and electronic equipment and the aeroplane main earth, where applicable, and as specified by the aeroplane constructor.

5.2.6  Static discharger wicks and the main aeroplane structure.

6       Electrical Properties of Composite Structure

6.1     In the case of lightning protection, for the partial conductors the method of surface protection will vary with the criticality of the structure in question. Deterioration of the means of protection or possible hidden damage to the material which may affect its structural integrity, need to be considered. While such materials provide a measure of electro-magnetic screening, the need for additional measures will be a function of the location of the material in relation to critical equipment and wiring in the aircraft. Particular attention will also have to be given to the protection required near fuel systems – e.g. fuel tanks.

For non-conducting materials which have no intrinsic lightning protection or screening properties, the measures taken will again depend on the relative locations of the material and critical systems or fuel and the possible loss of the components due to internal air pressures in the event of a strike.

6.2     The partial conducting materials should present no problem in dissipating P-static but problems can arise with the non-conductors. Depending upon the location of the material, protection may be required.

6.3     Electrical currents, other than lightning, can flow in some partial conducting materials and means may be required to limit this by provision of alternative current paths if the effect of large voltage drop is important or if such currents can damage the material.

6.4     Particular care has to be taken that all joints, permanent and temporary, are capable of carrying any currents which may flow particularly those resulting from lightning strikes. Structural damage and loss of screening capabilities may occur if these are not adequately controlled.

6.5     The adequacy of the material in supplying a ground plane for antenna may have to be considered. Again it will vary with the material and the radio frequency of the system.

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