MIL-HDBK-274 Electrical Grounding For Aircraft SafetyThis handbook has been prepared to provide U.S. Navy and Marine Corps aircraft maintenance and operations personnel with the background and technical details of the different types of aircraft grounds and how they are applied. In addition, this handbook provides background information for aircraft grounding, static electricity theory and how it affects aircraft, and techniques used for measurement of grounding points. This handbook is intended for use by all U.S. Navy and Marine Corps aircraft maintenance and operations personnel for the purposes of ensuring that aircraft are properly and safely grounded. This handbook is for guidance only and cannot be cited as a requirement.

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MIL-HDBK-274A

b. Power

(1) Ground Fault

(2) RF Electromagnetic Energy

(3) Lightning

4.4.1 Static energy sources. The static sources listed in 4.4 are defined and evaluated in the following paragraphs.

4.4.1.1 Triboelectric effects. These are generally associated with buildup of static charges on an aircraft in flight. High static voltages, however, can result from interaction at the contact surfaces of various materials in any type of relative motion, for example, wind-blown snow or dust particles striking a parked aircraft.  A conservative estimate of electrical current in a moderate wind-blown dust situation is 30 microamperes (µA).  This type of static electricity is also called Precipitation Static or P-Static.  Connecting the aircraft frame to a static ground point provides a conductive path back to ground for the static charges as they are generated.  A static ground point is defined as a ground point having less than 10,000 ohms resistance to Earth.

4.4.1.1.1 Triboelectric effects example. A helicopter exemplifies triboelectric effects. Static voltages in excess of 1,000 volts (V) are normally generated by a hovering helicopter. Personnel must use extreme care when approaching a hovering aircraft.  Strict observance of the prescribed aircraft manual procedure will prevent the serious injury certain to occur if the aircraft is touched prior to grounding.

4.4.1.1.2 Wind-blown dust storm. A conservative estimate of electrical current for a moderate wind-blown dust situation is 30 µA. In an ungrounded aircraft, this current flows from Earth to the snow, dust, etc., via the tires and airframe.  Thus, the potential between airframe and Earth is determined almost entirely by the tire resistance.  A worst-case value of 40 megohms (M ) for tire resistance is used to compute an airframe potential of:

Equation:                                                                                                                                (1) V = I·R

= (30 µA) (40 M ) = 1,200 V

Where:

V = potential between the airframe and Earth

I = current from the airplane to Earth

R = resistance from the airframe to Earth

(For further information on precipitation static refer to the MIL-STD-464.)

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