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

4.4.2.4.5 Charge transfer.  The charge transfer, Q, is defined as the integral of the time- varying current over its entire duration, or:

Equation:                                                                                                                                 (5)

t

Q =  I o

idt (Ampere- seconds)

Where:

Q = charge in coulombs t = time in seconds

i = current in amperes

4.4.2.4.6 Action integral.  The action integral of a current waveform is a measure of the ability of the current to deliver energy and is defined as the integral of the square of the time- varying current over its entire duration.

Where:

t = time in seconds

i = current in amperes

t     i2  dt (ampere2-seconds)

o

4.4.2.4.7 Aircraft grounding.  Grounding an aircraft offers some protection against the effects of lightning, even a direct strike.  A major direct lightning strike usually is preceded by less powerful leaders that determine the direction of the main strike.  Grounds can control the direction of the leaders away from the tires and consequently control the direction of the main strike.  In addition, if a direct strike occurs, the ground wire may vaporize contributing to the conductivity of the ionized path, and thereby minimize leakage currents through nearby personnel.

4.4.2.4.8 Lightning discharge through aircraft.  Lightning discharge through an aircraft to Earth is an extremely variable phenomenon.  Lightning voltages as high as 100 MV and currents up to 650 kA are cited in literature.  At such high levels, grounding will not afford the degree of protection or confidence factor attained for static electric protection.  Nevertheless, a safety ground will aid in protecting personnel and equipment to some extent, especially for the lower energy strikes.

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