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

4.5.8 Aircraft or capacitor discharge.  The rate at which a capacitor or an aircraft discharges is determined by its time constant and is given by the equation:

Equation:                                                                                                                                (7)

t = R·C

Where:

t = time in seconds

R = resistance of airframe to ground in ohms

C = capacitance of aircraft in Farads

A capacitor can be assumed to be completely discharged after five time constants or 5t.  Aircraft capacitance is a fixed quantity under a particular parked configuration.  Therefore, in order to reduce the discharge time, the resistance should be reduced.  Aircraft tires measured in the field have resistances as high as 40 M .  Aircraft capacitance values have been measured as high as

0.005 µF.  Using equation (7):

t = R·C

t= (40 x 106) · (0.005 x 10-6)

t = 0.2 second

5t = 1 second

4.5.9 Discoordination threshold levels.  Discoordination threshold levels of the human heart action have time durations as low as 0.2 second.  Thus, 1 second for the aircraft to discharge is unacceptably long.  If, for example, a maximum resistance of 10,000 ohms is considered, the same calculation yields a capacitance discharge time of 0.25 ms.  This is well below the 0.2 second critical value.  Therefore, it is necessary to reduce aircraft-to-ground resistance by grounding the aircraft, thus bypassing the aircraft tires and mooring chains.

4.6 Damage threshold levels.  The third variable needed for analysis is the threshold for injury or damage due to electrical effects.

4.6.1 Ordnance. A current level greater than 15 percent of the maximum no-fire value is the potential danger level during ordnance handling.  The ignition of EEDs in various actuating mechanisms is the most common danger area.

4.6.2 Sources of injury from shock.  There are two possible sources of injury to personnel from shock:

a. Involuntary reflex movements.  Involuntary reflex movements may occur, which can result in injury due to secondary effects, such as falling.

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