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

4.4.1.1.2.1 Charge accumulation. Wind-blown dust storm charges will accumulate until a sufficient time to discharge them has passed or a person touches the aircraft.  Twelve-hundred volts (1,200V) exceeds parameters for non-lethal shock to personnel and could cause a reflex action resulting in injury to personnel.

4.4.1.2 Induced static charge. Electrical storms involve the relatively slow movement of heavily charged clouds which set up an electrostatic field over a large area of the Earth's surface below the cloud.  The presence of an electrical field between an active storm cloud system and the Earth results in large induced charges on parked aircraft.  As shown on Figure 1, the negative charge in the cloud attracts a positive charge from the Earth onto the aircraft via the tires.  This charging usually occurs at a relatively slow rate which results in relatively small current flows that cause no damage to the aircraft.  If a person is in contact with the plane throughout the charging period, for example, as the charged cloud moves slowly overhead, he will probably feel no effect.  If the storm cloud loses its negative charges either by a lightning discharge to another cloud or to some spot on the ground, or even if the storm cloud merely moves on, the

electrostatic force of the negative charge holding the positive charge on the aircraft disappears. The positive charge on the aircraft will now return to ground via the tires.  Should the tire resistance be high, the discharge time may be long.  The positive charges are conducted from the ground up through the tires, building up charges of up to 60 kilovolts (kV) on the aircraft.  These are called induced charges.  Should a person now touch the aircraft and provide an alternate path to ground, the person will receive a shock which normally may do no more harm than to cause loss of balance, with perhaps a fall to the ground.  However, on rare occasions the shock could

be serious.  If the cloud above suddenly discharges to another cloud, the charged aircraft immediately discharges back to ground via whatever path is available, this includes the person touching the aircraft.  The person will now probably feel a sharp shock, the severity of which depends on the extent to which the aircraft was originally charged.  It can be seen that if the aircraft were connected to a static ground point (less than 10,000 ohms), the positive charge would take this path of least resistance back to ground and thus provide protection to personnel working about the aircraft.

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