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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5.7.2 Additional tie-down/grounding points.  Figure 14 shows additional tie- down/grounding points that may be encountered.   The manufactured unit shown on Figure 14 would normally be found in aircraft hangars while the field fabricated unit would be found on aircraft aprons.  The static ground receptacle shown on Figure 15 may be found at either location.  The grounding conductor may or may not be part of the installation.  Units with cast bronze covers held in place by chains should not be used on aircraft aprons as the covers may come loose and could contribute to Foreign Object Damage (FOD).

5.8 Resistance measurements for alternate grounding points. If conditions prevent the use of known aircraft grounding points, resistance checks using a two-point measurement technique should be performed to verify that the alternate location is an acceptable grounding point.  Prior to performing resistance measurements, the meter should be calibrated.  Routine checks of the reliability of the meter's readings are obtained by using test standard resistors.  A test standard resistor of 0.0025 ohm is typically supplied with the meter.  The calibration reading should be accurate within 5 percent of full scale.  If the reading is not accurate, resistor values within the meter should be changed.  For a detailed procedure of changing internal resistors, refer to the operating or maintenance manual for the meter.

5.8.1 Test method for resistance measurements of alternate grounding points. The test setup for performing resistance measurements is shown on Figure 16.  Perform this test as follows:

a. Connect the clamp type probe to the known aircraft grounding point.

b. Connect the second probe to the proposed alternate grounding location.  Make certain that the probe's two tips are making good contact with this location.

c.  Place "OHMS FULL SCALE" selector switch on highest range and change the range setting in descending order until largest deflection is obtained when the "TEST" switch is depressed.  Prior to depressing "TEST" switch, make sure meter is properly adjusted to the red line or full-scale position as determined by the range selector.

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