AIM 

4/20/23 

b. 

Medium intensity omnidirectional flashing white lighting system provides conspicuity both day and night 

on catenary support structures. The unique sequential/simultaneous flashing light system alerts pilots of the 

associated catenary wires. 

c. 

High intensity flashing white lights are being used to identify some supporting structures of overhead 

transmission lines located across rivers, chasms, gorges, etc. These lights flash in a middle, top, lower light 

sequence at approximately 60 flashes per minute. The top light is normally installed near the top of the supporting 

structure, while the lower light indicates the approximate lower portion of the wire span. The lights are beamed 

towards the companion structure and identify the area of the wire span. 

d. 

High intensity flashing white lights are also employed to identify tall structures, such as chimneys and 

towers, as obstructions to air navigation. The lights provide a 360 degree coverage about the structure at 40 

flashes per minute and consist of from one to seven levels of lights depending upon the height of the structure. 

Where more than one level is used the vertical banks flash simultaneously. 

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2

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4.  LED Lighting Systems 

Certain light

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emitting diode (LED) lighting systems fall outside the combined visible and near

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infrared 

spectrum of night vision goggles (NVGs) and thus will not be visible to a flightcrew using NVGs. 
The FAA changed specifications for LED

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based red obstruction lights to make them visible to pilots using 

certain NVG systems, however, other colors may not be visible. 
It is recommended that air carriers/operators—including Part 91 operators—who use NVGs incorporate 

procedures into manuals and/or standard operating procedures (SOPs) requiring periodic, unaided scanning 

when operating at low altitudes and when performing a reconnaissance of landing areas. 

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2

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Air Navigation and Obstruction Lighting