VASI systems with remote on-off switching must be operated when they serve the runway in use and where intensities are controlled in accordance with TBL 3-4-2 and TBL 3-4-3 except:
Step | Period/Condition |
---|---|
High | Day - Sunrise to sunset. |
Low | Night - Sunset to sunrise. |
Step | Period/Condition |
---|---|
High | Day - Sunrise to sunset. |
Medium | Twilight - From sunset to 30 minutes after sunset and from 30 minutes before sunrise to sunrise,* and during twilight in Alaska. |
Low | Night - Sunset to sunrise. |
*During a 1 year period, twilight may vary 26 to 43 minutes between 25 and 49N latitude. |
NOTE: The basic FAA standard for VASI systems permits independent operation by means of photoelectric device. This system has no on-off control feature and is intended for continuous operation. Other VASI systems in use include those that are operated remotely from the control tower. These systems may consist of either a photoelectric intensity control with only an on-off switch, a two step intensity system, or a three step intensity system.