How is the intensity step chosen for day, night and poor visibility?
Brighter when the day is bright or visibility falls, dimmer on a clear night, so the lights are seen but never dazzle.
The lights run on a small number of fixed steps, and the step is chosen from how bright it is outside and how far the pilot can see. Two rules do most of the work. A bright background needs brighter lights, so day takes a higher step than night. Poor visibility needs brighter lights, so fog, rain or low cloud move the step up. The lowest step belongs to a clear night, where a high step would dazzle.
The step comes from the aerodrome's step table, usually driven by RVR and the background light level. It is selected by ATC, advised by the system, or set automatically with ATC able to override. Every light on a circuit follows the CCR current, so related lights are moved together to keep the picture balanced. Because the table protects the pilot, a lost sensor or link should fall back to a defined safe step and alert ATC, and the step reached should be confirmed against the step requested.
Higher step, so the lights stand out against a bright background or through fog.
Lowest steps, because a high step would dazzle the pilot.
Frequently Asked Questions
Q1What is intensity step selection?
Choosing which fixed brightness step the CCRs run at any moment. The choice follows the conditions, mainly how bright it is outside and how far the pilot can see, and it sets one output current for the whole circuit.
Q2Why is the step higher by day than at night?
By day the lights compete with a bright background and can be washed out, so they need a higher step to stand out. At night the same brightness would dazzle, so the lowest steps are used when visibility is good.
Q3How does poor visibility change the step?
Fog, rain or low cloud scatter and absorb light, so the lights must be brighter to be seen from a distance. The step moves up as visibility falls, usually following a table built on RVR, with hysteresis and time delays so the lights do not flicker between steps.
Q4Who or what chooses the step?
ATC normally selects it from the tower control panel. The system may advise a step for ATC to accept, or set it automatically with ATC able to override, and a CCR can also be set locally. If the visibility input or link is lost, the control should fall back to a defined safe step and alert ATC.
Q5Why not run at full brightness all the time?
At night in good visibility full brightness dazzles the pilot and can hide other lights. It also shortens lamp life and wastes energy, and it breaks the balance between systems that are meant to match. Stepping down when conditions allow is part of good lighting, not a saving at the pilot's expense.
Q6Which standards apply?
ICAO Annex 14 requires intensity control of the lighting, and Doc 9157 Part 4 gives guidance on step tables, with national rules adding detail. The step table, the number of steps and the thresholds are set by the aerodrome, so confirm against the approved procedure and the standard in force.
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