What is RVR-to-intensity-step control?
How the RVR result is used to choose the brightness step of the runway lights — manual, advisory or automatic — and why the loop between the lights and the RVR needs hysteresis, an ATC override and a safe fallback.
RVR-to-intensity-step control uses the RVR (and the background light level) to choose the brightness step of the runway lights. In poor visibility the lights must be bright enough to be seen; in good visibility they must not dazzle. The step can be chosen by ATC, suggested by the system for ATC to accept, or set automatically with ATC able to override.
It is a closed loop. The step is chosen from RVR, and RVR is calculated from the step. Brighter lights give a longer calculated RVR, which could trigger a dim, which shortens the RVR again. Designs therefore use hysteresis (a gap between the brighten and dim thresholds) and time delays, so the lights do not flicker between steps.
The thresholds come from an aerodrome step table built on ICAO guidance. If the table, the link or the RVR input is wrong, the lights can be too dim for the real visibility, so the control must fall back to a defined safe step and alert ATC when RVR or the link is lost. Entry 13 covers the RVR sensor; the CCR steps themselves are in the CCR intensity steps entry.
Frequently Asked Questions
Q1What is RVR-to-intensity-step control?
It is the link between the RVR system and the runway lighting control: the RVR (with the background light level) is used to choose the brightness step of the runway lights. Lights need to be bright enough to be seen in poor visibility, and dim enough not to dazzle pilots in good visibility. The step can be chosen by ATC, suggested by the system or set automatically.
Q2Why is it a loop, and why does that matter?
The RVR computer uses the light intensity step to calculate RVR, and the control logic uses RVR to choose the step. Brighter lights give a longer calculated RVR, which could tempt the logic to dim, which shortens the RVR, and so on. Designs therefore use hysteresis, so the RVR needed to dim is clearly better than the RVR that caused the brighten, and time delays, so one reading does not change the step.
Q3Who decides the step: ATC or the system?
That is an aerodrome decision. In manual mode ATC selects the step. In advisory mode the system proposes a step and ATC accepts it. In automatic mode the RVR drives the step, and ATC can still override it. Many aerodromes keep ATC in control, so follow the procedures approved for your aerodrome.
Q4Where do the step thresholds come from?
From a step table set for the aerodrome and approved by the authority, based on the ICAO guidance on intensity settings in Annex 14 and the Aerodrome Design Manual (Doc 9157) Part 4. Different light groups, such as approach, centreline and edge lights, may follow different tables. The table is part of the approved configuration, so it should be changed only under change control.
Q5How does it connect to the AGL system?
The RVR computer sends RVR to the airfield lighting control and monitoring system, which applies the step table and commands the constant current regulators. The step of each CCR sets the lamp current, and the intensity setting of the lights then feeds back into the next RVR calculation. See the CCR intensity steps and intensity control entries for the steps themselves.
Q6What happens if RVR or the link fails?
The system should not be left on a low step by accident. A good design detects a lost or invalid RVR or a failed link, falls back to a defined safe step chosen by the aerodrome, normally erring towards brighter rather than dimmer, and alerts ATC and maintenance. The fallback and alarm should be tested at commissioning and again after any change.
Q7Is it safe to work on?
Changes to the step table, the thresholds or the link alter how bright the runway lights are in low visibility, so they need ATC and authority agreement and a documented test, not a quick edit. Work on the CCRs or the RVR system also needs the permit-to-work and aircraft-movement controls, and ATC must be told before anything is taken out of service.
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