In what order does the lighting change from normal to LVP and back?
Going in, the lights are checked and the standby supply made ready before LVP are in force. Coming back, ATC cancels only after visibility has stayed better for a margin and a delay.
A changeover is a sequence, not a switch. Each phase is declared by ATC and each state is confirmed before the next one starts. The aim is that no light, interlock or standby supply is left in an unknown state while the aerodrome is working to low-visibility minima.
The stop bars and the taxiway centreline are the part most often got wrong. A lit stop bar means stop, so the green lights beyond it must be off, and no changeover may leave the two out of step.
- Trigger. Visibility or cloud base reaches the limit set in the aerodrome’s LVP document. ATC decides, not the lighting.
- Prepare. ATC declares the preparation phase. The standby power is made ready and the critical areas are protected.
- Check. Lamps, circuits and interlocks are read back from the control system. A fault found now is dealt with before LVP are in force.
- Put in force. The LVP configuration is set: the full CAT II/III approach pattern, stop bars at the holding positions in use, and the step from the RVR table.
- Hold the interlock. The taxiway centreline beyond a lit stop bar stays off. ATC extinguishes a bar only to clear a movement.
- Monitor and downgrade. If serviceability falls below the minimum, the category or minima are reduced and ATC is told.
- Stand down. Visibility improves and stays better for a margin and a delay. ATC cancels LVP.
- Return and confirm. Stop bars return to normal use and the steps return to the table for the conditions. The state reached is confirmed against the state requested and logged.
Read back the state of the lights, circuits and interlocks, and log it, before the next phase.
A command sent is not a state reached. A lost link or a failed circuit would go unseen until an aircraft needs the lights.
Frequently Asked Questions
Q1What is the mode changeover sequence?
It is the ordered set of steps by which the airfield lighting moves from normal operations to the Low-Visibility Procedures configuration and then back again. ATC declares the phases, the lighting control system carries out the configuration and reports status, and the order and values come from the aerodrome's approved LVP document.
Q2What happens in the preparation phase?
ATC declares the preparation phase. The lights and critical areas are checked and the standby power is made ready, so that the full configuration can be put in force with every item healthy and monitored. Nothing is assumed: the status of lamps, circuits and interlocks is read back.
Q3Why are the stop bars and taxiway centreline interlocked during changeover?
A lit stop bar means stop, so the green centreline lights beyond it must be off. The interlock holds that rule on every change of mode, so a changeover never leaves green lights lit beyond a lit stop bar. ATC extinguishes a bar only to let an aircraft move.
Q4What if a light or circuit fails while LVP are in force?
If serviceability falls below the minimum for the operation, the category or minima are reduced and ATC is told. The lighting does not quietly carry on at a level the aerodrome has not approved. The minimum serviceability levels come from the approved LVP document and the regulator.
Q5Why does the return to normal wait?
Stand-down normally needs visibility to stay better for a margin and a delay before ATC cancels LVP. That stops the aerodrome switching in and out of LVP with every reading, and it is the same idea as the hysteresis in the RVR step table.
Q6Who controls the changeover, ATC or the system?
ATC declares each phase. The lighting control system then sets the lights, intensity steps and interlocks, and shows ATC and maintenance the state reached. The system should confirm the state reached against the state requested, and record the change in its event log. Follow the aerodrome's approved procedure for who does what.
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