What is interlocking?
The rules a control system checks before it accepts a command — blocking anything unsafe, conflicting or out of sequence, and saying why.
Interlocking is the set of rules in the control system that decide whether a command is accepted. When an operator sends a control command, the ALCMS checks it against the rules first. If the command would be unsafe, conflicting or out of sequence, it is blocked and the operator is told why. The rules fall into a few kinds: mode (equipment in local mode cannot be commanded remotely), conflict (selections that must not be active together), sequence (conditions that must be met first) and safety (work in progress inhibits switch-on).
The best-known lighting example is the stop bar and the lead-on lights beyond it (see Stop Bar Lights and Lead-on Lighting): while the stop bar is lit, the taxiway lights beyond it are kept off, so a pilot is never shown a lit route through a red stop bar. An interlock acts before the command, which is what separates it from an alarm. Its limit is just as important: an interlock is logic in software, so it never replaces physical isolation. A bypass or override removes the protection and should be used only with authority, for a limited time, and logged.
- Request. The operator sends a command from the control position in charge.
- Check. The ALCMS tests it against mode, conflict, sequence and safety rules.
- Decide. The command is allowed, or blocked with a reason.
- Act. An allowed command goes to the equipment; the status feedback confirms it.
- Resolve. If blocked, fix the condition named; do not look for a way around it.
Interlocking Explorer
Pick a command and see how the rules respond
The check BEFORE SENDING
What to do AND NOT DO
Teaching simulation. Rules and messages are illustrative; real systems are configured for each airfield.
Frequently Asked Questions
Q1What is interlocking?
Rules built into the control system that decide whether a command is accepted. An interlock blocks a command that would be unsafe, conflicting or out of sequence, and normally tells the operator why it was refused.
Q2What is a typical lighting interlock?
A stop bar and the taxiway lead-on lights beyond it: while the stop bar is lit, the lights beyond it are kept off, so a pilot is not shown a lit route through a stop bar. Others include blocking commands to equipment in local mode and preventing conflicting taxi routes from being selected together.
Q3How is an interlock different from an alarm?
An alarm reports that something has happened. An interlock acts beforehand and prevents the command from being carried out. An interlock that blocks a command may also raise a message or an event in the log.
Q4Can an interlock be bypassed?
Some systems allow an override for testing or abnormal operation, but it should be done only with authority, under the airport's procedure, for a limited time, and with the override logged. A bypass removes a protection, so the operator takes on the check the interlock would have done.
Q5Does an interlock make work on equipment safe?
No. An interlock is logic in software and it relies on the signals and settings being right. People are protected by physical isolation and lock-off at the equipment under the work permit, never by an interlock alone.
Q6What should I do when a command is refused by an interlock?
Read the message, because it names the rule. Fix the condition it describes, for example the equipment being in local or a conflicting selection being active, and then send the command again. Do not look for a way around it.
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