How does the ALCMS control the PAPI?
The control system switches and dims the glide-path lights and watches their health — the aiming itself is set on site.
A PAPI gives pilots glide-path information through the pattern of red and white lights from its light units (four on a PAPI, two on an APAPI). The ALCMS-to-PAPI link controls its power and brightness: the ALCMS commands the PAPI’s CCR to switch on and off and to change step, and the CCR drives the series circuit that feeds the units through their isolating transformers.
What the ALCMS does not do is aim the beams. The angles are set mechanically on site and checked at maintenance. Because a failed unit can make the indication misleading, the ALCMS monitors the circuit and, where the system supports it, the units, and reports faults rather than leaving them unseen.
- Switch. The PAPI is turned on with the runway and approach direction in use.
- Dim. The operator or schedule sets the intensity step for daylight and visibility.
- Drive. The CCR sets its current and the PAPI units light.
- Monitor. The CCR reports state, current and faults back to the ALCMS.
Frequently Asked Questions
Q1What is the ALCMS-to-PAPI link?
The control and monitoring path from the ALCMS to a PAPI. The ALCMS switches the PAPI on and off and sets its brightness through its CCR, and receives status and fault information back.
Q2What does the ALCMS control on a PAPI?
The power and brightness of the lights, through the PAPI's CCR and series circuit. It does not aim the beams or change the glide-path angle.
Q3Why is the aiming not controlled from the ALCMS?
The beam angles are set mechanically on site during installation and commissioning and checked at maintenance. The ALCMS only controls whether the lights are on and how bright they are.
Q4Why is PAPI status important?
The PAPI gives the pilot glide-path information from the pattern of red and white lights. If a unit fails, the pattern can be misleading, so failures are monitored and reported rather than left unseen.
Q5When is the PAPI switched on?
Normally with the runway lighting for the approach direction in use, as set by the airport's procedures. The ALCMS carries out the operator's selection.
Q6Is the PAPI brightness set separately from the runway lights?
It is usually adjusted in steps to suit daylight and visibility, but how it is grouped with other circuits depends on the airport's design and control philosophy.
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