PAPI Intensity Control

The PAPI Intensity Control is the component—or set of components—that manages the luminous intensity of the PAPI light units. It receives brightness commands from the air traffic control tower, the ALCMS, or a local manual panel, and translates these commands into appropriate electrical signals to the lamps or LED modules.

Intensity control is essential because the required brightness of PAPI varies dramatically with ambient light: during bright daylight, the system must be intensely bright to be visible against the sky; at night, lower intensities are needed to avoid glare. Typically, PAPI systems support 3 or 5 intensity steps, corresponding to day, twilight, night (and intermediate levels for 5‑step systems).

The intensity control function may be implemented as:

• Integrated into the PAPI Control Unit – as a set of relay or solid‑state outputs that switch the CCR’s current taps.
• Part of the Constant Current Regulator (CCR) – where the CCR itself has selectable current levels, and the control unit merely sends a selection signal.
• Standalone module – a separate device with its own user interface and communication, often used in smaller or portable installations.

The intensity control exists because PAPI's red/white indication has to remain equally readable at both ends of an enormous range of ambient brightness, and a fixed-output light source cannot do that. In full daylight, especially against a bright sky, a dim PAPI beam simply disappears from view before a pilot ever gets useful glide slope information from it; at night, that same high output would glare and could actually make the red/white contrast harder to judge rather than easier. Its function, then, isn't brightness for its own sake but legibility — keeping the pattern equally clear and unambiguous whether it's being read at midday or on a moonless night, which is what allows a single fixed installation to serve every hour of operation rather than only a narrow band of lighting conditions.

PAPI systems are designed to provide the optimal intensity for each ambient light condition. The table below shows typical current settings for a 5‑step system (most common for precision runways):

For 3‑step systems, typical currents are 2.8 A (night), 4.1 A (twilight), and 6.6 A (day). The actual current values may vary depending on the specific CCR and PAPI units. LED‑based PAPI systems may use PWM (pulse‑width modulation) or current‑controlled drivers, but the principle of stepped brightness remains the same.

PAPI intensity control is an essential feature of every PAPI system, regardless of the installation:

• Precision runways (CAT I, II, III) – integrated with the ALCMS for automatic brightness selection based on ambient light sensors or tower commands.
• Non‑precision runways – often with manual step selection via a control panel in the tower or electrical room.
• Heliports – with dedicated helicopter PAPI intensity control, possibly with different step values.
• Military airfields – where rapid intensity changes may be needed for tactical operations.
• Portable or temporary PAPI – using battery‑powered controllers with simple rotary switches for step selection.
• Retrofit upgrades – where an existing PAPI with a manual switch is upgraded to an automatic system with ALCMS integration.

The intensity control electronics sit in the CCR room or equipment shelter alongside the PAPI's power supply, wired into the constant current loop (or DC distribution) feeding the wing bar and, where fitted, into the ALCMS network or ambient light sensor providing automatic brightness commands. A manual control panel, where used instead of or alongside automatic control, is typically located in the tower or a nearby electrical room so controllers can select a step directly without needing shelter access. On portable PAPI, the equivalent controller is built into the unit itself with a simple rotary switch, keeping intensity selection at the fixture rather than routed back to a fixed control room. Wherever it sits, the controller's position keeps its control wiring to the CCR or driver circuit short, since that link carries the step commands that ultimately set the wing bar's brightness.

The intensity control process involves several steps, from command to light output:

• Command Generation: The desired brightness step is determined by the air traffic controller (based on weather, time of day, or pilot request) or automatically by an ambient light sensor feeding the ALCMS. The command is sent to the intensity control module.
• Signal Translation: The intensity control module converts the command into a control signal. In a traditional system, this is typically a set of relay contacts that select the appropriate tap on the CCR’s multi‑tap transformer. In modern systems, it may be a digital signal (e.g., Modbus) to the CCR to set the output current.
• CCR Output Adjustment: The CCR adjusts its output current to the selected value (e.g., 2.8A, 4.1A, 6.6A). This current is fed into the series circuit, and each PAPI unit receives the same current, which determines its brightness.
• LED‑Specific Control: For LED‑based PAPI systems, the CCR may still supply a constant current (e.g., 6.6A) and the LED drivers inside each unit use PWM to adjust brightness. In this case, the intensity control module sends a PWM signal or a separate 0–10V control signal to the drivers, independent of the series current. Some LED systems use a two‑wire power and data interface, where the intensity command is embedded in the power signal.
• Monitoring and Feedback: Advanced intensity control modules monitor the actual current or light output to verify that the correct intensity has been achieved. If there is a deviation, an alarm is generated.

Intensity control is the command layer that sits between a brightness decision and the light actually reaching the required step. An ATC instruction, an ALCMS schedule, or an ambient light sensor generates the desired brightness step, and the intensity control module translates that into whichever signal path the installation uses — a relay selecting a CCR tap for traditional series-current systems, or a PWM/0–10V signal to LED drivers for systems that hold the series current constant and vary duty cycle instead.

Because every PAPI unit on the loop receives the same current (or the same PWM command, on LED systems), intensity is inherently a loop-wide setting rather than a per-unit one — the module's monitoring and feedback function exists to confirm that the whole array actually reached the commanded step, not to adjust any single unit independently of the rest.

Where intensity control is integrated into the PAPI Control Unit, its physical form is a bank of relay or solid-state switching outputs mounted on the control unit's internal circuit board, wired to switch the CCR's current taps or a PWM driver stage in response to the selected step. Where it forms part of the CCR itself, the switching hardware sits inside the CCR's own enclosure, with the control unit reduced to sending a simple selection signal over a control cable. A standalone module takes the form of its own small sealed enclosure, housing a switching or PWM output stage, a local keypad or rotary selector for manual step selection, and a communications port for remote control, mounted either in an equipment room rack or a weatherproof field cabinet depending on the installation. In every configuration the switching components are rated for the full series-circuit current or LED driver load they carry, with heat-sinking or ventilation sized to the switching losses involved.