Low Visibility Operations Masterclass, Part 4

Visibility basics

Visibility during landings is heavily reliant on the contrast between the runway and its surroundings. Factors such as airborne particles: water droplets, fog, or snow: scatter and diffuse light, reducing contrast and making it harder for us to distinguish the runway. This is particularly evident when landing into the sun or when using landing lights in conditions like snow or fog. The reduction in contrast makes it difficult to identify crucial features such as a snow-covered runway or those in brightly lit urban areas.

In conditions of poor visibility, such as haze or fog, the scattering of light reduces the ability to distinguish the runway’s features from the environment, which can make landing decisions more complex.

Approach lighting systems

Approach lighting systems are essential tools for us to navigate low-visibility conditions, ensuring we can safely identify the runway and align ourselves properly during final approach.

Calvert 3-bar high-intensity approach lighting design

One of the most commonly used approach lighting systems is based on the Calvert 3-bar high-intensity design, the ICAO standard pattern for precision approach runways, found at major airports throughout Europe and much of the world. This system provides critical guidance during low-visibility approaches, where our ability to visually distinguish the runway from its surroundings may be significantly reduced by fog, haze, or darkness.

The Calvert 3-bar system typically features a series of high-intensity lights arranged in three bars that extend out from the runway threshold. As the aircraft approaches, these lights help us maintain the correct approach path by acting as visual markers. The bars serve two purposes:

1. Guidance for Alignment: The centreline and its crossbars show us the extended runway centreline, and give us a horizon reference for roll and a sense of distance to the threshold (the glide path itself comes from the ILS and the PAPI). This is especially vital during low visibility, when visual references to the ground or runway edges may be difficult to distinguish.
2. Runway Identification: These lights assist us in identifying the runway more clearly. Without such systems, distinguishing between the runway and the surrounding terrain, particularly in hazy or foggy conditions, can be extremely challenging.

This system works with the ILS, not instead of it: the ILS brings us down to the decision height, and the approach lights give us the visual references we need to continue from there.

US approach light system with sequenced flashing lights

In addition to the Calvert system, the US approach light system (ALS) is another common system used to support us in low-visibility operations. This system features sequenced flashing lights, which are arranged in a specific pattern along the approach path.

The sequenced flashing lights serve a dual purpose:

• Simplified Approach: They act as a simplified approach system by offering clear visual guidance along the final approach. The flashing pattern helps us visually confirm our position on the glide path and assures us that we are on track to land safely.
• Enhanced Visibility: By flashing in a sequence, these lights are more noticeable, cutting through haze or fog where steady lights might blend into the surrounding environment. The flashing effect makes the approach path stand out more clearly, reducing the likelihood of confusion or misalignment during critical phases of the landing.

Runway centreline lights: a visual cue for distance

Along the runway, the centreline lights are a crucial visual cue for alignment and for the runway remaining. They are white until 900 metres from the runway end, then alternate red and white from 900 metres to 300 metres from the end, and turn solid red over the last 300 metres, telling us, on the landing roll or the takeoff roll, how much runway is left.

Runway edge lights

Runway edge lights, which mark the lateral boundaries of the runway, are typically white. However, in the final 600 metres, these lights are sometimes highlighted with amber lights for better visibility during low-visibility conditions.

For ILS Cat 2 and Cat 3 operations, runway lights are set to high intensity, so that we can acquire and keep the visual references during low-visibility landings flown with the autopilot and autoland. We must stay alert for potential interference with the Instrument Landing System signals and should be ready to disconnect the autopilot if any disturbances are detected near the ground.

Taxiway lights