Runway Glidescopes: How Do They Guide Pilots Safely?

Airport runway glidescopes, or Visual Glide Slope Indicators (VGSIs), are ground devices that use lights to assist pilots in landing an airplane. The lights define a vertical approach path during the final approach to a runway and help pilots determine if the airplane is too high or too low for an optimal landing. There are several types of VGSIs, including Visual Approach Slope Indicators (VASIs), Precision Approach Path Indicators (PAPIs), and Pulsating Visual Approach Slope Indicators (PVASIs). These systems provide glidepath information that pilots can use to ensure adequate obstacle clearance and touch down within a specified portion of the runway.

Visual Glide Slope Indicators (VGSI)

There are several types of VGSIs, including:

• Visual Approach Slope Indicator (VASI)
• Precision Approach Path Indicator (PAPI)
• Pulsating Visual Approach Slope Indicator (PVASI)
• Three-Color Visual Approach Slope Indicator (T-VASI)
• Helicopter Approach Path Indicator (HAPI)

The most common type is the Visual Approach Slope Indicator (VASI), which consists of light units arranged in bars. There are two-bar and three-bar VASI systems, providing one or two glide paths, respectively. The basic principle of VASI is color differentiation between red and white lamps. Each light unit projects a beam of light, with a white segment in the upper part and a red segment in the lower part. The lights are arranged so that pilots see a combination of colors indicating their position in relation to the glide path.

Another commonly used system is the Precision Approach Path Indicator (PAPI), which is similar to VASI but with lights installed in a single row, typically on the left side of the runway. PAPI projects a pattern of red and white lights that provide visual approach slope information, assisting pilots in establishing a stabilized descent.

Tri-color visual approach slope indicators consist of a single light unit projecting a three-color (red, green, and amber) visual approach path. Below the glide path is indicated by red, on the glide path is indicated by green, and above the glide path is indicated by amber.

Pulsating visual approach slope indicators consist of a single light unit projecting a two-color visual approach path. The on-glide path indication is a steady white light, while the slightly below glide path indication is a steady red light. If the aircraft descends further below the glide path, the red light starts to pulsate, and the pulsation rate increases as the aircraft deviates further from the desired glide slope.

Instrument Landing System (ILS)

The Instrument Landing System (ILS) is a radio navigation system that provides precision guidance to aircraft approaching a runway. It is a ground-based system that supports a safe landing during instrument meteorological conditions (IMC) such as low ceilings or reduced visibility due to fog, rain, or snow.

ILS consists of two separate facilities that operate independently but come together in the cockpit to enable both lateral and vertical precision guidance. These are:

• Localizer (LOC) - This transmits VHF signals (108.1 MHz to 111.95 MHz) to provide aircraft with lateral guidance, ensuring the aircraft is aligned with the centre of the runway during the approach and landing phases of flight. The localizer antenna is positioned on the far end of the runway.
• Glide Slope (GS) - This transmits UHF signals (329.15 MHz to 335.0 MHz) to provide aircraft with vertical guidance, enabling a controlled descent to a runway. The glide slope antenna is typically located 750 to 1250 feet down the runway and 400 to 600 feet from the side of the runway's centreline.

Working together, these two facilities support a precision approach that allows aircraft to descend to a Decision Altitude (DA). At this point, the pilot must visually recognise the runway environment and either continue to land or execute a missed approach if the runway is not in sight.

ILS also includes optional approach lights and marker beacons to help pilots transition from instrument flying to visual flying and identify the runway's centreline.

Visual Approach Slope Indicator (VASI)

The VASI system works on the basic principle of color differentiation between red and white. Each light unit projects a beam of light, with a white segment in the upper part and a red segment in the lower part. When approaching the lights at the proper angle, meaning the pilot is on the glide slope, the first set of lights appears white, and the second set appears red. When both sets appear white, the aircraft is too high, and when both appear red, it is too low.

A mnemonic to remember the colors and their meanings is:

> White over White, you're high as a kite / you'll fly all night / check your height / you're out of sight / too much height.

> Red over White, you're alright / height's alright.

> Red over Red, you're dead / pilot's dead.

Some airports have three-bar VASIs that provide two visual glide paths to accommodate long-bodied aircraft. The first glide path is the same as the standard VASI, and the second is about 25% higher.

VASI installations are the most common visual glide path systems in use. They provide obstruction clearance within a 10-degree angle of the runway's extended center line and up to four nautical miles from the runway threshold. However, they are being phased out and replaced by Precision Approach Path Indicators (PAPIs), which are closer together and, therefore, more efficient to sight and maintain.

Precision Approach Path Indicator (PAPI)

The Precision Approach Path Indicator (PAPI) is a system of lights on the side of an airport runway that provides pilots with a visual indicator of their aircraft's position relative to the correct glide path for the runway. It is generally located on the left-hand side of the runway, approximately 300 metres (980 ft) beyond the landing threshold.

Each PAPI light unit consists of a light source, red filter, and lenses. A high-intensity beam is emitted from each unit, with the lower segment of the beam in red and the upper part in white. The transition between the two colours must be conspicuous, with the transition taking place over an angle not greater than three minutes of arc. This characteristic is key to the PAPI signal.

The PAPI guidance signal is formed by fixing the colour transition boundaries of the four units at different angles. The designated glideslope is midway between the second and third light unit settings. The lowest angle is used for the unit furthest from the runway, and the highest for the unit nearest to it.

When an aircraft approaches the runway, the angles of its approach determine whether the light beams appear red or white to the pilot. If the aircraft is approaching on the correct glide path, the four-light PAPI system will show two red lights and two white lights. A steeper approach will increase the number of white lights, while a shallower angle will increase the number of red lights.

The ratio of white to red lights seen depends on the angle of approach to the runway. Above the designated glide slope, a pilot will see more white lights than red. Below the ideal angle, more red lights than white will be visible. At the optimum approach angle, the ratio of white to red lights will be equal.

The PAPI system is simple to maintain and works for all aircraft approaching a runway. This means that if several aircraft are approaching, they can all make adjustments in good time and ensure a smooth landing every time, without any last-minute changes to the glide path.

The Precision Approach Path Indicator system was first devised in 1974 by Tony Smith and David Johnson at the Royal Aircraft Establishment in Bedford, England. Since then, it has played a vital role in aviation safety, including being used by NASA's Space Shuttle for its safe landing.

Pulsating Visual Approach Slope Indicator (PVASI)

The PVASI is similar to the Visual Approach Slope Indicator (VASI) but consists of only one piece of equipment. It uses a mix of red and white, steady and pulsating light to indicate the pilot's height on approach. A steady white light indicates that the aircraft is on the correct glide path, while a pulsating white light means the aircraft is too high. If the aircraft is slightly below the glide path, the pilot will see a steady red light. As the aircraft descends further below the glide path, the red light will start to pulsate.

The pulsating rate of the light increases as the aircraft deviates further from the desired glide slope, either above or below. This system helps pilots make the perfect approach and landing by providing vertical guidance down to the runway. While the runway centreline provides lateral guidance, the PVASI ensures the aircraft is at the correct height.

The PVASI is not as common as other visual glide slope indicators like the VASI or PAPI. As of May 2022, there were only 84 PVASIs installed at 51 airports and heliports in the United States and its territories, according to Federal Aviation Administration data. In comparison, there were 6730 PAPIs and 623 VASIs.

Frequently asked questions