What is VASI Lights: 9 Complete Quick Facts

What is VASI Lights? | FAA VASI Lights

VASI Light System

The visual approach slope indicators, acronym VASI lights, are light units installed on the edge of an airport runway threshold, i.e., rear of an airfield runway. It functions to provide descent guidance information during approach through visual aids. These lights may be seen from up to 8 kilometers away during the day and up to 32 kilometers away at night. They provide the pilot with a quick knowledge of their vertical position from the runway, depending on how far away they are. For trainee pilots, witnessing how the VASI works for the first time and realizing how useful it is nothing short of a miracle. It’s a fundamental system, yet the quantity of assistance it provides pilots is incredible.

How do VASI Lights work?

VASI assembly can be made up of 2, 4, 6, 12, or 16 light units grouped near the middle and distant bar respectively. Most VASI installations have two bars, one close and one far, and can have 2,4, or 12 light units. Some VASIs have three bars, close, medium, and distant, which give a different visual glide path for high cockpit aircraft. This installation may be made up of 6 or 16 light units. VASI installations, typically comprised of two, four, or six light units, are situated on one side of the runway, generally the left. The system, which includes 12 or 16 light units, is placed on both sides of the runway.

The whole system of VASI Light involves a single pair of lights located around 7 meters from the runway’s commencement. Each light is intended to seem white or red depending on the angle from which it is seen. Generally, four light units are located on the runway’s left side in the shape of two wing bars. These bars are known as the up wind and down wind wing bar, respectively.

 When the pilot approaches the lights at the correct angle, indicating that the pilot is on the glideslope, the first set of lights becomes white, and the second set of lights looks red. This means that the upwind bar is red, and the downwind bar is white. In other cases, if both bars are white, the aircraft is overly higher; and if both bars are red, the aircraft is too low.

The individual light boxes that form a VASI work comprise 2-light bulbs separated by a solid barrier; The red light is located beneath the white light. The box is positioned on the ground at the desired angle for the light to shift. VASI lights generally operate 24 /7 and in daytime, they might  be seen for at least 3 to 5 miles and around 20 miles at night.

How to read VASI Lights?

When the plane falls below the angle at which the lights are aligned, the lights of a VASI shift from white to red when observed from the cockpit. The top bar is slightly raised above three degrees, while the lower bar is slightly lowered below three degrees. If the pilot is on a three-degree glide path, the further bar should be red, and the closer bar should be white.

There is a popular mnemonic for memorizing the colors and their meaning:

“White over White, you’re high as a kite.

Red over White, you’re alright.

Red over Red, you’re dead.

White over Red, you’re under the head.”

The last option isn’t genuinely feasible unless the VASI installation is faulty. For example, the white and red filter glass on the light is installed up side down by mistake, or the pilot is flying up side down by accidental scenario.

How are VASI Lights designed?

The VASI’s primary concept is color discrimination between red and white. A light beam is emitted with white in the upper segment and red in the bottom, from every light unit. The light units are situated in way that the pilot utilize the VASI when approaching runway, will able see the light signal :

vasi lights
VASI Lights; Image Source: Jeff Gibson – JGib10 at English WikipediaVasi-1, marked as public domain, more details on Wikimedia Commons

Aside from VASIs or alternate glide path systems, pilots have no means of knowing if they are flying too high or too low when approaching an airport. It doesn’t matter in many instances, especially in tiny, easy-to-maneuver aircraft. Pilots are trained to approach the runway and land at both steep and shallow angles efficiently. Therefore, a pilot should ideally be able to manage it.

Runways vary in various forms and sizes, and approaching an unfamiliar runway might throw a pilot’s senses off. Some runway sight illusions can confuse even the most seasoned pilots. When observed from the final approach, an upsloping runway shows that the plane is flying too low. A downslope runway has the opposite effect, giving the pilot the impression that they are flying too high.

The last reason VASI lights are so helpful is because of the surroundings outside of the airport. When a pilot approaching a designated public utility airport, the approach path is guaranteed to be unobstructed. A 3o glides path stretching out and up from the runway threshold is used to construct the conventional approach path and if something blockade comes into contact with this glide path, the landing threshold will be moved farther down the runway till the approaching is straightforward. No instrument can tell a pilot flying by sight where it is, except for the VASI lights.

Types of VASI Lights

2 bar VASI Lights

The 2-bar VASI light is made up of 2 parallel light bars stacked on top of one another along the sides of a runway. These installations give a single visible glide path typically vertically set at 3 degrees. VASI lights have visibility in the range of 3 to 5 miles throughout the day time and up till 20 miles during the night. Precise obstacle clearance is extended between a range of 10 degrees of the extended runway centerline and up to 4 nautical miles from the threshold of the runway.

3 bar VASI Lights

The three-bar VASI light adds one extra light bar. At bigger airports, a three-bar VASI offers two visible glide paths. The near and center bars give the lower gliding path, typically set at 3o; the middle and far bar provide the upper glides path, which is usually 0.25o high. The higher glide path is created so that pilots in very tall and huge planes can avoid wheel landing before the runway threshold.

Although standard glide path angles are 3 degrees, some sites may require angles as high as 4.5 degrees to provide adequate obstacle clearance. Precise obstacle clearance similar to the 2 bar VASI lights, is provided. These VASI variants can have 2, 4, 6, 12, or 16 light units.

Tri-color VASI Lights

A single light unit constitutes the whole Tri-color VASI Light systems that utilizes a three-color visual approach rout, forecasted into the final approach region of the runway. This is where the indicator is placed. For example, the indicator for the glide path below is red, the indication for the glide path above is amber, and the signal for the glide path above is green. The range of these indicators varies from 0.5 mile in the day time, up to 5 miles at night, in regards with visibility circumstances.

Pulsating VASI Lights

Similar to the tri-color VASI, pulsating visual approach slope indicator also comprises of a single light unit but forecasts a 2-color visual approach route onto the final approach region of the runway in regards with its actual placement. A continuous white light indicates that you are on the glide path. Steady red light is displayed slightly below the glide path indicator. The red light begins to pulse as the airplane drops farther below the glide path. For example, the glide path indicator seen above is a pulsing white light as the aircraft moves more above or below the ideal glideslope, the pulsing rate rises.

PAPI Lights

A precision approach path indicator (PAPI) works as a visual aid responsible for offering guidance info to support a pilot in acquiring and to maintain the right vertical approach to an airport or airfield. It is typically positioned around 300 meters beyond the runway’s landing threshold on the left side of the runway.

PAPI Lights; Image Source: Original uploader Tswgb. Edit by Abuk SABUKPAPI Jersey Airport, marked as public domain, more details on Wikimedia Commons

What’s the functional difference between PAPI and VASI?

A precision approach path indicator (PAPI) is not much different from the VASI system, except their placement in a single row of four lights, often on the runway’s left side. However, depending on the runway/taxiway arrangement, the PAPI might be positioned on the right. 

PAPI employs four groups of two lights, positioned adjacently, to give the pilot five various degrees of accuracy (R = red; W = white):

[R R R R]Below Glideslope
[R R R W]Slightly Below Glideslope
[R R W W]On Glideslope
[R W W W]Slightly Above Glideslope
[W W W W]Above Glideslope
PAPI Lights Reading: Red & White Differentiations; PAPI and VASI Lights Differences

How to fly with reference to VASI lights?

VASI Lights Approach

Vertical guiding lights are an excellent reference for you to utilize on descent whether you are flying during the day or at night. If the lights indicate that you’re on a glide path on the bottom leg of your pattern, you’ll probably finish up slightly low. Turns have a faster rate of descent, so you’ll drop altitude faster than a straight-in descent.

Utilizing VASI Lights signals on the final approach ensures flying a steady approach to the runway and not chasing the glide path. While there can be high and low extremities to a stabilized approach, the 3-degree glide path enables a consistent technique to approach the runway through perfect landing.

VASI Lights Landing

During instrument check rides, the move of cutting the power and descending for the runway. The method is fundamentally unstable. When reaching the flare, stop utilizing references from vertical guidance.

To accurately understand the height, the focus should be switched to references such as the end of the runway or the runway borders. The closer the approach is to the lights, the more sensitive the VASI Lights or PAPI lights get, and it might be risky to try to follow them at this point. Simply put, VASI lights should not be pursued over the runway at low altitudes.

Fresnel Lens Optical Landing System (FLOLS)

The FLOLS also facilitates continuous automated operation and Pilots are assisted of an approaching aircraft with optical landing by referencing the proper glide slope angle. This system has 2 sets of horizontal datum lights with normal orientation to the approach route, 2 vertical bars of waves off light, 2 double types cut-lights, and a source light indication system made up of 5 normally aligned cell assemblies.

Land based FLOLS; Image Credits: anonymous, IFLOS Mk 14, marked as public domain, more details on Wikimedia Commons

Each assembly is validated with one source light, one one Fresnel and lenticular lens, these lenses should be installed on the left side of the runway, approximately 10 feet from the pavement’s side and 750 feet from the runway end, and are configured to provide the glide slope to the pilot.

Which airfields have VASI lights?

By far, the most popular glide path systems in the world are VASI lights and PAPI lights. They are situated at each runway in all existing world’s major airports. Absence of a glideslope alignment system can be witnessed in only very small or private airports. VASIs are the only means for the pilots to recognize if they are flying overly higher or overly low-level when approach to an airport. It doesn’t matter in many instances, especially in tiny, easy-to-maneuver aircraft.

Other Runway Lights

Approach Light Systems (ALS)

The primary mechanism of transitioning from instrument flight to visual flight for landing is an approach lighting system(ALS) and the sophistication and design of an approach light system for a specific runway are determined by operational needs. It consists of a series of light bars, strobelight, or a mix of the two and extend out ward from the approach end of an airport runway end.

Approach Lights; Image Source: GaritzkoApproach Lighting System Bremen 1, marked as public domain, more details on Wikimedia Commons

ALS is often assigned to a runway using an instrument approach procedure (IAP). The air traffic control tower is in charge of the runway illumination and for Pilot-controlled illumination, which the pilot may turn on through radio, may be placed at non-towered airports. The brightness of the lights is influenced by the time of the day. Approach Light Systems continuance from the landing threshold into the approach area for a distance of 2400 to 3000 ft (approximately) for precision instrument runway, whereas it drops down to 1400 to 1500 feet (approximately) for non precision instrument runway.

Runway Edge Light Systems

Runway edge lighting highlights the runway edges during periods of compromised visibility. These  are white, except when it is substituted by yellow on instrumental runway, on the least between the last 2,000 ft and half the runway length, that establishes a landing danger zone. The runway lights illuminate red to signal the end of the runway to leaving aircraft and illuminate green on runway’s out ward direction from the end to signal the threshold.

These light systems are classed based on the amount of intensity they can produce:

  1. High-intensity runway lights (HIRL).
  2. Medium-intensity runway lights (MIRL).
  3. Low-intensity runway lights (LIRL).
Runway Edge Lights; Image Credits: kuhnmiLanding at Zurich International AirportCC BY 2.0

The runway edge lights are majorly clear or white but there are few outliers to offer extra information to pilots in specific situations. These are designed to perform in a bi-directional manner towards the least between the last 600 meters (2,000 ft) and one-half of the usable runway length. They appear white to a pilot arriving from the runway’s short end, but yellow to a pilot approaching from the other rear, signifying that the runway is reaching its conclusion.

Runway End Identifier Lights (REILs)

Many airports have runway end identifier light(REILs) placed to allow quick and accurate identification of the approaching end of a specific runway. This REILs comprises two synchronised flashing light, one on either side of the runway-threshold. REILs can function omnidirectionally or unidirectionally concerning the approach area.

It is recommended to install the runway end identifier lights in the following ways:

  1. At the threshold of a non-precision approach runway necessitating further evidence on the threshold or presence of other unsuitable approach lighting aids.
  2. At places of permanent displacement of the runway threshold from its extremity or provisional displacement from the normal position, and at times of need for additional evidence to determine the threshold.
  3. Symmetric regular positioning has to be maintained along the runway centreline, in line along the threshold, and approximately 10m yonder each line of runway edgelights.
  4. It is meant to flash white lights at a rate of 60-120 times/min.

Esha Chakraborty

I have a background in Aerospace Engineering, currently working towards the application of Robotics in the Defense and the Space Science Industry. I am a continuous learner and my passion for creative arts keeps me inclined towards designing novel engineering concepts. With robots substituting almost all human actions in the future, I like to bring to my readers the foundational aspects of the subject in an easy yet informative manner. I also like to keep updated with the advancements in the aerospace industry simultaneously. Connect with me with LinkedIn - http://linkedin.com/in/eshachakraborty93

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