physics

Visual approach slope indicator

Visual approach slope indicator is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Visual approach slope indicator rather than just read about it. In short: The visual approach slope indicator (VASI) is a system of lights on the side of an airport runway threshold that provides visual descent guidance information during final approach. These lights may be visible from up to 8 kilometres (5.0 mi) during the day and up to 32 kilometres (20 mi) or more at night.

Key takeaways

Visual approach slope indicator belongs to physics; place it in that map before memorising details.
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Connect Visual approach slope indicator to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Visual approach slope indicator from memory before moving on to harder problems.

Reference excerpt

The visual approach slope indicator (VASI) is a system of lights on the side of an airport runway threshold that provides visual descent guidance information during final approach. These lights may be visible from up to 8 kilometres (5.0 mi) during the day and up to 32 kilometres (20 mi) or more at night.

Standard VASI

Basic visual approach slope indicators consist of one set of lights set up 7 metres (23 ft) from the start of the runway. Each light is designed so that it appears as either white or red, depending on the angle at which it is viewed. When the pilot is 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. This used to be the most common type of visual approach slope indicator system; however, it is being phased out and replaced by precision approach path indicators (PAPIs), which are closer together and therefore more efficient to sight and maintain. A mnemonic to remember the colors and their meaning 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. (White over Red is not possible.) Simple VASIs are obsolete, having been deleted from ICAO Annex 14 in 1995, however T-VASIS and AT-VASIS are still specified. T-VASIS is defined as twenty light units symmetrically disposed about the runway centre line in the form of two wing bars of four light units each, with bisecting longitudinal lines of six lights". AT-VASIS is an abbreviated form of T-VASIS, defined as "ten light units arranged on one side of the runway in the form of a single wing bar of four light units with a bisecting longitudinal line of six lights."

Tri-color VASI

A tri-color system consists of a single-light unit projecting a three-color 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. When descending below the glide path, there is a small area of dark amber. Pilots should not mistake this area for an “above the glide path” indication.

Pulsating VASI

The pulsating visual approach slope indicator (PVASI) is a system similar in purpose to a VASI, but only consists of one piece of equipment. It indicates the pilot's height on approach with a mix of red and white, steady and pulsating light. Pulsating white light indicates that the aircraft is too high, whereas steady white indicates being on the glide path. Steady red light is meant to show a position only slightly below glide path, with pulsating red light being indicative of a position well below glide path. This type of system is less prevalent than other visual glide slope indicators like regular VASIs or PAPIs: 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, compared to 6730 PAPIs and 623 VASIs.

Dual VASI Some airports serving long-bodied aircraft have three-bar VASIs that provide two visual glide paths to the same runway. The first glide path encountered is the same as provided by the standard VASI. The second glide path is about 25 percent higher than the first and is designed for the use of pilots of long-bodied aircraft.

See also Index of aviation articles Approach lighting system (ALS) Leading lights Optical landing system (OLS) Precision approach path indicator (PAPI) Pilot controlled lighting (PCL) Runway edge lights (HIRL, MIRL, LIRL) Runway end identifier lights (REIL)

References

FAA Aeronautical Information Manual, Chapter 2 (Aeronautical Lighting and Other Airport Visual Aids), Section 1 (Airport Lighting Aids)

External links FAA Aeronautical Information Manual

Illustrations

Worked examples

Example 1 — a first encounter with Visual approach slope indicator

Start with the simplest possible case. Write down what Visual approach slope indicator claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Visual approach slope indicator before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Visual approach slope indicator ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Visual approach slope indicator

In research: Visual approach slope indicator appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Visual approach slope indicator in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Visual approach slope indicator is common in secondary-school and first-year university syllabi. It links to neighbouring topics British inventions, Optical communications, Visual glide slope indicators, so understanding it makes those chapters shorter.
In everyday life: Look for Visual approach slope indicator outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Visual approach slope indicator in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Visual approach slope indicator means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Visual approach slope indicator out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Visual approach slope indicator in simple terms?

Why does Visual approach slope indicator matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Visual approach slope indicator?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Visual approach slope indicator.