Guide Slope & Visual Slope Guidance Systems

What is a Guide Slope?

A guide slope (or glide slope/glide path) is the precise, standardized vertical descent angle that aircraft follow during final approach and landing. The most common angle is 3 degrees, providing a balance between safe obstacle clearance, manageable sink rates, and efficient runway use. This angle is defined by international standards from the International Civil Aviation Organization (ICAO) and the Federal Aviation Administration (FAA).

Guide slopes ensure that aircraft approach the runway at a trajectory that avoids terrain and obstacles, while allowing smooth transitions to the landing phase. The slope is referenced from a point 50 feet above the runway threshold—a margin that accommodates even large aircraft and obstacles like approach lights or airport fencing.

At airports with advanced navigation aids, the guide slope is electronically transmitted via the Instrument Landing System (ILS) Glide Slope, allowing precise approaches even in poor visibility. For visual landings, pilots rely on Visual Slope Guidance Systems (VSGS), which provide clear, real-time feedback using color-coded lighting arrays.

Visual Slope Guidance Systems (VSGS): Overview

Visual Slope Guidance Systems are optical aids installed at airports to help pilots identify their position relative to the optimal glide path. These systems are essential for both day and night operations, supporting safe landings even at airports without precision electronic aids.

VSGS are visible from at least 5 nautical miles (NM) during the day and up to 20 NM at night. The angular coverage is carefully engineered, providing cues within at least 10 degrees of the runway centerline, and typically from 0.25 to at least 4 NM on final approach.

Main types of VSGS:

• Visual Approach Slope Indicator (VASI)
• Precision Approach Path Indicator (PAPI)
• Pulsed Light Approach Slope Indicator (PLASI)
• Tri-Color VASI

Each system uses unique optics and light arrangements to provide pilots with unmistakable, standardized signals—often summarized by mnemonics like “red over white, you’re all right” (VASI) or “two red, two white” (PAPI).

VASI: Visual Approach Slope Indicator

The VASI system consists of two or three horizontal light bars, typically placed on the left side of the runway. Each bar contains several light boxes, with colors and angles carefully aligned for reliable indications.

Two-bar VASI indications:

• Both bars white: Above the glide slope
• Near bar white, far bar red: On the glide slope (usually 3 degrees)
• Both bars red: Below the glide slope (unsafe)

Three-bar VASI: Used for airports handling large aircraft, with the upper two bars set for a slightly steeper path. This allows both small and large aircraft to follow safe, appropriate slopes.

VASIs are designed for visibility up to 20 NM at night and 4 NM by day, within a 10-degree sector of the extended runway centerline. ICAO and FAA standards require backup power and frequent alignment checks to ensure reliability.

PAPI: Precision Approach Path Indicator

PAPI is a modern visual slope system, typically employing four lights in a single row near the runway’s edge.

PAPI indications:

• Four white: Far above glide path
• Three white, one red: Slightly high
• Two white, two red: Correct glide path
• One white, three red: Slightly low
• Four red: Significantly below glide path

PAPI’s finer gradation allows pilots to make precise adjustments, making it the system of choice at major airports. Each light is meticulously calibrated, and the system provides reliable cues up to 20 NM at night and 3.4 NM by day.

PLASI: Pulsed Light Approach Slope Indicator

PLASI is a compact, single-unit system projecting steady or pulsed red and white lights:

• Steady white: On the glide slope
• Pulsing white: Above the glide slope (pulse rate increases with deviation)
• Steady red: Slightly below glide slope
• Pulsing red: Well below glide slope (pulse rate increases with deviation)

PLASI is extremely precise and ideal for airports with limited space or unique approach angles. Maintenance includes regular photometric and alignment checks.

Tri-Color VASI

The Tri-Color VASI uses a single light unit that changes color (amber, green, or red) based on the pilot’s approach angle:

• Amber: Above glide path
• Green: On glide path
• Red: Below glide path

Tri-Color VASI is cost-effective for smaller airports, but its narrow transition zones and cockpit-height sensitivity limit its use for larger aircraft.

How Visual Slope Guidance Systems Work

VSGS rely on precisely angled lenses, filters, and mirrors to project colored beams at specific vertical angles. The arrangement ensures that pilots see color changes exactly at the desired approach angle. For instance, in VASI, the color transition from white to red is engineered to coincide with 3 degrees, with a tolerance of ±0.25 degrees.

PAPI achieves even finer gradation by setting each light to a slightly different angle, allowing pilots to make subtle corrections. PLASI and Tri-Color VASI use electronic modulation or color mixing for feedback, though with fewer gradations.

Calibration considers cockpit height, aircraft type, and terrain, and regular maintenance ensures brightness, color fidelity, and alignment meet international standards.

Airport Lighting Systems in Context

Visual slope guidance is part of a broader airport lighting ecosystem that includes:

• Runway Edge Lights: Outline runway boundaries (white/yellow)
• Threshold Lights: Mark runway start/end (green/red)
• Runway Centerline Lights: In precision runways (white/red)
• Approach Lighting Systems (ALS): Extend from threshold, providing transition cues
• Taxiway Lights: Guide ground movements (blue/green)
• Obstruction Lights: Mark hazards (red/white beacons)

Modern systems use LED technology for reliability, color fidelity, and energy efficiency. Lighting can be remotely controlled by ATC or pilots (via Pilot-Controlled Lighting).

Practical Use: How Pilots Interpret and Respond

Pilots integrate VSGS cues into their approach procedures:

• VASI: “Red over white, you’re all right.” Adjust descent if both bars are red (too low) or white (too high).
• PAPI: “Two red, two white.” Adjust for more reds (too low) or more whites (too high).
• PLASI: Adjust to achieve steady white; pulsing lights indicate deviation.
• Tri-Color VASI: Maintain green; amber (too high), red (too low).

Cross-checks with instruments and procedures are essential, especially in challenging conditions. Stabilized approaches—constant rate of descent, airspeed, and configuration—are emphasized for safety.

Comparison Table: VASI, PAPI, PLASI, Tri-Color VASI

Regulatory Standards

All visual slope guidance systems must comply with ICAO Annex 14 and FAA standards. These define photometric requirements, installation geometry, maintenance schedules, and coverage, ensuring harmonized safety and reliability worldwide.

Summary

A guide slope is the heart of safe aircraft approaches, and visual slope guidance systems are the pilot’s real-time link to that optimal path. Whether using VASI, PAPI, PLASI, or Tri-Color VASI, these systems provide clear, standardized indications, dramatically reducing landing risks and enhancing efficiency for airports of all sizes.

For airport operators, investing in modern VSGS—and maintaining them to international standards—is a critical step toward safer, more reliable airfield operations.