Glide Slope: Electronic or Visual Approach Slope Guidance

A glide slope is a precisely defined vertical flight path used by aircraft during final approach to a runway, most commonly set at a 3-degree angle above the horizontal. The glide slope ensures a controlled, stabilized descent, providing safe obstacle clearance and facilitating a safe landing in both visual and instrument meteorological conditions (VMC and IMC). Glide slope guidance is a core element of instrument approach procedures and is referenced in international aviation standards such as ICAO Annex 10 and Annex 14.

Glide slope information is delivered through two main methods:

• Electronic systems: Including the Instrument Landing System (ILS), satellite-based approaches like Localizer Performance with Vertical Guidance (LPV), Barometric Vertical Navigation (Baro-VNAV), and Ground-Based Augmentation System (GBAS/GLS).
• Visual aids: Such as the Visual Approach Slope Indicator (VASI) and Precision Approach Path Indicator (PAPI).

The glide slope is depicted on approach charts, cockpit instruments, and runway visual aids, enabling pilots to verify and correct their descent profile throughout the approach phase. Strict adherence to the glide slope ensures compliance with published minimums, obstacle clearance, and safe threshold crossing heights, supporting all categories of approaches from basic precision (Category I) to advanced autoland operations (Category III).

Instrument Landing System (ILS) Glideslope: Electronic Precision Approach

The ILS glideslope is the most widely used electronic approach aid in commercial and general aviation. It provides vertical guidance to pilots by transmitting a radio beam at a fixed angle (typically 3 degrees) from an antenna located near the runway threshold.

• How it works: The glideslope transmitter emits two overlapping beams using the 328.6 to 335.4 MHz frequency range. Aircraft receivers interpret signal differences, displaying a ‘glideslope needle’ on cockpit indicators. When the needle is centered, the aircraft is on the correct descent path. Deviations cause the needle to move, prompting pilot corrections.
• Coverage: Standard ILS glideslope provides usable signals up to 10 nautical miles from the runway, vertically from 0.45 to 1.75 degrees above the horizontal, and laterally up to 1.75 degrees left or right of the centerline. Advanced installations may reach up to 18 NM.
• Precision: ILS supports Category I, II, and III approaches, with decreasing decision heights and visibility minima, enabling landings in very low visibility, including autoland.

Operational procedures:
Pilots tune the correct ILS frequency, verify the Morse code identifier, and set the published course. Descent on the glideslope begins only after the aircraft is established on the localizer and glideslope, with continuous cross-checks against published altitudes and DME fixes to ensure safety.

Caution: False glideslope lobes can appear above the main beam. Pilots must intercept the glideslope from below and confirm altitude at key approach fixes.

Satellite-Based Glide Slope Guidance: LPV Approaches

Localizer Performance with Vertical Guidance (LPV) approaches use satellite navigation systems and augmentation (like WAAS in the US, EGNOS in Europe) to create a stabilized descent path that closely resembles ILS precision.

• How it works: The glidepath is generated using GPS data, enhanced by ground reference stations for high accuracy. The angle is typically 3 degrees, with angular sensitivity increasing near the runway.
• Classification: LPV is an APV (Approach with Vertical Guidance) – not a “precision approach” by ICAO, but operationally very close to ILS Category I in terms of minima (as low as 200–250 feet AGL).
• Cockpit indications: Pilots load the RNAV (GPS) approach with LPV minimums. Lateral and vertical deviation indicators (often a “glideslope needle”) on the primary flight display provide real-time feedback.
• Advantages: No ground-based transmitters are required, increasing airport accessibility. LPV is less affected by terrain or signal reflections.

Limitations:
LPV minima may be higher than ILS at some locations, and service is subject to satellite and augmentation system integrity. Pilots must check NOTAMs for availability before use.

Barometric VNAV and GLS (GBAS Landing System): Alternative Electronic Glide Paths

Barometric Vertical Navigation (Baro-VNAV)

• Principle: Uses the aircraft’s altimeter (corrected for local pressure) to calculate a vertical descent profile.
• Usage: Common in RNAV (GPS) approaches with LNAV/VNAV minimums.
• Limitations: Less accurate than ILS or LPV; sensitive to pressure errors and temperature. Pilots must monitor conditions, especially in cold weather or at high elevations, to ensure obstacle clearance.

Ground-Based Augmentation System (GBAS/GLS)

• Principle: GBAS ground stations at airports receive GPS signals, compute corrections, and broadcast them to aircraft, enabling highly precise vertical and lateral guidance.
• Advantages: Multiple approaches from a single installation, less susceptible to site limitations than ILS, and can exceed ILS precision.
• Deployment: Becoming more common at major airports in Europe, the US, and Australia.

Visual Glide Slope Guidance: VASI and PAPI

Visual Approach Slope Indicator (VASI)

• System: Arrays of two or three horizontal light bars aligned beside the runway. Each bar projects red or white light at fixed angles.
• Interpretation:
  • White over white: Above glide path
  • Red over red: Below glide path
  • Red over white: On glide path
• Coverage: Visible up to 5 NM during the day and 20 NM at night, within ±10° of centerline.
• Role: Purely visual, for VFR or supplementing instrument approaches in good weather.

Precision Approach Path Indicator (PAPI)

• System: Four focused lights in a row beside the runway.
• Indications:
  • Four white: Well above
  • Three white, one red: Slightly above
  • Two white, two red: On glide path
  • One white, three red: Slightly below
  • Four red: Well below
• Coverage: Usable up to 5 NM by day, 20 NM by night, within ±10° of centerline.
• Accuracy: Finer resolution than VASI; especially useful at airports with complex terrain or non-standard runway slopes.

Other Visual Glide Path Systems

• Tri-Color VASI: Single light unit changing color (green = on-path, amber = high, red = low), suitable for smaller airports.
• Pulsating Visual Approach Slope Indicator (PVASI): Single/dual lights, with steady or pulsating red/white beams indicating glide path position.
• Alignment of Elements: Simple ground markers used at basic airfields to visually indicate the correct approach angle.

All visual aids are designed to provide quick, interpretable cues for pilots to maintain safe, stabilized descents, but are for visual conditions only.

Comparison Table: Electronic and Visual Glide Slope Guidance

Regulatory and Operational Standards

• ICAO Annex 10: Specifies technical and operational requirements for electronic navigation aids (ILS, GLS).
• ICAO Annex 14: Covers aerodrome visual aids, including VASI and PAPI.
• FAA AIM and FAR 91.129(e)(3): Outline pilot responsibilities for using visual glide path indicators and maintaining the correct approach slope.
• Approach Charts and NOTAMs: Publish all required altitudes, threshold crossing heights, and minimums for each approach.

Regular flight and ground checks ensure the correct alignment and performance of all glide slope systems. Pilots must check NOTAMs for system availability and comply with all regulatory procedures on approach.

Cockpit Procedures for Glide Slope Approaches

ILS and LPV Approaches

1. Pre-approach: Review the approach chart, tune the correct frequency (ILS) or load the RNAV procedure (LPV), set the published inbound course.
2. Intercept: Establish on the localizer/final approach course. Initiate descent when the glide slope/glidepath needle centers.
3. Descent: Adjust pitch and power for the target rate of descent (about 600–800 fpm at 90 knots for a 3-degree slope).
4. Cross-check: Compare indicated altitude with published values at DME fixes or step-down points.
5. Decision height/altitude: At the minimum, continue the approach only with the required visual reference. If not, execute a missed approach.

Visual Glide Path Aids

• Acquire lights: On final, identify the VASI or PAPI. Adjust approach to maintain the correct color combination.
• Maintain glide path: Stay at or above the visual glide path until a lower altitude is necessary for landing, per regulations.
• Corrections: Make smooth, incremental pitch changes for minor deviations.

Real-World Applications

• Low-visibility ILS Approach: Airliners landing in fog at a major airport use ILS guidance to descend safely to the runway, with precise glideslope information enabling landings where visual cues are absent.
• LPV Approach at Regional Airport: Business jets and turboprops use satellite-based LPV approaches for stabilized descents at airports lacking ILS, increasing access and safety in marginal weather.
• VASI/PAPI in Visual Conditions: General aviation pilots use VASI or PAPI for stabilized approaches, ensuring safe obstacle clearance and minimizing the risk of runway undershoot.

Pilots, air traffic controllers, and airport operators must understand and effectively utilize glide slope systems—both electronic and visual—to ensure safe, efficient, and compliant aircraft landings in all weather conditions.