Precision Approach: Instrument Approach with Vertical and Lateral Guidance

A Precision Approach (PA) is a cornerstone of modern aviation safety, enabling aircraft to land safely even when weather conditions make visual landings impossible. By providing both lateral (side-to-side) and vertical (up-and-down) guidance, precision approaches guide pilots from the final approach fix directly to the runway threshold, ensuring a stabilized descent path and minimizing risks such as Controlled Flight Into Terrain (CFIT). Precision approaches are mandatory for most commercial operations under Instrument Flight Rules (IFR) and are a testament to decades of technological innovation and international standardization.

Key Components and Systems of Precision Approaches

Instrument Landing System (ILS)

The Instrument Landing System (ILS) is the most widely deployed precision approach system, consisting of two main components:

• Localizer (LOC): Provides lateral guidance, aligning the aircraft with the runway centerline.
• Glideslope (GS): Provides vertical guidance, typically at a 3-degree descent angle to the touchdown point.

ILS installations may also include marker beacons or Distance Measuring Equipment (DME) for range information, and high-intensity approach lighting systems (ALS) to enhance visual cues during the final stage of approach. ILS is classified into Categories I, II, and III, each supporting progressively lower decision heights and Runway Visual Ranges (RVR), enabling landings in increasingly poor visibility.

ILS Categories

• CAT I: Minimum 200 feet DA, 550 meters RVR
• CAT II: Minimum 100 feet DA, 300 meters RVR
• CAT III (a/b/c): Down to 0 feet DA and as low as 75 meters RVR, supporting full autoland

These categories require specific airport infrastructure, certified aircraft equipment, and crew training. ILS remains the gold standard for precision approach technology, but its cost, maintenance, and sensitivity to terrain and obstacles are prompting a gradual shift toward satellite-based solutions.

GBAS Landing System (GLS)

The GBAS Landing System (GLS), or Ground-Based Augmentation System, is a modern advancement that uses GNSS (primarily GPS) signals, augmented by ground-based correction data, to provide precision approach guidance. Unlike ILS, a single GLS installation can support approaches to multiple runways and enable flexible, curved, or offset approach trajectories.

GLS offers lower infrastructure costs, immunity to signal distortion from terrain or buildings (a limitation of ILS), and the ability to serve complex airport layouts. As GNSS technologies mature, GLS is expected to play an increasingly important role, particularly at airports where ILS installation is impractical.

Precision Approach Radar (PAR)

Precision Approach Radar (PAR) is primarily used at military airports and as a backup at some civil aerodromes. Instead of electronic signals processed by the aircraft, PAR involves air traffic controllers providing real-time, verbal course and glidepath corrections to pilots based on high-resolution radar data. While labor-intensive, PAR is invaluable for training, operational testing, and locations lacking other precision aids.

Critical Parameters in Precision Approaches

Decision Altitude/Height (DA/DH)

DA/DH is the altitude (DA, above mean sea level) or height (DH, above the runway threshold) at which a pilot must decide to continue the approach and land if the runway is in sight, or execute a missed approach if not. Published DA/DH values are strictly enforced and determined based on obstacle clearance, system performance, and airport infrastructure.

Runway Visual Range (RVR)

Runway Visual Range (RVR) quantifies visibility along the runway, as seen from the cockpit at the centerline. RVR values are measured by transmissometers positioned along the runway and reported at key points (touchdown, midpoint, rollout). RVR is a decisive factor for approach and landing minima, with lower values required for higher ILS categories.

Approach Lighting Systems (ALS)

A High-Intensity Approach Lighting System (ALS) is crucial for transitioning from instrument to visual flight during the final approach. ALSF-1 and ALSF-2 configurations are required for CAT II/III operations, providing sequenced flashers and high-intensity lights extending into the approach path.

Types of Approaches: Precision vs. Non-Precision

Precision Approach (PA)

• Provides: Both lateral and vertical guidance
• Systems: ILS, GLS, PAR
• Minima: Decision Altitude/Height (DA/DH)
• Usage: Major commercial and military airports, low-visibility conditions

Non-Precision Approach (NPA)

• Provides: Lateral guidance only (e.g., via VOR, NDB, LOC, RNAV LNAV)
• Vertical Guidance: None; requires descent to Minimum Descent Altitude (MDA)
• Minima: Higher than PA, increased risk in poor weather or complex terrain

Approach with Vertical Guidance (APV)

• Provides: Both lateral and vertical guidance, but not to full precision standards (e.g., LPV, LNAV/VNAV via SBAS like WAAS)
• Regulatory Status: Not classified as a precision approach by ICAO/FAA, but offers significant safety improvements over NPA

Supporting Technologies and Infrastructure

Marker Beacons

Marker beacons are legacy ground-based transmitters indicating key points on the approach, such as the outer marker (FAF), middle marker (approximate DA for CAT I), and inner marker (threshold for CAT II/III). They are increasingly replaced by DME or GPS fixes.

Localizer (LOC) and Glideslope (GS)

• Localizer (LOC): VHF transmitter aligned with runway centerline for lateral guidance.
• Glideslope (GS): UHF transmitter providing a vertical path (usually 3 degrees) to the touchdown zone.

Minimum Descent Altitude (MDA)

MDA is the lowest altitude to which descent is authorized on NPAs or circle-to-land maneuvers. Unlike DA/DH, pilots may level off at MDA and continue until the missed approach point before deciding to land or go around.

Autoland

Autoland systems enable fully automated landing, rollout, and stopping of the aircraft, used especially in CAT III conditions. Autoland relies on ILS CAT III or GLS approaches, advanced autopilot, and redundant systems.

Operational Procedures and Regulatory Framework

Precision approaches are designed, charted, and published in Aeronautical Information Publications (AIPs) according to criteria in ICAO Doc 8168 (PANS-OPS), FAA TERPS, and regional equivalents. Procedures include:

• Initial Approach: Alignment and descent to the final approach fix
• Intermediate/Final Approach: Intercepting and following the lateral and vertical path to DA/DH
• Missed Approach: Published escape path in case of inadequate visual reference or system failure

All commercial flight crews must be trained and authorized for precision approaches, especially for low-visibility operations (CAT II/III), which require extra airport infrastructure and airline/crew certification.

Safety and Technological Evolution

Precision approaches have dramatically reduced approach and landing accidents worldwide. Technological advances, including the transition from ground-based to satellite-based systems, continuous performance monitoring, and integration with advanced avionics, continue to improve both safety and airport capacity.

Summary Table: Precision Approach Essentials

Conclusion

A Precision Approach is the backbone of safe, reliable air transport in all weather conditions. By providing both lateral and vertical guidance through advanced ground or satellite-based systems, it ensures that aircraft can land safely even when pilots cannot see the runway until the final moments. Precision approaches are subject to rigorous international standards, require certified infrastructure and crew training, and continue to evolve with the latest navigation and automation technologies. Their adoption and ongoing modernization are crucial for global aviation safety and efficiency.

For more details on implementing or upgrading precision approach capabilities at your airport or within your fleet, contact our aviation systems specialists or request a demonstration of our certified solutions.