All-Weather Operations (AWO) – Aviation Operations Glossary

Introduction

All-Weather Operations (AWO) constitute a specialized area of aviation, enabling aircraft to operate safely and efficiently even when weather conditions significantly reduce or eliminate visual references. As global aviation demands punctuality and safety regardless of prevailing meteorological conditions, AWO procedures, systems, and training have become critical to both commercial and general aviation. This glossary serves as a comprehensive resource for professionals tasked with developing, implementing, or performing all-weather operations, referencing international and national standards such as ICAO Doc 9365, EASA CS-AWO, and FAA AC 120-29A.

Scope

AWO terminology applies to fixed-wing aircraft operating under Instrument Flight Rules (IFR), spanning commercial air transport and general aviation. It covers the full operational spectrum: surface movement, take-off, departure, approach, and landing, whenever visual cues are restricted. The glossary details requirements for equipment, operational approval, and aerodrome infrastructure, focusing on instrument approaches from Non-Precision (NPA) to Precision (CAT I/II/III) and Approach with Vertical Guidance (APV).

Glossary of Key Terms

All-Weather Operations (AWO)

Definition:
All-Weather Operations (AWO) are aircraft movements—on the ground or in flight—performed when meteorological conditions reduce or eliminate the pilot’s visual reference due to fog, snow, rain, low cloud, or darkness. AWO enable safe, regular operations by integrating certified navigation aids, standardized procedures, specialized crew training, and advanced airborne/ground systems.

AWO are regulated, requiring that aircraft, flight crews, and airports meet strict technical and procedural criteria. This includes certified approach and landing systems (e.g., ILS, GBAS), operational approvals for low-visibility minima, and implementation of Low Visibility Procedures (LVP) at airports. Advanced technology, such as autoland and enhanced vision systems, expands AWO’s reliability and scope.

Example Use:
Airline operators at fog-prone airports rely on AWO to maintain schedules, using CAT III approaches in visibility as low as 75 meters RVR, provided all approval and technical requirements are met.

Aerodrome Operating Minima (AOM)

Definition:
Aerodrome Operating Minima (AOM) define the lowest meteorological conditions (visibility, RVR, and cloud ceiling) under which take-off or landing may occur at a specific airport. For take-off, AOM are typically expressed as RVR or visibility; for landing, as combinations of RVR and Decision Altitude/Height (DA/H) or Minimum Descent Altitude/Height (MDA/H), depending on approach type.

AOM are collaboratively established by operators, regulators, and airports, considering aircraft performance, navigational aids, and crew qualification. They are published in flight manuals, operations manuals, and approach charts, and must be checked by crews before every instrument approach or departure.

Example Use:
A pilot reviewing approach charts confirms that reported RVR exceeds the published minimum before commencing an ILS approach.

Precision Approach and Landing Operations

Definition:
Precision Approach and Landing Operations use ground or satellite-based navigation aids to provide both lateral and vertical guidance for approach and landing. They allow lower minima than non-precision approaches and are categorized as CAT I, II, or III based on achievable minima.

Systems commonly supporting precision approaches include Instrument Landing System (ILS), Microwave Landing System (MLS), and Ground-Based Augmentation System (GBAS). These operations require certified systems, redundancy, and trained crews.

Example Use:
A CAT II ILS approach enables landing in 300 meters RVR with a 100-foot decision height.

Approach and Landing Operations with Vertical Guidance (APV)

Definition:
APV procedures provide both lateral and vertical guidance for approaches, but do not meet the full technical standards for precision approaches. Common examples are Baro-VNAV and Localizer Performance with Vertical Guidance (LPV) enabled by Satellite-Based Augmentation Systems (SBAS). APV approaches lower the risk of controlled flight into terrain (CFIT) and offer lower minima than non-precision approaches.

Example Use:
A regional airport lacking ILS enables safer approaches by publishing LPV procedures using EGNOS or WAAS.

Non-Precision Approach and Landing Operations (NPA)

Definition:
NPA procedures provide only lateral guidance (e.g., from VOR, NDB, or localizer) without electronic vertical guidance. Pilots manage vertical descent using step-down fixes or continuous descent, observing a Minimum Descent Altitude/Height (MDA/H). NPAs generally have higher minima than approaches with vertical guidance.

Example Use:
A pilot flies a VOR approach to MDA, descending further only if visual cues are acquired.

Categories of Operation

Category I (CAT I) Operation

Definition:
A precision approach with a Decision Height (DH) not lower than 60 meters (200 feet) and RVR not less than 550 meters. CAT I is the baseline for precision approaches worldwide.

Example Use:
Most commercial runways with ILS support CAT I approaches, allowing safe landings in moderate low visibility.

Category II (CAT II) Operation

Definition:
A precision approach with DH lower than 60 meters (200 feet) but not lower than 30 meters (100 feet), and RVR not less than 300 meters. Requires enhanced ground infrastructure, certified aircraft, and specially trained crews.

Example Use:
Major hubs enable landings in lower visibility using CAT II approaches and advanced autopilot systems.

Category IIIA (CAT IIIA) Operation

Definition:
Precision approach with DH lower than 30 meters (100 feet) or no DH, and RVR not less than 175 meters. Supports autoland in very low visibility.

Example Use:
Aircraft equipped for CAT IIIA routinely land at international airports during dense fog.

Category IIIB (CAT IIIB) Operation

Definition:
Allows approaches with DH lower than 15 meters (50 feet) or no DH, and RVR as low as 50 meters (airport and aircraft dependent). Requires fail-operational autoland and surface movement guidance.

Example Use:
CAT IIIB operations keep airports open in the lowest practical visibility.

Category IIIC (CAT IIIC) Operation

Definition:
Theoretical zero-visibility operation with no DH and no RVR minimum. Totally autonomous airborne and ground systems would be required; not yet implemented in practice.

Supporting Systems and Procedures

Instrument Landing System (ILS)

Definition:
ILS is a ground-based system providing precise lateral (localizer) and vertical (glide slope) guidance. It is the primary system for CAT I, II, and III approaches worldwide.

Example Use:
Aircraft tune to the ILS frequency and follow the electronic path to touchdown during low-visibility landings.

Microwave Landing System (MLS)

Definition:
MLS uses microwave frequencies for precision guidance, allowing flexible approach paths and supporting complex airport layouts. Adoption is limited due to ILS prevalence and rise of satellite-based systems.

Example Use:
Used at select airports with complex approach requirements.

Ground-Based Augmentation System (GBAS)

Definition:
GBAS enhances GNSS accuracy and integrity near airports, enabling multiple precision approaches (GLS) and supporting future higher-category minima.

Example Use:
Major airports deploy GBAS to support precision approaches for multiple runways.

Global Navigation Satellite System (GNSS)

Definition:
GNSS includes GPS, GLONASS, Galileo, and BeiDou, providing worldwide navigation. In AWO, GNSS is augmented by SBAS or GBAS for required accuracy during approaches.

Example Use:
Modern aircraft use GNSS for en route navigation and RNAV/RNP approaches.

Required Navigation Performance (RNP)

Definition:
RNP specifies the accuracy and integrity requirements for aircraft navigation on a given route or procedure. RNP approaches, often with vertical guidance (RNP APCH), support all-weather accessibility with built-in performance monitoring and alerting.

Example Use:
Airlines use RNP approaches to access airports surrounded by challenging terrain in low visibility.

Summary

All-Weather Operations (AWO) are central to safe, efficient, and reliable aviation in today’s complex weather and operational environments. By leveraging certified systems, rigorous training, and robust procedures, AWO minimize delays and diversions while maintaining the highest safety standards in adverse conditions. Whether through ILS, GBAS, APV, or RNP procedures, the future of AWO is increasingly digital, flexible, and resilient.

For more information on implementing or upgrading your AWO capabilities, contact our team or schedule a demonstration.