ICAO Doc 8168 – Procedures for Air Navigation Services – Glossary

Aircraft Operations

Aircraft operations, as defined by ICAO Doc 8168, encompass piloting, navigation, and management of aircraft in compliance with established flight procedures and operational requirements. These operations are categorized as visual flight rules (VFR) or instrument flight rules (IFR), each governed by specific procedures to ensure safety and efficiency. Doc 8168 addresses all phases of flight—departure, en-route, arrival, approach, holding, and missed approach—for both fixed-wing and rotary-wing aircraft. It specifies obstacle clearance, navigation accuracy, airspace protection, and operational minima, creating a harmonized framework for safe and standardized international civil aviation.

Obstacle Clearance Altitude/Height (OCA/H)

Obstacle Clearance Altitude/Height (OCA/H) represents the minimum altitude or height above a reference point (typically aerodrome elevation or runway threshold) required to guarantee adequate clearance from obstacles during critical flight phases. OCA (altitude) is referenced to mean sea level; OCH (height) is referenced to aerodrome or runway elevation. OCA/H is calculated for each segment of instrument approaches, missed approaches, and holdings, considering terrain, obstacles, navigation aid accuracy, and aircraft performance. Pilots must adhere to published minima derived from OCA/H to avoid controlled flight into terrain (CFIT) and ensure regulatory compliance.

Procedure Design Gradient (PDG)

The Procedure Design Gradient (PDG) is a specified climb or descent gradient used in the design of departure, missed approach, and sometimes approach procedures to ensure aircraft maintain adequate obstacle clearance. The standard PDG for fixed-wing aircraft in Doc 8168 is 3.3% (3.3 meters climb per 100 meters horizontal), while helicopters typically use a 5.0% gradient. If obstacles require a steeper PDG, this is clearly published for operator awareness. PDG calculations factor in maximum takeoff weight, engine-out performance, atmospheric conditions, and obstacle proximity, ensuring all aircraft using the procedure can meet safety requirements.

Minimum Sector Altitude (MSA)

Minimum Sector Altitude (MSA) is the lowest altitude providing at least 300 meters (1,000 feet) of obstacle clearance within a specified sector, typically a 25 NM radius around a navigation aid or waypoint. MSAs serve as emergency reference altitudes for pilots in the event of navigation or communication failure. They are depicted on approach charts and divided into sectors based on compass bearings. While not intended for routine navigation, MSAs guarantee safe terrain separation during unexpected situations and are recalculated with any significant changes in the obstacle environment.

Standard Instrument Departure (SID)

A Standard Instrument Departure (SID) is a pre-planned, published IFR procedure for efficient and obstacle-free departures from an aerodrome. SIDs are designed according to Doc 8168 criteria, specifying navigation performance, climb gradients, track or heading instructions, and altitude restrictions. SIDs account for terrain, airspace, noise abatement, and traffic flow, using the standard PDG unless higher gradients are needed. They are published in national Aeronautical Information Publications (AIPs) and improve airspace capacity, predictability, and safety.

Omnidirectional Departure

An Omnidirectional Departure is a procedure permitting departure in any direction from a runway when the surrounding terrain and obstacle environment allows. Doc 8168 requires procedure designers to assess obstacles within a defined radius, applying the standard PDG unless specific sectors require a higher gradient. Omnidirectional departures are suitable for aerodromes in flat, obstacle-free areas, offering flexibility while still mandating that pilots maintain minimum climb gradients and altitude restrictions for safety.

Standard Terminal Arrival Route (STAR)

A Standard Terminal Arrival Route (STAR) is a published IFR arrival procedure that standardizes the transition from en-route airspace into the terminal area. STARs in Doc 8168 include waypoints, altitude and speed restrictions, and navigation requirements. They account for local airspace structure, traffic flow, noise abatement, and obstacles. STARs may be conventional or RNAV-based, supporting safe, predictable, and efficient arrivals at busy or complex airports.

Area Navigation (RNAV)

Area Navigation (RNAV) allows aircraft to fly any desired path within the coverage of navigation aids or within the limits of self-contained systems. Doc 8168 details criteria for RNAV procedures, including navigation performance, protected airspace, and obstacle clearance. RNAV supports Performance-Based Navigation (PBN), enabling route flexibility, reduced reliance on ground-based aids, and enhanced capacity and efficiency with maintained or improved safety margins.

Obstacle Clearance

Obstacle clearance is the principle of maintaining defined vertical and horizontal margins between aircraft and obstacles (terrain or man-made structures) during all flight phases. Doc 8168 prescribes minimum margins based on phase of flight, navigation accuracy, and procedure type. Procedure designers analyze terrain and obstacles to calculate these margins. Obstacle clearance prevents CFIT and ensures that, even with navigation or system failures, aircraft remain safely separated from hazards.

Holding Procedures

Holding procedures are standardized flight patterns keeping aircraft within defined airspace while awaiting further ATC clearance. Doc 8168 provides criteria for holding pattern dimensions, protected airspace, turn radii, and timing to guarantee obstacle clearance and traffic efficiency. Holding procedures may use ground-based navigation aids or RNAV waypoints and are tailored to aircraft category and environmental complexity.

Helicopter Procedures

Helicopter procedures in Doc 8168 address the unique performance and operational needs of rotary-wing aircraft, especially under IFR or challenging conditions. These include point-in-space (PinS) approaches, heliport procedures, and specialized criteria for gradients and minima. Helicopter procedures accommodate diverse landing sites, maneuverability, and variable obstacle environments, ensuring standardization and safety comparable to fixed-wing operations.

Simultaneous Operations on Parallel or Near-Parallel Runways

Simultaneous operations use two or more runways for independent or dependent arrivals and departures. Doc 8168 specifies runway spacing, approach and departure separation, navigation aid alignment, and ATC procedures. Types include independent, dependent, and segregated operations, with minimum runway spacing for independent approaches typically set at 1,035 meters. These procedures increase airport capacity and efficiency, especially at major hubs.

Noise Abatement Procedures

Noise abatement procedures minimize aircraft noise impact on airport communities. Doc 8168 covers preferential runway use, specific departure and approach routes, climb/descent profiles, thrust management, and operational restrictions during sensitive periods. Procedures are tailored to local environmental needs, updated regularly, and require collaboration among stakeholders to balance efficiency with environmental stewardship.

Application of ICAO Doc 8168 in Procedure Design

Procedure designers use Doc 8168’s criteria to develop safe, efficient, and standardized instrument procedures. The process includes terrain and obstacle analysis, navigation aid assessment, and calculation of OCA/H, PDG, and protected airspace. Procedures are validated through simulation, inspection, and trials, then published in AIPs. Ongoing review ensures continued compliance and adaptation to operational changes.

Implementation and Operational Use

Each ICAO Contracting State implements Doc 8168 procedures through national regulation and AIP publication. Flight crews and ATC are trained on interpretation and application. Operators ensure aircraft performance meets requirements for climb gradients, navigation, and obstacle clearance. Differences from ICAO standards are published in the AIP for transparency and international harmonization.

Amendment and Record-Keeping

Doc 8168 is periodically amended to reflect new technology, operational practices, and safety requirements. Amendments are approved by the ICAO Council, with records maintained in each edition. Users are responsible for staying current and ensuring compliance. The amendment process involves broad consultation to ensure practicality and global applicability.

Summary Table: Key Terms and Application Context

Glossary of Additional Terms

• Aerodrome Elevation: The highest point of the landing area, used as a reference for vertical measurements in procedure design and charts.
• Decision Altitude (DA)/Decision Height (DH): The specified altitude/height in a precision approach at which a missed approach must be initiated if visual reference is not established.
• Missed Approach Procedure: A published procedure for safe transition if an approach cannot be completed to landing.
• Visual Flight Rules (VFR): Regulations under which a pilot operates an aircraft in weather conditions clear enough for visual reference.
• Instrument Flight Rules (IFR): Regulations for flight operations outside visual weather conditions, requiring compliance with instrument procedures.
• Protected Airspace: Airspace established around a procedure to keep aircraft clear of obstacles and other traffic, defined by design criteria.
• Terminal Arrival Altitude (TAA): The minimum altitude providing at least 1,000 feet of obstacle clearance within a defined area during ENR-to-approach transition.
• Procedure Turn: A maneuver prescribed when a course reversal is needed to align the aircraft with an approach segment.
• Continuous Descent Operation (CDO): An optimized, continuous descent approach procedure minimizing noise and emissions.

This glossary is intended for educational and operational reference. For regulatory compliance and technical procedures, always refer to the latest official ICAO publications and your national authority’s Aeronautical Information Publication (AIP).