Airroute
See Airway
Airspace (MTCD)
A continuous volume of defined dimensions.
Airspace Capacity
The number of aircraft that can be fitted into ATC sectors, keeping in mind aircraft separation and safety standards, area navigation direct routings and other factors (EATCHIP, 1996).

Airspace Management A generic term covering any management activity provided for the purpose of achieving the most efficient use of airspace based on actual needs and, where possible, avoiding permanent airspace segregation. Airspace Management Cell A joint civil/military cell responsible for the day-to-day management and temporary allocation of national or sub-regional airspace within its jurisdiction for a specific time period, by means of a standard message format. Airspace Management Cell (AMC) A centralised body providing the day-to-day management and temporary allocation of the airspace. Airspace Management Cell Manageable Area An area subject to management and allocation by AMC at level 2 (pre-tactical level).
Note : Under the TSA Concept, these manageable areas are either formal structures entitled TSAs or R&D Areas that are manageable at Level 2 in the same way as TSA Airspace Region A defined volume of airspace.
Airspace Reservation A defined volume of airspace normally under the jurisdiction of one aviation authority and temporarily reserved, by common agreement, for exclusive use by another aviation authority.
Airspace Restriction Any changes to the limits, structure and/or availability of airspace. Airspace Structure 1- A division of the airspace designed to accommodate the safe operation of aircraft during a specific phase of flight.
2- In the context of the FUA Concept, ‘Airspace Structures’ include Controlled Airspace, ATS Route, Danger Area, Restricted Area, Prohibited Area, Temporary Segregated Area, Cross-Border Area.
Airspace Structure Data Set of data defining the airspace structure on which the functions comprising an automated ATS system are based. Airspace Use Plan Message An ASM message of NOTAM status notifying the daily decision of an Airspace management Cell on the temporary allocation of the airspace within its jurisdiction for a specific time period, by means of a standard message format. Air-to-Ground Communication One-way communication from aircraft to stations or locations on the surface of the earth. Airway A control area or portion thereof established in the form of a corridor equipped with radio navigation aids. Airways and Airport Surveillance Radar A primary radar capable of detecting aircraft and weather to a range of approximately 150 nautical miles with the capability of being used for long range (en-route) and short range (terminal and airport) surveillance and vectoring.

Airworthiness Directive Issued by the FAA, AD's apply to equipment and/or structural components which are found to be operationally unsafe or defective. Compliance with AD's is mandatory. Non-compliance after a specified period of time, deems an aircraft unworthy for flight. Alert Limit (GNSS) Alert Limit is, for the phase of operation in progress, the distance which the GNSS position error cannot exceed without a GNSS integrity alert being given to the user. Alert Phase A situation wherein apprehension exists as to the safety of an aircraft and its occupants. Alerting Service A service provided to notify appropriate organisations regarding aircraft in need of search and rescue aid, and assist such organisations as required. Alignment A MURATREC sub-process which calculates the misalignment parameters for PR/SSR combined plots. All Weather Operations Panel The AWOP recommended material on technical and operational aspects of Microwave Landing System/Area Navigation (MLS/RNAV) operations for inclusion in various ICAO documents. The panel now also addresses the possible application of Global Navigation Satellite System (GNSS) in approach, departure and landing operations. Allan Variance Sigma Allan Variance, internationally known as two sample sigma. It is computed as follows: Form the squares of the second differences, add them, divide by 2 times the number of terms and form the square root. Allocation, allocate Distribution of frequencies, SSR Codes, etc. to a State, unit or service. Distribution of 24-bit aircraft addresses to a State or common mark registering authority. Almanac Data A set of parameters similar to the more precise ephemeris data, used for approximating the GPS satellite orbits. Alone Track Error A fix error along the flight track resulting from the total error contributions. Along Track Acceleration Time derivative of the Ground Speed. Along Track Distance The distance along the desired track from the waypoint to the perpendicular line from the desired track to the aircraft. Alpha Convergence Period (Symbol Tau (alpha)) Mean time to reach a RMS value such that : RMS< RMScv + alpha + RMSpv (0<alpha<1). Alpha Convergence Period (Symbol Tauij (alpha)) The mean time needed after a change in Mode Of Flight from i to j, to render a RMS error value less than alpha times the ultimate RMS error for the Mode Of Flight. Alpha Convergence Period (Symbol TauOk(alpha)) Mean time after Track Recovery (symbol 0) needed to render a RMS state error value lower than alpha times the ultimate RMS error for the Mode Of Flight k (symbol k) a> 1.