The scheme whereby specific flight levels may be assigned to specific route segments within the route network.
Source: ICAO EUR Doc 009: Guidance material on the implementation of a 300 m (1000 ft) vertical separation minimum in the European RVSM airspace
The flight level allocation scheme (FLAS) sets out the main principles of flight level allocation within a portion of airspace, or at the border between two airspaces.
FLAS is described in relevant documents, such as Letters of agreements between ATS units, AIPs, AIMs, etc. The tactical application of similar solutions (e.g. temporary use of opposite flight levels after coodination between two controllers from different units) is not considered as FLAS. The goal of FLAS is to establish basic, robust rules to be followed in order to achieve safe operations in terms of vertical separation. The controllers may, after a successful coordination, deviate from the FLAS (e.g. regarding a particular flight, for a period of time, etc.).
Applications of FLAS include:
Allocation of levels along the border between two ATS units. While the standard allocation of cruising levels, set out in Annex II (i.e. odd levels eastbound, even levels westbound) is considered inappropriate, the two neighbouring units may agree upon an alternative, e.g. Unit A transfers traffic at odd levels and Unit B uses even levels.
Allocation of specific levels (or level bands) for certain situations. These are often defined for aircraft departing from or arriving to an aerodrome that is close to the boundary. Normally, the FLAS will be constructed in such a way as to ensure vertical separation between departures and arrivals in case of radio communication failure. Normally, arriving aircraft would be cleared to a level that is above the one assigned for the departures. Crossing the other aircraft's level would only be cleared after successfully establishing communicaiton and when there is reasonable certainty that horizontal separation would be maintained until the two aircraft are vertically separated again. The level assignment does not necessarily follow the eastbound/westbound standard. Examples:
Arrivals to XXXX are to be passed to Approach at level XXX (or within the level band between XXX and YYY)
Departures from XXXX are to be passed to Area Control at level XXX (or below level XXX or within the level band between XXX and YYY)
Arrivals to XXXX are to be passed to the next Area Control Unit (within which XXXX is situated) at levels below XXX (or within the level band between XXX and YYY)
Level allocation based on time of day. For example, flights over the North Atlantic are normally easbound in the morning and westbound in the afternoon. Therefore, both odd and even levels are normally available for flights in the prevailing direction. This principle can be applied whenever the flow of traffic follows similar daily routines to increase the number of available levels.
Allocation of levels at the border between RVSM and non-RVSM airspace
Contingency FLAS (often abbreviated as cFLAS). An example of this is the temporary suspension of RVSM operations due to a wide area affected by severe turbulence. In such case, neighbouring ATS units may agree to the use of the available levels (e.g. Unit A would use FL 310, FL 350 and FL 390 and Unit B would use FL 330m FL 370 and FL 410). Having appropriate arrangements in place (e.g. written down in a Letter of agreement) would ease the transition process in such situations.