The Flight Level Allocation Scheme (FLAS) refers to a system where specific flight levels are designated for certain route segments within a route network. This structured method of assigning altitude helps maintain safe vertical separation between aircraft. It is especially important when transitioning between different airspaces or coordinating flights along shared borders. According to the ICAO EUR Doc 009, which provides guidance on the implementation of the 300-meter (1,000-foot) vertical separation minimum in European RVSM airspace, FLAS plays a key role in ensuring efficient and safe air traffic management.
Purpose and Description
FLAS sets out clear rules for assigning flight levels in specific airspace sectors. These rules are usually outlined in official documents like Letters of Agreement (LoAs) between air traffic service (ATS) units, national Aeronautical Information Publications (AIPs), and other Aeronautical Information Management (AIM) materials. These rules serve as a baseline. While they provide structure, controllers are allowed to deviate from them when needed—as long as coordination with relevant units takes place. For instance, a controller might assign a flight a different level temporarily if it’s deemed safe and both sides agree.
Notably, FLAS refers only to established, documented procedures. Temporary or tactical decisions—like two controllers agreeing to use opposite flight levels for a single situation—don’t count as part of the formal FLAS. The main goal of FLAS is to introduce consistent and dependable rules that support safe vertical separation, rather than handling ad hoc situations.
Key Applications of FLAS
There are several common ways FLAS is used in day-to-day air traffic operations:
Border Level Allocation
When two neighboring ATS units manage adjoining airspaces, they may choose to override the standard cruising level rules described in Annex II—which assign odd levels for eastbound traffic and even levels for westbound traffic. For example, if this standard setup doesn’t fit local traffic needs, the units may agree to alternate responsibilities. Unit A might handle odd levels while Unit B handles even ones, regardless of direction.
Arrivals and Departures
FLAS can also define what levels aircraft should be transferred at during arrivals and departures:
- Arrivals to an airport may need to be handed over to approach control at a fixed level (e.g. FL 100) or within a specific altitude band (e.g. between FL 90 and FL 110).
- Departures from an airport may be passed to area control at or below a certain level, depending on traffic flow and coordination agreements.
- In some cases, arriving aircraft must be handed over to another area control unit at levels below a set value, ensuring smoother transitions across sectors.
Time-Based Level Allocation
FLAS can also be flexible based on the time of day. One well-known example is the North Atlantic airspace, where traffic flows predominantly eastbound in the morning and westbound in the afternoon. To match these daily patterns, both odd and even levels might be assigned to aircraft flying in the dominant direction. This approach allows more aircraft to operate efficiently by maximizing the use of available levels.
RVSM Border Transitions
In regions where aircraft are moving between Reduced Vertical Separation Minimum (RVSM) airspace and non-RVSM airspace, FLAS helps define the appropriate flight levels for smooth handovers. Since separation standards differ between these airspace types, it’s important that level assignments are clearly planned and agreed upon.
Contingency FLAS (cFLAS)
In exceptional circumstances, such as severe turbulence affecting a wide area, regular RVSM procedures might be temporarily suspended. This is where Contingency FLAS (cFLAS) comes into play. Neighboring ATS units may agree on alternative level allocations to maintain safe operations. For example, Unit A might handle flights at FL 310, FL 350, and FL 390, while Unit B manages traffic at FL 330, FL 370, and FL 410. These arrangements are easier to implement when documented ahead of time in letters of agreement, ensuring smooth transitions during emergencies.
Final Thoughts
The Flight Level Allocation Scheme is a foundational part of managing complex airspace structures. By setting predefined rules for altitude assignments, it supports both efficiency and safety in air traffic control. While there’s room for flexibility when needed, having a reliable system in place helps controllers, pilots, and air navigation services work together seamlessly—even when traffic is heavy or conditions are challenging.
