Pre-flight planning: route selection, ATS route structure, levels and airspace
Domain 3 examines route and level selection: how the ATS route structure is organised, how airspace restrictions constrain the route, and how a cruising level is chosen and validated.
Route structure
- ATS routes are published in the AIP and en-route charts with designators whose letter prefix identifies route type and region (for example L, M, N, P for RNAV routes, A, B, G, R for conventional regional routes).
- Each segment carries a minimum en-route altitude or minimum flight altitude giving terrain and obstacle clearance; grid MORA gives 1,000 ft clearance, 2,000 ft in mountainous areas.
- One-way airways, direction restrictions, conditional routes (CDR) with published availability, and organised track systems (such as the North Atlantic OTS) all constrain the choice.
- Airspace restrictions: prohibited, restricted and danger areas, temporary reserved or segregated areas, military exercise areas, and NOTAM/AIP SUP changes must be checked along the whole route, including alternates and diversion routes.
Level selection
- IFR cruising levels follow the ICAO table on magnetic track: 000–179° odd flight levels, 180–359° even, from the transition altitude upward; RVSM applies FL290–FL410 at 1,000 ft separation.
- The level must be above the route minimum, within the airway direction rule and RVSM approval, and is then chosen for the most favourable wind component and lowest fuel burn.
Worked example
Track 275°M, so an even level applies. Options are FL300 with wind 300/80 and FL340 with wind 290/110. At FL300 the wind is 25° off track, giving headwind 80 × cos 25° ≈ 72 kt. At FL340 it is 15° off track, giving 110 × cos 15° ≈ 106 kt. FL300 offers the smaller headwind, so FL300 is filed, provided the route MEA and airway direction allow it.
Traps
- Using true track instead of magnetic track for the level table.
- Forgetting that some states apply a quadrantal rule below a stated level.
- Choosing a level without checking the airway MEA, one-way status or CDR availability.
- Checking NOTAMs only for the destination and not for the route and alternates.