How do I choose the correct IFR cruising flight level?

In aviation and real-world flying, choose an IFR cruising flight level that is legal for the route and direction, clears every published minimum, suits the aircraft’s weight and performance, and accounts for winds and weather. File the best usable level, not merely the highest one, and expect ATC to assign another.

How do I calculate a suitable IFR cruising level?

Build the level from legal constraints first, then optimise it for aircraft performance, wind and ride quality. We use this order:

1. Establish the vertical reference. Below the transition altitude, fly an altitude referenced to local QNH. After passing the transition altitude in the climb, set standard pressure; the lowest usable flight level is at or above the transition level. The transition level can change with pressure, so an altitude such as 17,000 ft does not automatically become FL170. Our explanation of how altitude, flight level and the transition layer differ covers the altimeter-setting change in detail.
2. Find the controlling minimum. Check every airway minimum, route-segment restriction, terrain requirement and applicable procedure constraint. The highest relevant minimum controls that part of the route. An off-airway direct leg needs its own obstacle-clearance assessment under the rules of the FIR being crossed.
3. Apply the route’s level-allocation rules. Consult the applicable charts and flight-information rules rather than assuming that “odd east, even west” works everywhere. Direction is commonly based on magnetic track, although some national rules use different wording or schemes. Published one-way airways and mandatory route levels take precedence.
4. Check aircraft capability. Use the AFM, POH or approved performance data to check weight, temperature, climb time, service ceiling, maximum operating altitude, oxygen or pressurisation limits and icing capability. Do not select a level the aircraft will reach only as descent should begin. For turbine twins crossing high terrain, engine-out drift-down and escape requirements can be more restrictive than all-engines cruise performance.
5. Compare winds and weather. A slightly lower level may save fuel when it avoids a strong headwind or an unnecessarily long climb. Consider icing, turbulence and temperature as well as wind. Do not treat climbing above forecast thunderstorms as a valid avoidance plan; convective weather requires adequate lateral clearance.
6. Confirm airspace and equipment requirements. RVSM airspace commonly extends from FL290 through FL410 and permits 1,000 ft vertical separation between suitably approved aircraft. Real-world aircraft and operator approval requirements apply. Some FIRs also use metric levels or special direction allocations.
7. File a practical request and alternatives. Make sure the aircraft can reach the requested level after departure restrictions and remain there long enough to justify the climb. Plan acceptable levels above and below it because ATC may be unable to approve the first choice.

Where an ICAO flight-plan field expects coded level notation, F340 means FL340, while A090 means an altitude of 9,000 ft. Some simulator planners instead expect a value in feet, so check what the field is asking for.

Which odd or even flight level should I use?

Under the common ICAO semicircular allocation, IFR tracks from 000° through 179° use odd flight levels, while tracks from 180° through 359° use even flight levels.

This is a planning convention, not permission to fly the level. National regulations, controlled airspace, airway direction, RVSM status and an ATC clearance can all override the basic table. In United States controlled airspace, for example, the ATC-assigned altitude or flight level governs; elsewhere, particular routes may reverse or replace the usual allocation.

Use the route’s track or course as specified by the local rule, not the aircraft’s wind-corrected heading. A strong crosswind can produce a heading on the other side of north or south without changing the appropriate cruising-level direction.

Should I choose the optimum or maximum flight level?

Choose a level with useful performance margin, not the aircraft’s absolute ceiling. The maximum permitted or achievable level is a limit; the optimum level is where the aircraft is expected to operate efficiently at its present weight and conditions.

• Piston aircraft: allow enough climb performance for turbulence, downdraughts and rising terrain. The published service ceiling is rarely a sensible routine cruise target, and oxygen requirements may impose a lower practical limit.
• Pressurised turboprops: compare time-to-climb and fuel burn with winds and route length. A high level can lose its benefit on a short sector.
• Jets: request a level near the calculated optimum while retaining margin below the maximum. A heavy jet may begin lower and request step climbs as fuel burn reduces its weight.

Suppose a westbound jet has a magnetic track of 245°, a route minimum of FL190, an optimum near FL350 and an initial maximum achievable level of FL355. Under the common direction scheme, FL340 is the practical request: FL350 is the wrong directional allocation, while FL360 is initially above the aircraft’s capability. A later step to FL360 may become possible.

Simulator-generated cruise levels still require checking. In Microsoft Flight Simulator, our guide to selecting and checking cruise altitude in the built-in planner explains how to compare its suggestion with terrain, routing and aircraft performance.

Why did ATC assign a different flight level?

ATC may change the requested level for traffic separation, airway restrictions, sector capacity, crossing traffic, military activity or equipment limitations. A filed level is only a request; the cleared level is the one to fly.

If the clearance exceeds the aircraft’s capability or creates another safety problem, advise ATC that you are unable and request an alternative rather than accepting it. Simulator ATC can also issue inefficient or poorly timed level changes, especially after rerouting, so cross-check every instruction against aircraft performance and published minima. Our complete simulator IFR workflow explains how the cruise clearance fits between departure, en-route changes and descent.

Common IFR cruising-level mistakes

The planning error we see most often is choosing an attractive cruise number first and checking the route afterwards. Avoid these specific traps:

• Confusing an altitude on QNH with a flight level on standard pressure.
• Applying an odd/even rule from the wrong country, FIR or airway.
• Using aircraft heading instead of the track or course specified by the applicable rule.
• Checking terrain near the airports but overlooking high ground on an en-route direct leg.
• Treating the service ceiling or maximum FMS level as a normal cruising target.
• Assuming a higher level is always faster without comparing climb cost and forecast winds.
• Trying to overfly convection instead of planning lateral avoidance.
• Keeping the original level after a reroute changes the track, minimum altitude or airway restrictions.