# How do pilots decide what their cruising altitude will be?

Danny Beckett
• How do pilots decide what their cruising altitude will be? Danny Beckett

When filing a flight-plan for a long-haul high-altitude IFR flight, how do pilots select a specific cruising altitude? Typically in the range of 25,000 - 35,000 ft.

• Cruise altitude may be assigned by ATC for separation (IFR especially, or in other controlled airspace -- since, in the US, these altitudes would be Class A and controlled airspace). For example, when you request a DUAT(S) briefing, it will usually give suggested ATC routing and also altitude assignments (along with what's most frequently used).

The altitude you request somewhat depends on aircraft performance, I would imagine, but obviously any ATC clearance must be adhered to unless amended. Hopefully one of the commercial pilots can fill in more of the blanks, though.

• I guess you know the FAR rules for picking discrete values.

When it comes to performance, with a jet you want to fly as high as possible. The only reason not to do so is total distance; on a short hop there is not enough flying time to climb all the way up.

The higher you fly (in the troposphere) the colder the air is, which makes the thermodynamic cycle of all air-breathing engines more efficient. Also, the air gets thinner, so your friction drag is reduced at the same TAS. The specifics depend on the thrust over airspeed, and generally you want to fly at a lift coefficient between $\sqrt{\frac13C{\pi}A}$ and $\sqrt{C{\pi}A}$, where $C$ is the zero-lift drag and $A$ the wing's aspect ratio.

The higher value is perfect for piston engines and props, and the lower one for turbojets (think fighter aircraft engine). With a turbofan you will be between both values. To yield more precise results, both formulas will be longer, but these are the major factors for optimum lift coefficient.

Since there is an optimum cl for maximum range, you might want to climb continuously during the flight to compensate for the lower mass (due to fuel burn) with lower air density, so in reality you adjust altitude in steps, in accordance with traffic control.

• As has been stated, ATC will assign you an altitude, taking into account your requested altitude, traffic conditions, and of course the FAR.

Peter's formulas look reasonable to me from a design standpoint. The aircraft designer will use something like that to calculate typical values, which will be included in the aircraft's operating handbook.

The table is listed by pressure altitude and gross weight. At each combination there will be performance values such as:

• Average N1 (for a turbine)
• Max TAT for thrust rating
• IAS Knots
• Mach number
• ISA fuel flow LB/HR/ENG
• ISA TAS Knots

The flight distance will determine the approximate gross weight, which corresponds to an altitude and cruise speed that provide the best efficiency. Here is an example from a virtual 737 handbook, with the values I listed. The optimum performance is in blue. Higher gross weights have been removed for clarity.

Airlines may have their own performance tables, choosing a custom balance of efficiency and speed. Dispatchers for the airline will take the flight information and decide on a cruising altitude for filing the flight plan, which the pilot will then receive. As you can see, as the aircraft burns fuel and gets lighter, it will be more efficient at higher altitudes. This is the step climb that Peter mentioned, where the aircraft will climb to higher altitudes as the flight progresses when they are cleared by ATC.

They may also take into account weather factors such as winds aloft and turbulence.

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• At some early stage in the flight, why will commercial pilots announce the cruising altitude of the flight?

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• When filing a flight-plan for a long-haul high-altitude IFR flight, how do pilots select a specific cruising altitude? Typically in the range of 25,000 - 35,000 ft.

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