General:

Generally speaking, the higher a jet aircraft flies, the more efficient it will be, and the lower the fuel flow.
In other words specific fuel consumption (SFC) is less. That statement is true up to a point. After a certain flight level called the optimum flight level SFC gets worse as fuel flow increases again. This is due to the increased drag penalty above the optimum flight level as a result of the excessive angle of attack (AoA) required to create enough lift to support the aircraft. Eventually a flight level will be reached at which the AoA will reach stall values.

Aircraft normally cruise with a positive nose up attitude, in swept wing jets this typically approximately 2-3 degrees is the optimum to provide sufficient lift, while limiting drag. You may have noticed cabin staff struggling to pull the meal service trolley toward the front of the plane (ie: uphill).

Optimum altitude is a function of the fuel flow and the TAS. TAS reduces as an aircraft climbs at a fixed Mach number into the cold air environment.
Fuel flow (FF) decreases with altitude up to a point, then increases thereafter.

Practical example using the B727. Refer to the Mach 0.80 schedule on B727 manual page 3-93 extract below.

EMZW 70, 000 kg.      Mach 0.80

FL340 is optimum FL irrespective of ISA deviation.
(ie: SAR is lowest at FL340, due to best TAS/FF combination).

Unfortunately FL340 is NOT an IFR hemispherical level. We could use FL350 if travelling WEST, and FL330 travelling EAST, to get as close as possible to the optimum level, and lower the zone fuel burn off (nil wind).

Note that SAR at ISA +20 is only about 2% higher than in ISA. A good rule is that SAR varies approximately 1% per 10 degrees above/below ISA. Put another way, if enroute cruise temperature is ISA+/-
10C, not correcting for TAS or FF for ISA deviation will produce a FBO in error only 100 kg on a 10, 000 kg
(1, 000 nm) flight.

To assess which IFR FL is closest to the optimum for a particular cruise zone, you simply compare the optimum GW at the IFR levels with the estimated mid zone weight (EMZW). Whichever IFR level is closest in terms of GW is the optimum IFR FL. Refer to example below.

Example 1. Tracking EAST at FL330/M 0.80. EMZW for zone is 70, 000 kg. Temp all levels ISA.

What is the optimum IFR FL for the zone. (Refer page 2-14).

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Difference 8, 900 kg

Difference 4, 200 kg

In this case you will burn less fuel in the zone if you remain at FL330. Checking altitude capability at ISA shows that FL370 would infact not be available until the aircraft GW had reduced to 69, 400 kg, which would occur beyond the mid point of the zone.

Airline crews attempt to be within 2, 500 ft of optimum FL at any time if traffic is not a problem, and winds and turbulence are not adverse.

You should check that at the ISA deviation quoted that optimum FL does not exceed the engines max cruise thrust limits. Selecting a lower mach number may assist.

Example 2. Tracking WEST at FL310/M 0.79. EMZW for zone is 66, 000 kg. Temp all levels ISA +10.

What is the optimum IFR FL for the zone. (Refer page 2-14).

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Difference 1, 400 kg

Difference 15, 600 kg

In this case you will burn less fuel in the zone if you climbed to FL350. Checking altitude capability at ISA shows that FL350 is available before the mid point of  the zone.

Example 3. Tracking EAST at FL330/LRC. EMZW for zone is 72, 000 kg. Temp all levels ISA -5.

What is the optimum IFR FL for the zone. (Refer page 2-14).

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Difference 12, 200 kg

Difference 300 kg

In this case you will burn less fuel in the zone if you remain at FL330. Infact FL330 is obviously about perfect to limit the zone fuel burn. Checking altitude capability at ISA shows that FL370 becomes available when the aircraft GW has reduced to 70, 100 kg, though you would only climb to that level if winds, turbulence or
traffic was a problem at FL330.

Slow traffic ahead at your level could see you have to slow down or climb above optimum to allow you to pass that aircraft. Slow traffic can significantly ruin your day, especially on long haul flights where crew duty times can be exceeded. It can also raise the fuel burn, and in doing so reduce your range, which might have been a critical factor before the slow traffic problem arose.

Now attempt the optimum FL assignments.

Optimum FL assignments

Assignment 1. Tracking WEST at FL310/M 0.82. EMZW for zone is 69, 000 kg. Temp all levels ISA +5.

What is the optimum IFR FL for the zone. (Refer page 2-14).

a. FL310              b. FL330            c. FL350                d. FL370

Assignment 2. Tracking EAST at FL290/M 0.84. EMZW for zone is 81, 000 kg. Temp all levels ISA -5.

What is the optimum IFR FL for the zone. (Refer page 2-14). Ensure max cruise thrust limits are not exceeded.

a. FL290              b. FL330            c. FL370                d. FL350

Assignment 2. Tracking EAST at FL290/M 0.79. EMZW for zone is 82, 000 kg. Temp all levels ISA.

What is the optimum IFR FL for the zone. (Refer page 2-14). Ensure max cruise thrust limits are not exceeded.

a. FL290              b. FL330            c. FL370                d. FL350

Suggested answers

1. c
2. a
3. a