There are times when it is a good idea to climb to a higher altitude whilst enroute. This could be to get an increased tail wind/decreased head wind, to reduce the fuel flow, or to escape from turbulence. Either way, the new flight level you wish to cruise at may not be available immediately, and you may have to delay the step climb until the aircraft GW is reduced to a suitable level.

We will be referencing page 2-14 of the B727 manual when undertaking step climb calculations.

Points to remember: Manual page 2-2a, para 2 states that an abbreviated method to be used in step climb calculations is to allow 50 kg/1, 000 ft of step climb to account for the extra fuel burnt in the climb. No distance need be calculated for the step climb zone. (ie: the aircraft is suddenly at the new level having burnt 50 kg of fuel per thousand feet. No climb time need be allowed for either. A step climb is different from a “drift climb” technique in that the step climb is deliberately delayed until the point when a climb can be made directly to the new level at climb speed (Mach 0.78) at climb thrust. A drift climb occurs at cruise power, and cruise speed. These are usually stand alone questions in ATPL exams, though you may incorporate them in a large normal operations flight plan if you feel it would reduce the total flight fuel.

Example 1.

A B727 is tracking from Auckland (New Zealand) to Hobart on ERC H5 route L513. At TopC (150 DME Auckland) FL310 the aircraft begins to cruise at Mach 0.80 at a gross weight of 75, 000 kg, in ISA +10 conditions. The wind is 115 kt HEAD.

Turbulence begins to increase as a result of a forecast upper level jetstream with a core centred at FL330.

The Captain wishes to climb into the smooth air reported at FL350 as soon as possible.

Assuming the aircraft continues at Mach 0.80/FL310 until a climb to FL350 can be commenced, and that Mach 0.80 schedule will be used on reaching FL350 where the temperature is ISA +10, at what minimum distance past TopC can the climb to FL350 be commenced ?

Points to note:

• A common mistake is to subtract the step climb fuel from the TopC GW instead of adding it. This will almost definately mean a fail in this case.
  
• Be aware of whether it is a head or tail wind. Another way to fail.
  
• When finding the TopC GW at the new level, remember to use the ISA deviation/Mach number combination at the new level.
  
• If you are asked to step climb as soon as possible, and no cruise Mach number for the new level is quoted, enter page 2-14 and extract the cruise schedule that has the highest GW. This will reduce the fuel burn off required, and in doing so shorten the distance to the start of climb point. Typically M 0.79 will allow a step climb soonest, even sooner than Long Range Cruise (LRC).

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