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Aircraft Flight Planning Topic 9.

Drift Climbs


This procedure is often preferred to the step climb type, provided other air traffic is not a problem.

The flightcrew could plan a drift climb, which means holding the same cruise Mach number but de-coupling the altitude hold autopilot function. This allows the aircraft to drift climb, typically at 100-200 ft/min as the aircraft weight reduces due to fuel burn off.

The crew would watch the groundspeed readout during the drift climb, and request whichever level gave them the highest groundspeed. Assuming no traffic separation problem exists, ATC would normally allow that level, even non-hemispherically if necessary.

Drift climb technique is particularly useful on long sectors, such as trans-Pacific, or trans Atlantic.

For our ATPL purposes we will assume we will cruise hemispherically.

Example 1.

A B727 is tracking from Curtin (Western Australia) to Brisbane via ERC H1 routes A576, T11, G326.

Position at 2127 UTC:

Overhead Alice Springs VOR FL330 Mach 0.80 Wind by INS 330M/85 kt TAT -22

Aircraft GW 72, 700 kg.

FL370 wind/temp reported as 270M/100 kt/ISA

If a drift climb to FL370 is begun overhead Alice Springs, how many miles from Alice Springs would the aircraft reach FL370 ?

Points to remember:
  • Use the FL, wind and ISA deviation that is half way between your initial and final cruise altitudes when calculating fuel flows and ground speeds (eg: drift climb from FL330 to FL370, use FL350 wind/temps).
  • For EMZW, use the weight half way between the GW at start of climb, and that GW at TopC.

Step 1. From B727 manual page 2-14 find max arrival GW FL370/M 0.80/ISA is 69, 400 kg.

Step 2. Find FBO required to reach FL370 by subtracting FL370 GW from fix GW at Alice Springs.



Step 3. Find EMZW during climb.

69, 400 + 72, 700

= 71, 050 kg (say 71T)

Step 4. Calculate TAS,GS, FF,SGR at FL350

Step 5. Divide climb FBO of 3, 300 kg by SGR to get miles to TopC.

3, 300 kg divided by 7.543 kg/gnm = 438 gnm. Answer !

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