Copyright Avfacts 1998.
Drift climb calcs page 1.
Drift Climbs
General:
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
Aircraft GW 72, 700 kg.
FL330
Mach 0.80
Wind by INS 330M/85 kt
TAT -22
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 ?
•
•
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.
GW 69, 400 kg
FL350 wind/temp
85 kt/ISA
From
Curtin
FL330
2127 UTC
GW 72, 700 kg
EMZW
71T
ISA
FL370 to Brisbane
M 0.80
Mach 0.80
TAS 461 kt (FL350)
GS 546 kt (FL350)
FF 4, 119 kg/hr (FL350)
SGR 7.543 kg/gnm
Climb FBO 3, 300 kg
Alice Springs
437 gnm
Copyright Avfacts 1998.
Drift climb calcs page 2.
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.
Fix GW
72, 700 kg
TopC GW
-69, 400 kg
Climb FBO
3, 300 kg
Step 3. Find EMZW during climb.
69, 400 + 72, 700
2
=
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 !
Example 2.
A B727 is tracking from Alice Springs to Perth via ERC H2 route J64.
Position at 0727 UTC:
Overhead Ayres Rock NDB VOR
Aircraft GW 78, 700 kg.
FL310
Mach 0.79
Wind by INS 280M/65 kt
OAT -52C.
FL350 wind/temp reported as 260M/30 kt/ISA +5C.
If a drift climb to FL350 is begun overhead Ayres Rock, at what time would the aircraft reach FL350 ?
To Perth FL350
M 0.79
TopC 75, 500 kg
0810.5 UTC
EMZW
77T
ISA+5
FL330
36 kt/ISA
0727 UTC
78, 700 kg
M 0.79
TAS 459 kt
GS 423 kt
FF 4, 409 kg/hr
SGR 10. 423 kg/hr
Climb FBO 3, 200 kg
ETI 43.5 min
FL310 from
Alice Springs
ISA-6
307 gnm
Ayres Rock
Copyright Avfacts 1998.
Drift climb calcs page 3.
Step 1. From B727 manual page 2-14 find Max arrival GW FL350/M 0.79/ISA+5C is 75, 500 kg.
Step 2. Find FBO required to reach FL350 by subtracting FL350 GW from fix GW at Ayres Rock.
Fix GW
78, 700 kg
TopC GW
-75, 500 kg
Climb FBO
3, 200 kg
Step 3. Find EMZW during climb.
75, 500 + 78, 700
2
=
77, 100 kg (say 77T)
Step 4. Calculate TAS, GS, FF, SGR at FL330.
Step 5. Divide climb FBO of 3, 200 kg by SGR to get miles to TopC.
3, 300 kg divided by 10.423 kg/gnm = 307 gnm. Answer !
Step 6. At an average groundspeed of 423 kt it will take 43.5 minutes to reach FL350.
Arrival at FL350 calculated to occur at 0810.5 UTC. Answer !
Now attempt the drift climb assignments.