Climb optimisation

[Bula]

Just a question i've been trying to look at but am dumbfounded if I can find an answer.

On climb, strong jet streams at upper levels.

On climb is it better to,

1. maintain the FMS calulated climb speed (284/.77 kts for example) or

2. BRoC (somewhere between 250 and 275 into .76 until up there) if strong tailwinds at upper levels, and/or

3. in the case of strong headwinds at upper levels maintain FMS climb to around FL200 then accelerate to 300-320/.78.

Cheers

[Intruder]

Maintain the FMS climb speed. You want to get to altitude in the least time possible, so you can get to most efficient cruise fuel flow. Make your wind adjustments in cruise.

[Wizofoz]

Actually, I think No.s 2 and 3.

There is no climb forcast page, so I Think the FMC assumes linear variation between current wind and wind at altitude. As this is not the case with a jetstream, climbing as quickley as possible into the tailwind, or getting as many track-miles as possible before climbing into the tailwind, would seem the most efficient way to fly.

[lalbak]

Something you can do is look at your groundspeed next time you climb. Even with a headwind your GS will increase as you climb and as soon as you start descend it will start dropping again. This makes it sound logical that you'd want to climb as fast as possible. From personal experience this is the case with turboprops, not sure if it's also the case for jets climbing to FL300+.

[Ashling]

If fuel efficiency is your sole driver then option 2 works best for either headwind or tailwind although your final cruise level may differ in each case. Cost Index 0 will give you BROC and in any case it should be a selectable option on your FMC.

What complicates things is Cost Index and that is governed by company policy so at the end of the day you fly your companies cost index and accept the climb speed it gives you.

[Bula]

Thanks for your replies. The A320 does have a climb forecast page, but alas the box seems a little perplexed even though it allows for wind variation.

From what I can gather reading a few books on aircraft performance it might have something to do with the aircrafts constant acceleration during the climb, and a reduction in that acceleration when maintaining a constant MACH > 30 000 feet and the modern use of the Constant IAS and Mach climb method, but i cant seem to find the info i'm looking for.

The FMGC variations seem to be only at the upper levels and related directly to Mach, which seems to make sense due to fuel burn and TAS at lower levels. Seeing it takes around 15 minutes to climb 30 000 and a furher 8 minutes for the last 7 000 feet in the A320, where a 0.01 mach change = 6-8 kts of TAS.

So.. from what I can gather,

Get up there to reduce fuel burn and > TAS.

Allow the CI and it applicable MACH variance to counter for the upper level wind ie Targeting a Slower MACH with strong tailwinds, and a higher MACH with headwinds.

Now.. heres another one.. how about descents :)

[waren9]

Program the box best you can. Use history winds if available, or prior to departure put in your (forecast) climb winds that reflect what you anticipate.

Use Cost Index as per company policy. Fly managed speed.

The FMGC generated speeds will reflect what speed to fly. In the case of strong headwinds it wil command approx M0.01 faster per 100kts headwind and vice versa for a tail wind.

Same answer for descents. The better you program the box, the more accurate the answer.