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-   -   Saving climb fuel (https://www.pprune.org/tech-log/642487-saving-climb-fuel.html)

LLClimb 5th Sep 2021 14:22

Saving climb fuel
I've found that you can climb at speed faster than max rate climb speed (Vy) and save fuel. When you climb at faster speed, you will get a longer ground distance. So if you climb from the same starting point, to the same target point at higher altitude, a climb at lower speed will need to fly level after reaching target altitude to get to target point. When adding fuel used from both climb leg and cruise leg, the total fuel used can be higher than fuel used from the direct climb to target point at higher speed. Is this a common knowledge? Is it used in some useful applications?

Capt Scribble 5th Sep 2021 16:35

Doh! You had better tell Airbus.

JRK 5th Sep 2021 20:21

When I climb I am usually too busy watching the magnificent scenery outside to bother about such a thing.

Locked door 5th Sep 2021 20:48

Max rate of climb is min fuel? Since when?

Cost index zero is min fuel (at least at my outfit) and CI 20 is min cost. Neither produces a climb speed at Vy unless itís a coincidence and both vary dependent on the wind.

Banana Joe 5th Sep 2021 20:49

I let VNAV do the job, according the CI entered and TOC wind. If I am requested best rate, I have the option on the CLB page.

Banana Joe 5th Sep 2021 20:50

Locked door

I remember being told at ground school during my ATPL that climb at full thrust actually saves fuel as you spend less time at climb power and more at cruise power. But you increase maintenance costs.

Capt Scribble 5th Sep 2021 21:16

Faster speed, increased drag, lower for longer and climb thrust all increase fuel burn. I'm not clever enough to do the math but the FMGC is.

Occy 5th Sep 2021 23:20

Vmo minus a few knots, you get there quicker and ATC thinks youíre bad ass and not to be trifled with. Mike drop.

SoulMinties 5th Sep 2021 23:39

Our company SOP was to use CLB rather than de-rate even after a TO-1 or -2 departure. All our ops were at CI15.

KRviator 6th Sep 2021 00:15

The difference being, the level segment at TOC is done at a far higher density altitude than your suggested high-speed cruise climb through the lower altitudes. Higher DA = Lower pressure = lower drag.

Lower altitude = higher air pressure = lower TAS for given IAS = more drag = more fuel used in that climb. Best bet as a rule of thumb? Ignoring wind, get as high as you can, as quickly as you can, and stay there as long as you can are the keys to saving fuel. In normal ops? put your desired CI in the box and let it do the thinking for you. It can do it faster and better than I ever could.

LLClimb 6th Sep 2021 04:08

There is a climb speed, called “Lean and Long Climb (LLC)” speed that maximizes fuel saving from a direct climb to a target point at higher altitude, over an indirect climb with minimum fuel climb plus maximum range cruise to the same target point. I’m preparing an article on LLC to submit to AIAA Journal. I just need to know if I’m repeating a work someone else has done already.

vilas 6th Sep 2021 04:56

It's different than what you think.

SpamCanDriver 6th Sep 2021 13:16

Archive mole

Amen Brother 🙏

BizJetJock 6th Sep 2021 16:13

We don't have CI on our little brain FMSs, but for the Challenger 604/605 series 10 minutes with the Flight Planning manual tells you that according to Bombardier you use less fuel to a point along track by climbing at higher speeds than by climbing at max rate then cruising. But what does the manufacturer know, eh?

tubby linton 6th Sep 2021 16:32

Climb quickly into a tailwind and slowly into an increasing headwind seems to work.

RVF750 6th Sep 2021 17:05

We don't have the choice. Just CI and ECON all the time. Policy of dumbing down to prepare us to fly the French things soon.

tdracer 6th Sep 2021 20:10

It may not be universal, but I saw a study ~20 years ago that for the 757/PW2000, derated climb was a false economy. Derated climb increased fuel burn, and it didn't do anything positive for the engines. In fact, some derated climbs could cause the compressor stators to operate in a range that could cause harmonics that could crack the stators.

LLClimb 7th Sep 2021 05:30

Optimum speed?

Very interesting. Does Bombardier recommend a speed to climb to a point?

BizJetJock 7th Sep 2021 11:53

No they don't specifically recommend a profile. They just give fuel/time/distance tables for 2 profiles - 250/.72 (best climb) and 300/.78 (high speed).
When you compare the fuel to climb at 300/.78 with the fuel to climb at 250/.72 then cruise to the same point you get a lower fuel burn. Not by a lot, but it is definitely not more!

Capt Fathom 7th Sep 2021 12:05

There is a climb speed, called “Lean and Long Climb (LLC)” speed
Not something I have heard of.
I just follow the company standard procedures which no doubt closely follow the manufacturers procedures.
There is always times when you can save some fuel based on your experience and common sense…. but without rewriting the Performance Manual!

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