A320 : fuel efficient descent

[airsp]

Hello,

I'm flying currently Cost Index at 10.
The managed speed during descent is 250kt.

I'm wondering what the best speed is during the descent for fuel efficiency

250kt, 280kt, 300kt, GD ?

[Natstrackalpha]

Cost Index Most cost effective ops.

In:

Fuel

Time

Maint.

The graph at the top is the A320 family (319/320/321)
The graph below is the 340 (I think)

[vinayak]

That's odd... In my present airline we are using cost index 4 and it gives us a speed of 270 IAS

We used to do .78/300 for the longest time until the FDM showed using the managed 270 knots was saving fuel.

Also, increasing speed or decreasing speed for tactical advantage is alway benificial in a situation where it could enhance traffic flow.

For instance an airplane like q400 or ATR72 we would increase speed if we are at almost the same distance to land, minimising his delay and enhancing the traffic flow.

Hope that helps...

[HazelNuts39]

From Airbus 'Getting to Grips with Airplane Performance':

To minimize overall fuel consumption during flight, a low cost index must be used. As the descent phase is performed at idle thrust, it is advantageous to maximize its duration, from a fuel consumption standpoint. This is achieved at a low descent speed, which depends on the aircraft type (e.g. 250 knots for the A320 family). In any case, the descent speed must remain above green dot.
CI = 0 ⇒ IASECON = Minimum descent speed (depends on A/C type)

[OPEN DES]

With CI=0 some a320's default to 250 as minimum, others 270.
Theoretically for minimum fuel (ci=0) you'd descend at green dot, but this is highly impractical. Also don't forget that we aim for minimum cost, rather than minimum fuel.

[HazelNuts39]

Open Des,

Please excuse my ignorance, I'm just curious:

I also thought that green dot would be minimum fuel. What makes it impractical?

True that cost index mimizes cost, by weighing the cost of fuel against time-dependent costs. My understaniding is that CI=0 minimizes fuel, there are no time-dependent costs. Am I wrong?

[OPEN DES]

You're right in your assumptions.
Impractical in the sense that a green dot descent in terms of ATC planning, traffic flow etc is impractical.

[OPEN DES]

The simplified minimum fuel flight goes as follows:
Single engine taxi-out
TOGA take off with minimum certified flap setting (maintain TOGA for 5min AEO)
Accel at 400ft (legal minimum) to best rate of climb speed (CI=0) with MCT.
Cruise at OPT level flying max range speed (CI=0)
Idle descent at green dot (NOT CI=0 as this defaults to 250/270)
Decelerated approach stable at xxxx ft
Landing with minimum certified landing flap
APU start 2 min after touchdown
Single Eng taxi-in

Ofcourse this is all very academic albeit simplified but good to keep in mind.

[Natstrackalpha]

Also don't forget that we aim for minimum cost, rather than minimum fuel.

Ok, am learning too . . .

Do you mean, the longer you are up there it costs more but your fuel flow is lower . . .? Thereby, if you skittle on down and get slotted into the flow of things (traffic) then you burn more . . (?) . . but you get there quicker . . .?

but anyway, can you explain what you mean here, please . . .(thirsting for knowledge)

OK. And your point is exactly? That guys are not flying fuel efficient in your company?

Am not really making any point just musing. I`m just saying there are a lot of different descent patterns and at the end of the day they seem to be as fast and effecient as the other as the bus tends to burn less fuel and the amount in descent is "academic" in the sense that it is not all that much fuel, unless you are going to descend level out for long periods, descend then level out again.

While we are here though, I worked out the difference between say Mach0.81 and .78 is only going to give you a saving over 1.5 hour of 60 miles, but you will have burned the extra and been merely 60 miles back or ahead depending on the your Machno.

That the bus is so refined in this context that it is already fuel effecient as a design aircraft and to try to tweek anymore out of it is like adding sugar to candy floss.

As it can back calc the descent from T/D to ROD whether it is 280 then 250 then green dot or whatever so that in an ideal world of no traffic, no weather and nothing to hit, it will descend at idle maintaining a seemless 3 degree descent - my only qualm is the early reversion (for want of a better word) to approach speed - solution: go Selected until you are ready for the approach (speed).

Rather than evaluate the possibility of saving fuel in the descent by concentrating on a particular speed, it might be more cost effective to consider the descent profile and descent management instead.

[OPEN DES]

I mean to say that the cost of fuel has to be seen in the context of other costs. That's why such a thing as cost-index has been invented.
The minimum fuel flight as I described above is a relatively slow flight, in reallife the fuelsaving does not offset the cost of time, engine wear (toga!), traffic flow requirements etc.

[Ghost_Rider737]

It depends on what your performance and idle factors are on the status page.

Cost index varies according to those numbers.

Newer engines have lower idle and performance factors I think.

Our A320s and have IAE V2500 engines.

CFM ....possibly different.

So CI 0 at one airline may have different speeds to another airline using different or older engines.

[OPEN DES]

Surely the resulting managed speed target depends on perf and idle factors. But no matter how you look at it: an idle green dot DESCENT at max L/D will lead to the lowest SFC and thus trip fuel, regardless of engine type etc. This should coincide with CI=0, but on the A320 the lowest econ des speed is limited to 250/270.

[OPEN DES]

OK. And your point is exactly? That guys are not flying fuel efficient in your company?

[HazelNuts39]

OPEN DES,

I realize that, as you say, the 'minimum fuel flight' in your post #8 is 'academic'.

But, considering that the reduction in tripfuel obtained by descending at green dot instead of some higher speed is due to lengthening the descent distance, i.e. starting the descent earlier, I'm wondering if the FMC allows you to calculate the top-of-descent point for a green dot descent?

[OPEN DES]

Hi Hazelnut,

Yes you can change the SPD LIM/ALT default from 250/10000 to GD/XXXXX. You will however need to take into account the 1kt/1000ft increment above FL200. This technique is sometimes used to finetune the managed descent profile when an early speed reduction below 250kt is given.
This is getting very academic though....

[mikedreamer787]

At TOPD chop an engine.

Descend flight idle at best L/D (green dot).

THAT is a fuel efficient descent. Academic
but fuel efficient.

[flyingchanges]

Why stop there, start the APU and shut both down.

[J.L.Seagull]

Sorry to be a bore and get serious again, but...

Here's something that I would like to throw into the mix:

Flying a COST INDEX is not about minimizing costs, or choosing the most cost effective way to fly. My understanding is that it makes estimating the cost of a flight more predictable. It gives an index (a factor, or a multiplier) for the bean counters to multiply the seat-kms flown.

So, it basically allows for more accurate profit/loss predictions (for the ops guys and the shareholders), which makes it easier to run the business.

For the CI concept to be used effectively, there should be a specific CI for each route and A/C type on that route. What say? Ideas?

Going back to the original post, OPEN DES's flight in post 8 is theoretically the best (while remaining legal)assuming that ATC and traffic would not interfere with your flight profile.

One more thing: You could do away with taxiing altogether though, and simply be towed to and from the runway! e-taxi and other such systems might actually become a reality sooner than we think!

[andreminella]

CI involves much more than only fuel.
it is included how much your company spend per flying hour and many more.
At the company which I work, some pilots reduce cost index when the ETA is earlier than the planned trying to reduce cost, with fuel saving. I asked to engineering if this action would reduce cost, they told me that the lowest cost is flying the CI for that route.

[FLEXPWR]

Hazelnuts39,

Setting cost index to zero should not get you flying at green dot. You should be higher by 15-20 knots out more depending on altitude and winds.

If you look at a typical L/D curve for jet aircraft, it is quite flat close to L/D max. Flying at GD+20 would cost the same than flying at GD. Maybe not the same exactly, but no noticeable difference, so Airbus chooses some speed higher than GD.