Fuel savings slow speed in descent ?

[Mach trim]

Why do you save fuel going slowly like 265 knots in descent ?

The aerodynmics behind it are ? I knew it once but have forgotten and am too lazy to look it up.

Thanks for helping me out.

In a A-320 ?


As opposed to staying high and doing a higher speed descent

p.s.
By the way this forum is great, I have learned a lot here and a ways to go

[TO MEMO]

Ok here goes...

Lets suppose you`re flying CI 200, you would be cruising at say M.81 on an A320 and descent speed would be around 330 kts.

If flying CI 10 (a common value these days) your descent speed would be around 250 kts.(less noisy too, you can reduce the headsets volume, thereby saving your hears)

On the first case descent will take around, lets say, 18 minutes (wild guess, not refering to FCOM) and in the 2nd case it will take, lets say, some 27 minutes. There is a 9 minute difference from the 1st case. So that`s 9 minutes of engines operating in idle, instead of cruise thrust.

Let`s see that in detail. An average A320 fuel consuption with engines in idle is 12 kgs per minute and in cruise 40 kgs per minute. So 9 minutes in idle means 108 kgs used. 9 minutes of cruise thrust means 360 kgs. But of course if you were flying CI 200, you`re flying faster, so instead of 9 more minutes in cruise, it will actually be around 3 or 4 minutes. But 3 minutes at cruise thrust represent 120 kgs of fuel, that`s already more than the 108 kgs of fuel for those extra 9 minutes of descent. Also CI200 will make you fly less time, but average fuel flow will be higher than with lower cost index. You also have to take that into account.

To be accurate you have to refer to FCOMs, inflight performance.

From personal experience, yes it does make a difference. It also makes a difference in fuel consumption if you do constant descent approaches. ie making an approach to London will take less fuel, thanks to their beautiful concept of CDAs than in Milan or Paris, for example.

Safe flying

Cheers

[kijangnim]

Greetings,
slower speed means more time in the sky, means longer flights, means Cost increase such as maintenance, flight crew, over flying taxes, head wind ( the longer you stay in it the more you suffer from it)

[misd-agin]

Want to know what your aircrafts L/D speeds are? Check out the dual engine failure glidespeed.

Typically around Vref + 90 kts (give or take a bit).

[OBK!]

kijangnim...it's taken into account...it's whole idea of cost index.

[kijangnim]

Greetings OBKThe subject is low speed descent, so cost index ZIPPO

[Intruder]

it's taken into account...it's whole idea of cost index.

Cost Index is designed to offset simple fuel cost with all other operational costs. If you want strictly the lowest fuel burn (as indicated by the OP), you use ECON with CI=0. Then you will get airspeed and altitude calculations for max range, adjusted for wind.

[TO MEMO]

Question posed in top slot:
"...Why do you save fuel going slowly ... in descent ? The aerodynamics behind it ...?"

It`s not a question of aerodynamics! It`s a question of fuel flow, as I explained in the 2nd post! It`s a question of gaining more or less time with engines on idle.

Try this...
Do one flight one day, using descent speed 250 from TOD till approach. Take note of the number of minutes and fuel used from TOD to landing.

The next time make a descent at 330 Kts from TOD till approach. Take note of the same figures!

Then compare... you will see why...

[misd-agin]

To Memo -Unfortunately your method is incorrect. The fuel and time savings have to include the additional miles you flew to get to the closer TOD for a 330kts descent. You're much closer to the airport, by more than 20-30 n.m., or more, if you can descend at 330 kts vs. 250 kts. Getting closer cost you time and fuel.A better way would be to figure out what the time and fuel was from a fixed point, say 100 n.m. to 10,000. Try the descent at both 250kts and 330kts.That said, if you're trying to save time the descent page is the most efficient way to save time vs. additional fuel burned.

[TO MEMO]

misd-agin, you`re correct! I may have not expressed myself correctly, but what you wrote, is what I actually ment.

Cheers

[St. Ex]

"Greetings,slower speed means more time in the sky, means longer flights, means Cost increase such as maintenance, flight crew, over flying taxes, head wind ( the longer you stay in it the more you suffer from it) You were being asked about FUEL SAVINGS: "Why do you save fuel going slowly like 265 knots in descent ? The aerodynmics behind it are ? I knew it once but have forgotten and am too lazy to look it up.Thanks for helping me out."Going slow will surely save you fuel but will cost you in terms of time. This is the concept of COST INDEX.

[kijangnim]

Greetings,
We can debate for centuries, however what happens in the real world?
Tomorrow within a 100% users prefered trajectories, block altitude airspace then lower speed to glide at idle thrust (still debatable fuel Flow) then saving 20 nm to 30 nm from cruise segment, tranformed into descent segment, yes you will save some fuel providing that there is no error on your TOD position.
Today different ball game, you cannot do what you want in the airspace, and ATC manages their airspace as per their rules and creteria.
Cost index or no cost index it is a fact.

[jetjockey737]

Hi Guys and Gals

I am sure that I saw a lower fuel flow today at idle des at 320 kts than we got a idle econ desc. ( 256kts) at FL390 and 380 respectively. Day five of earlies though so I didnt pay it too much attention. Was I imagining things?...please be gentle!!!

[Rick Studder]

This certainly is a question of aerodynamics, and has everything to do with the lift to drag ratio. Parasitic drag increases with the square of your increase in speed. Disregarding headwinds, any time you fly faster than best L/D ratio you waste fuel. Therefore lower descent speeds, closer to best L/D, save fuel.

[Antunes]

head wind ( the longer you stay in it the more you suffer from it)

You can see your speed going up when you have strong headwind - the aircraft does it to reduce the amount of time you're exposed to that wind.

In opposite way, if you experience strong tailwind your mach number will be very low. As an example, yesterday flight was flown at Mach .64 @ FL350 with 50kt tailwind (CI=8 on an A319)

During descent, as Rick Studder said, it's better to fly near the best L/D ratio (green dot). The lower the cost index, the closer you'll be to green dot speed.

Getting to grips with fuel economy, published by Airbus, is a great literature for this subject. I think you may find it on smartcockpit.

[misd-agin]

There are opportunities to be at idle from TOD to configured. Limited opportunities but they do exist.

Last night, FL390 to 800'AGL at full idle. Approx. 135 n.m. at idle. Fun.

Today, idle TOD until 1000' AGL. Granted ATC keep us slightly high so we needed partial speedbrakes to get back on profile(already near barber pole with too much energy, ergo partial speedbrakes(1/4?) for several minutes).