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MUC79
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Speed Reduction For Better Blockfuel Performance?

Wed Apr 20, 2011 5:17 pm

With rising fuel prices, more and more airlines tweak their cost factors for lower speeds and better fuel consumptions, or at least they tell their pilots to have an eye on the velocity. Air Berlin did this for example in 2009 with their A330s, saving a significant amount of fuel. If I remember correctly, JetBlue started off with this in 2006, more airlines followed.

However, all of these approaches only reduced cruise speeds marginally, let's say by 1 or 2%. This would lead to additional flying times of few minutes on short range. The whole procedure reaches is limits when departure delay needs to be re-captured, connecting flights are waiting, or the schedule is just too tight to loose these minutes in the air. So usually the last flight of the day is predestined to be taken a little more easy.

What do you think would a significant speed reduction of let's say of -10% on long-haul help in terms of block fuel? Let's leave aside all the schedule and delay issues and time dependent cost elements of the DOC. Is there a point, where flying even slower would worsen the overall fuel performance? An how would this be linked to the "cruise bucket"?

Thanks a lot in advance for your thoughts!
 
wilco737
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RE: Speed Reduction For Better Blockfuel Performance?

Wed Apr 20, 2011 5:27 pm

Many airlines did that. We reduced the cost index (so the cruise speed) as well to save fuel. But you cannot do it even further, because then the flight time is a lot longer and you need more fuel and pilots and F/A's get more paid as they are longer in the air. So the truth is in between.

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vikkyvik
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RE: Speed Reduction For Better Blockfuel Performance?

Wed Apr 20, 2011 5:35 pm

Quoting MUC79 (Thread starter):
Is there a point, where flying even slower would worsen the overall fuel performance?

Yes, there is. In a basic sense, the drag curve is U-shaped. There's a minimum drag point, and on either side, the drag (and hence fuel burn) increases.

As you go faster, drag increases due to friction (parasite drag) and eventually due to supersonic flow.

As you go slower, you have to keep increasing the angle of attack to maintain level flight, which increases the lift coefficient, which increases induced drag.

Typically (I think, but could be wrong), airliners cruise faster than their minimum drag point, so there can be a fuel-consumption benefit through flying slower.
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LAXintl
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RE: Speed Reduction For Better Blockfuel Performance?

Wed Apr 20, 2011 5:43 pm

Many airlines with more modern cockpits operate using what is called 'Cost Index' which is basically the cost of time versus the cost factors such as fuel. As fuel cost rise and take on a larger place over other cost, the cost index drops, which in essense reduces the cruise speed.
The goal with Cost Index is to produce the lowest overall trip cost by balancing the various factors.
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aviopic
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RE: Speed Reduction For Better Blockfuel Performance?

Wed Apr 20, 2011 6:09 pm

Quoting MUC79 (Thread starter):
An how would this be linked to the "cruise bucket"?

A lot can be found in studies like performed by Nasa and/or Dutch NLR and probably many more.

They show designs with reduced cruise speed to M 0.72, nearly un swept wings, removal of slats and different fuselage shape.
All together this must lead to a +/- 50 % fuel burn reduction.
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prebennorholm
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RE: Speed Reduction For Better Blockfuel Performance?

Thu Apr 21, 2011 2:01 am

Quoting MUC79 (Thread starter):
Is there a point, where flying even slower would worsen the overall fuel performance? An how would this be linked to the "cruise bucket"?

Yes, there is always an optimal economic cruise speed for any plane. It is basically when the wing is operation at the angle of attack which gives the best lift to drag ratio.

But it is never a single speed for any given plane. The higher you cruise, the faster the economic speed. The heavier you are, the faster the economic speed. And visa verse.

Then the whole picture is complicated by the wind. With headwind it is beneficial to fly a little faster since you spend less time battling the wind. And visa verse. A comparison of true airspeed (from indicated airspeed and altitude) and ground speed (from GPS) will indicate your headwind/tailwind component and go into complicated calculations about optimal speed on different sectors of a long range flight.

A major issue, however, is to have the optimal altitude assigned by ATC. A long range plane will often change to higher altitudes as fuel is burned and weight is reduced.

Wind is often known beforehand to be different at different altitudes. That may make you prefer an altitude which is somewhat lower or higher than the otherwise optimal altitude.

At the end of the day those calculations are further complicated by how important it is to arrive fairly timely. On most charter flights an hour or two late is a non-issue, while opposite when on a scheduled flight you carry lots of pax who are fetching connecting flights at your destination.
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MUC79
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RE: Speed Reduction For Better Blockfuel Performance?

Thu Apr 21, 2011 7:54 am

Thanks to all of you for your replies! That already helped a lot and give me some hints were to further look at!

A further search brought up this article, which explains things quite well in my opinion:
http://www.boeing.com/commercial/aer...es/qtr_4_07/AERO_Q407_article5.pdf

[Edited 2011-04-21 01:34:31]

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