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Old 19th Nov 2012, 15:57   #1 (permalink)
ona
 
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Question Fuel consumption question: holding vs reducing speed

Hi everybody,

I would like to know if you can shed some light on this matter . As an air traffic controller, I have often wondered which is best for fuel economy, reducing or doing a few more miles, which could be a path stretching or a holding pattern.

For example, consider an aircraft flying at FL 330 which is ten minutes before its top of descent and flying at M80. And imagine we need , in order to sequence that particular aircraft for the approach, to have him loose 20 NM. Consider the two following choices, both of which would make the aircraft loose 20 NM (about 3 or 4 minutes):

Choice 1: reduce speed during the whole descent phase, reduce speed to M77 instead of M80 at first, and then below FL 250 reduce speed to KT 220 instead of KT260/KT280.
Choice 2: let aircraft fly at own speed, and then loose the 3/4 minutes at a TMA entry point or clearance limit, by doing a full holding pattern at approximately FL 150.

Is one of the choices clearly better for fuel economy, or is there no universal truth and it depends on aircraft type?

Thanks in advance!
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Old 19th Nov 2012, 18:57   #2 (permalink)
 
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altough i'm not flying on the line that long so my opinion can be wrong i would go for the slower speed.
Our company recently lowered the speeds we have to fly in order to save fuel so i'm quite sure it will save fuel to fly slower instead of entering the hold

it's also less work for us
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Old 22nd Nov 2012, 15:04   #3 (permalink)
ona
 
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Thanks, it is clearly less work,

Your company's policy does mean it is less fuel. However, I guess there must be a limit... I mean, would flying at KT220 for 150 NM out be less fuel? Of course, I always mean clean acft, if you lower flaps or anything I am sure it is more fuel.

I know it is not the same, but with a car you do need more gas if you go 20 mph than if you go 55 mph...
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Old 22nd Nov 2012, 17:32   #4 (permalink)
 
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Depending on ACFT types, in most cases a slight reduction in speed should be the obvious choice especially on the descent. A decrease in speed will initiate descent so as long as it's not an excessive reduction and the ACFT was to descend anyway I would go that route.

Last edited by WhichWayIsNorth; 22nd Nov 2012 at 17:33.
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Old 23rd Nov 2012, 03:18   #5 (permalink)
 
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I think the best way is reduce speed

From my viewpoint, the different type will cause the same result, which is the same time to start approach. That's means it does not matter what kind of method you let the aircraft do, and they will cast the same time in the air.

So, the time is same, but let the aircraft reduce speed they will not consume lot of fuel when decent, because their could get energy from the gravity.

For the holding pattern, just one round will cast 2mins at low altitude will cast a lot of fuel for the moden fan turbine engine aircraft, especially some aircraft has just lower their flap at 10 degress or more.

And compare with the path of the two type, holding pattern will cast at least four more turns. For the moden jet, their turning diameter are very large and when turning polit will increase throttle to maintain the altitude.

The conclusion is:if possible, do not let aircraft join holding pattern.

And why we use holding pattern?

Think about the first way, it will cast lot more time to get thought FL, and a greater speed for approach. That will cause inconvenience for ATCs, especially in busy area.
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Old 23rd Nov 2012, 08:14   #6 (permalink)
 
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Reduce speed better

IMHO I think it is better to reduce speed during
the descent rather than hold. Holding requires more
attention as I have to ensure the acft leaves the
hold +-30secs. If there are 4 guys holding
much more attention is required. What I would like to know is how much to
reduce in terms of Mach No. to lose 1 min. Then I can either
vector out or reduce further before they hit F260 and below.
Is there any quick calculation?

Last edited by veloo maniam; 24th Nov 2012 at 00:42.
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Old 23rd Nov 2012, 20:36   #7 (permalink)
 
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In general it is better to reduce speed. A third option is, airspace constraints and traffic permitting, to initiate the descent earlier. A lower altitude will result in a lower true airspeed for a given calibrated airspeed. As a bonus the fuel flow will reduce as well. Winds will of course affect the outcome and can be used in your advantage. Descending someone out of that jetstream will hold him back as well.

At lower levels the fuel flow is generally lowest just above flap speed. The worst trick you can pull is to vector someone around the countryside 3000 feet and 170 knots. To overcome the added drag from the flaps needed to keep the thing in the air, a lot of fuel will be burned.
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Old 24th Nov 2012, 07:53   #8 (permalink)
Per Ardua ad Astraeus
 
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Quote:
As a bonus the fuel flow will reduce as well.
- no. Once you drag your arse in those extra miles at the lower level it goes up!

Whilst 'holding' is reasonably fuel efficient, it does take place (normally) at lower levels, therefore reducing towards endurance speed at cruise altitude is always preferable in fuel terms (and gives you longer to finish brekkie).

The only 'variable' is that with a 'seasoned eye' you might judge that the EATs will suddenly dissipate in which case being in the hold rather than toddling towards it is the best place to be.
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Old 30th Nov 2012, 13:23   #9 (permalink)
 
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Turns waste fuel, so if slowing down early saves a turn that would help. Descending in the hold at as low a continuous rate as possible is good too.
Speaking just for 74 s then 230 is a good clean speed, 190 good for base leg. This saves putting out an extra stage of flap.
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Old 13th Dec 2012, 05:53   #10 (permalink)
 
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Jeepers!

Leaving aside engine factors for the moment (about which I know little ), lowest fuel consumption occurs by definition at the speed for best L/D ratio, Vmd (min drag). This is commonly shown as a 'green dot' on FMS-equipped aircraft. This is also the speed for best endurance (another way of saying min fuel burn).

Slower than Vmd means more drag and therefore more fuel burn. Ditto faster.

It is therefore axiomatic that the most economical profile when required to lose time, all other factors being equal, is to reduce speed to meet the RTA as far as green dot. Any additional time required to be lost should be done in vectoring or holding at green dot.

Note: This assumes level flight and does not account for compressibility factors. In jet aircraft fuel burn typically increases at lower altitudes, but this is balanced by reduced TAS at green dot (i.e. the aircraft can go slower at green dot). Wind gradient is also a factor to consider. Therefore there is a balance between all these factors but I don't know enough detail to reach any authoritative conclusion as to the best vertical profile to fly, but the normal rule is stay as high as possible for as long as possible.

Last edited by Philthy; 13th Dec 2012 at 05:59.
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Old 13th Dec 2012, 07:25   #11 (permalink)
 
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Always the first option.

As Plithy says, let the a/c stay as high as possible for as long as possible, and ask the crew to slow down to the speed you need. If that speed is below max finesse for the aircraft, they will (or at least should) let you know pretty smartly.

Holding at low and medium level is to be avoided as far as practical.

Last edited by Trim Stab; 13th Dec 2012 at 07:25.
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Old 22nd Dec 2012, 21:45   #12 (permalink)
 
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Well, following some enquiries and discussion in another part of the forum it turns out that the best level for minimum fuel consumption at green dot/Vmd/holding speed is around FL250 for most jets.

The rule 'as high as possible for as long as possible' applies in cruising flight where you are flying for range. This gives the best TAS/fuel burn ratio. In the delay scenario, you are flying for endurance and the lower level is better. There are also some handling benefits at lower levels. Interestingly though, unlike cruise there does not seem to be a huge difference in fuel burn at Vmd at different levels.

One other interesting point to come out is that Boeing, at least, adds 5% to predicted fuel burn for flying a holding pattern (as opposed to in a straight line). This is because additional thrust is required in the turns.

So what can we in ATC take from all this?

When faced with a long delay the optimum profile is:
  • descend to around FL250
  • reduce speed to Vmd
  • absorb as much of the delay in a straight line as possible (this means early advice of a long delay is highly desirable)
  • if necessary to hold, extended length patterns reduce fuel burn
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Old 23rd Dec 2012, 00:37   #13 (permalink)
 
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Surely descending early at min speed will mean less fuel burn because you are spending less time at cruise when fuel burn is higher per minute than when in descent at min thrust?

Or to put it another way high speed descents use more fuel although they save time?
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Old 27th Dec 2012, 23:39   #14 (permalink)
 
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Bob,

In my post above, I probably should have put the first two points in reverse order. Slow to Vmd first, then drop down to FL250. There is an additional saving here from the recovery of potential energy in the descent at idle thrust also.

Furthermore, for short range flights if advice of a long delay can be provided early enough, there is a sizeable fuel saving to be had from stopping the climb at FL250 instead of climbing up to normal cruise altitude.

Philthy
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Old 28th Dec 2012, 00:05   #15 (permalink)
 
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Put it this way.

Take a point 150nm from the R/W. Aircraft at FL390, if allowed to descend normally will maintain that level for around another 30nm and then descend at idle thrust until the approach. If the approach is going to be delayed, then fuel is required for the hold.

If you request a min speed descent, then the descent point moves further away, so the aircraft may descend 150nm from the field (at idle thrust the whole time), saving 30nm at cruise thrust and maybe avoiding a future hold - The crew does need to be advised early though!

so case a) Extra 30nm at cruise thrust and fuel for holding.
case b) Engines at idle the whole time.

No brainer!

Oh and I've been known to descend from that level at 220kt the whole way - A light aircraft helped! Just discuss with the crew what is possible because a whole load of factors can come into play. The other tactic can be to ask the crew to arrive at a waypoint at a time...
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