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frostyj
Topic Author
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### Faster Flight But Longer Distance?

Just tracking flights before my flight to Washington. It looks like the route is exactly 7 hours, funny thing is that the route from Heathrow is barely longer, sometimes quicker.

You'd think it would be about 35:45 minutes longer from Heathrow over Dublin???

For example,

Are flights from Heathrow permitted a faster crusing speed or something? It seems that almost every flight I track from ther is either identical or faster. You would think that 300 miles would have an impact?

How can a flight taking off from Dublin be beat from a flight from Heathrow taking off at the same time? It doesn't make sense. A plane can't fly 300 miles in 10 minutes!

[Edited 2015-07-11 12:02:43]
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CplKlinger
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### RE: Faster Flight But Longer Distance?

Could be many different things - winds, ATC expediting, slot controls, even the average speed. You cover 42 miles at 220 knots in 10 minutes, but just over 100 at mach .84 in 10 minutes. Average that over a 7 hour flight, and it would be easy to pick up the distance.

Viscount724
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### RE: Faster Flight But Longer Distance?

You're also overlooking the 777 is faster than the 757.

OldAeroGuy
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### RE: Faster Flight But Longer Distance?

 Quoting Viscount724 (Reply 2):You're also overlooking the 777 is faster than the 757.

Yes, Long Range Cruise Mach for the 757 is 0.80 while the 777 is 0.84 or 5% higher.
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BravoOne
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### RE: Faster Flight But Longer Distance?

Don't forget that when you apply either tail or headwinds you come up with either a NAM (Nautical Air Miles) number that will in turn reflect the added value of the tail/headwind.

Leezyjet
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### RE: Faster Flight But Longer Distance?

Probably because the flights from LHR reach cruising altitude in plenty of time before joining the NAT Tracks, so are more likely to get a higher level than ones climbing out of Dublin to join the tracks. You don't really want a/c climbing in the NAT Track area, so I guess the ones from Dublin end up with the lower levels that are not quite as efficient.

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MSJYOP28Apilot
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### RE: Faster Flight But Longer Distance?

The 757 is close to the limit of its range on DUB to IAD. Thus the dispatcher would have likely planned the flight to be as slow as possible in order to conserve fuel. A faster flight plan eats up more fuel. This can cause a fuel stop for the 757 westbound. LHR to IAD on the 777 is no where near the max range of the 777 and more normal cruise speed would be likely planned. The only time you would see an airline today planning mach 84 or 85 on a 777 is if they were trying to make up time due to headwinds, longer weather avoidance route or a late flight. Ideally mach 81-83 will be plan and flown to save fuel. Likewise, mach 77-79 is typical for the 757/767 if schedule integrity is not an issue.

Of course, pilots and ATC can mean the plan doesn't always get flown but on a fuel critical trip like DUB to IAD you would see almost all the crews being more willing to fly the plan as pilots don't like making fuel stops. ATC also knows from the carriers which flights are more fuel and weight critical and will often try to work the flow for those types.

frostyj
Topic Author
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### RE: Faster Flight But Longer Distance?

This seems like the best explanation thanks. Although i'm not sure the route is the limit as Delta airlines operate a flight from JFK to PSA at 4,200 miles, 800 miles longer.
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speedbird128
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### RE: Faster Flight But Longer Distance?

 Quoting MSJYOP28Apilot (Reply 6):as slow as possible in order to conserve fuel

It has been my understanding from my years in ATC and asking the crews that flying below the LRC mach number will not results in any range gains, but the opposite actually, in that the fuel burn will be lower, but the distance covered reduced more so...

Another example of going to the extremes, is RYR decided to fly at M0.72/220 to increase fuel savings. This resulted in fewer directs, which meant more hours on the airframe, for next to no fuel savings... It was quickly abandoned.
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mmo
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### RE: Faster Flight But Longer Distance?

 Quoting MSJYOP28Apilot (Reply 6):The 757 is close to the limit of its range on DUB to IAD.

That route is no where near the limit of it's range!

 Quoting MSJYOP28Apilot (Reply 6):The only time you would see an airline today planning mach 84 or 85 on a 777 is if they were trying to make up time due to headwinds, longer weather avoidance route or a late flight. Ideally mach 81-83 will be plan and flown to save fuel

Really, our ECON cruise is .84 on the 777. I don't know where you are getting your information from, but it is wrong on so many issues.

 Quoting Speedbird128 (Reply 8):It has been my understanding from my years in ATC and asking the crews that flying below the LRC mach number will not results in any range gains, but the opposite actually, in that the fuel burn will be lower, but the distance covered reduced more so...

LRC is just that. At LRC you get the most mileage on a specific amount of fuel. When LRC is selected, into a headwind will result in a faster speed, while with a tailwind, you will have a slower speed. You are taking advantage of the winds. Slowing down is just putting the aircraft on the other side of the power curve and costing you a lot of fuel.
If we weren't all crazy we'd all go insane!

MSJYOP28Apilot
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### RE: Faster Flight But Longer Distance?

 Quoting mmo (Reply 9):That route is no where near the limit of it's range!

DUB-JFK often takes 60-70K fuel in the summer time on a normal day with some loads approaching 75K when bad thunderstorms are forecast. Max tankage on the 757 is about 76K depending on fuel density. This is summer time numbers with lighter headwinds and light cargo payloads on this route. It would be more fuel if the payloads were higher. So not quite max range but as I said its no doubt getting close to it. In the winter with stronger headwinds, you will take even more fuel. DUB-IAD is a longer flight than DUB-JFK and would likely need more fuel than DUB-JFK. And thats not just in enroute burn numbers. American carriers need to carry a reserve of 30 mins plus 10% of total time it takes to fly to the destination which can be reduced to 5% percent reserves or broken into two flights with a re-dispatch on 10% reserves. The longer flight would increase the required reserve fuel on DUB-IAD regardless of which reserve fuel you used.

mmo
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### RE: Faster Flight But Longer Distance?

DL has run a 757-200 from FRA-DTW which is a still air range of 3616NM, DUB-JFK is 2763NM and DUB-IAD is 2958NM. Again, the DUB operations are no where near the max range of the aircraft. Having flow the 757 years ago, I can assure you there is plenty of wiggle room on the DUB ops. So the difference between the JFK and IAD operations is only 195NM which is only 24-25 minutes. Not a big difference only about 2200LBS at the very most.
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Starlionblue
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### RE: Faster Flight But Longer Distance?

 Quoting mmo (Reply 9):LRC is just that. At LRC you get the most mileage on a specific amount of fuel.

LRC is a few percent faster than max range speed. For a 1% increase in fuel burn (LRC versus max range speed) you get 3-4% more speed, meaning less time on the airframe and crew. Thus you save in the long run. LRC also gives better peed stability compared to max range.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

speedbird128
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### RE: Faster Flight But Longer Distance?

 Quoting mmo (Reply 9):Slowing down is just putting the aircraft on the other side of the power curve and costing you a lot of fuel.
 Quoting Starlionblue (Reply 12):LRC is a few percent faster than max range speed. For a 1% increase in fuel burn (LRC versus max range speed) you get 3-4% more speed, meaning less time on the airframe and crew. Thus you save in the long run. LRC also gives better peed stability compared to max range.

Clear, thanks guys   As I thought. Flying super slow gets you nowhere.
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Starlionblue
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### RE: Faster Flight But Longer Distance?

 Quoting speedbird128 (Reply 13): As I thought. Flying super slow gets you nowhere.

:D

Flying at Minimimum Drag Speed, which is lower than Max Range Speed, does have uses. It is the speed at which endurance is maximized. Thus useful for holding or any kind of loitering such as maritime patrol. But as you say that gets you nowhere.

Flying slower than Minimum Drag Speed increases drag, and thus fuel burn, while further decreasing speed. This is only useful for approach and landing.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

speedbird128
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### RE: Faster Flight But Longer Distance?

 Quoting Starlionblue (Reply 14):Minimum Drag Speed

Would this be equated to green dot Speed?
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glen
Posts: 354
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### RE: Faster Flight But Longer Distance?

 Quoting speedbird128 (Reply 15):Would this be equated to green dot Speed?

Green dot speed on Airbus equals to best lift-to-drag ratio but is not far away from minimum drag speed. As far as I know within maximum 10 knots.
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Pihero
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### RE: Faster Flight But Longer Distance?

 Quoting Starlionblue (Reply 12): For a 1% increase in fuel burn (LRC versus max range speed) you get 3-4% more speed, meaning less time on the airframe and crew.

To be perfectly pedantic - and obnoxious - LR is defined as the schedule that achieves 99% of the Max Range, so it's a bit less than 1% burn increase.

 Quoting glen (Reply 16):Green dot speed on Airbus equals to best lift-to-drag ratio

Green Dot has consistently been mistaken by non A drivers :
1/- Green Dot = max L/D, therefore ==> best climb gradient ( used on a single engine out final segment or used on the *Expedite* zoom climb on the 320/330 ) ;

2/- It is also the best *drift down speed* used in case of one engine out in cruise over mountaneous terrain.

3/- It is, finally,j the speed used for holding in normal operations ( as Glen says, it's not very far from the min drag speed ; it is by all intents and purposes assimilated with min drag speed ).
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Starlionblue
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### RE: Faster Flight But Longer Distance?

 Quoting Pihero (Reply 17):Quoting glen (Reply 16): Green dot speed on Airbus equals to best lift-to-drag ratio Green Dot has consistently been mistaken by non A drivers : 1/- Green Dot = max L/D, therefore ==> best climb gradient ( used on a single engine out final segment or used on the *Expedite* zoom climb on the 320/330 ) ; 2/- It is also the best *drift down speed* used in case of one engine out in cruise over mountaneous terrain. 3/- It is, finally,j the speed used for holding in normal operations ( as Glen says, it's not very far from the min drag speed ; it is by all intents and purposes assimilated with min drag speed ).

If I understand it correctly, green dot is also the managed speed target when approach mode has been activated in the FMS.

 Quoting Pihero (Reply 17):2/- It is also the best *drift down speed* used in case of one engine out in cruise over mountaneous terrain.

And best drift down speed is technically best climb gradient speed, in this case resulting in the least negative gradient.

 Quoting Pihero (Reply 17): Quoting Starlionblue (Reply 12): For a 1% increase in fuel burn (LRC versus max range speed) you get 3-4% more speed, meaning less time on the airframe and crew. To be perfectly pedantic - and obnoxious - LR is defined as the schedule that achieves 99% of the Max Range, so it's a bit less than 1% burn increase.

No worries. If I'd known that detail I would have been perfectly pedantic as well.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

Pihero
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### RE: Faster Flight But Longer Distance?

 Quoting Starlionblue (Reply 18):If I understand it correctly, green dot is also the managed speed target when approach mode has been activated in the FMS.

yes ; flaps up.
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speedbird128
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### RE: Faster Flight But Longer Distance?

 Quoting Starlionblue (Reply 18):when approach mode has been activated

At what point is that typically activated ?
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Pihero
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### RE: Faster Flight But Longer Distance?

 Quoting speedbird128 (Reply 20):At what point is that typically activated ?

When you've pushed the *APPR* switch on the FCU... LOC and GS are *starred*, i.e *Armed* on your FMA.
The plane slows down to Green Dot speed...
Once you've selected Flaps config 1, the *S* speed will become the target... *F* in config 2... and Vapp with landing flaps.
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Starlionblue
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### RE: Faster Flight But Longer Distance?

 Quoting Pihero (Reply 21): Quoting speedbird128 (Reply 20): At what point is that typically activated ? When you've pushed the *APPR* switch on the FCU... LOC and GS are *starred*, i.e *Armed* on your FMA. The plane slows down to Green Dot speed... Once you've selected Flaps config 1, the *S* speed will become the target... *F* in config 2... and Vapp with landing flaps.

From what I've seen, approach mode will auto-activate if you haven't activated it "by hand" in the FMS by a certain point. I think once you've passed the Initial Approach Fix. You activate it by hand in the FMS in the Perforrmance page.

Before approach mode is activated, the plane flies at 250 knots under 10000 feet or at a another speed if there is a constraint on the STAR. Once approach is activated the plane flies at green dot in a clean configuration.

Makes life very easy.

[Edited 2015-07-21 06:21:16]

[Edited 2015-07-21 06:21:47]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

speedbird128
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### RE: Faster Flight But Longer Distance?

 Quoting Pihero (Reply 21):When you've pushed the *APPR* switch on the FCU... LOC and GS are *starred*, i.e *Armed* on your FMA. The plane slows down to Green Dot speed... Once you've selected Flaps config 1, the *S* speed will become the target... *F* in config 2... and Vapp with landing flaps.

Thanks, but that's the *how*. Not typically *when*.

 Quoting Starlionblue (Reply 22):From what I've seen, approach mode will auto-activate if you haven't activated it "by hand" in the FMS by a certain point. I think once you've passed the Initial Approach Fix. You activate it by hand in the FMS in the Perforrmance page. Before approach mode is activated, the plane flies at 250 knots under 10000 feet or at a another speed if there is a constraint on the STAR. Once approach is activated the plane flies at green dot in a clean configuration. Makes life very easy.

Does sound easy, and obviously the green dot varies on airframe weight at the time, so is not a static value.

So, 20NM out? 15? 10? When you get GS or LLZ alive? If ATC says reduce to minim clean, that would be the green dot?
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Pihero
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### RE: Faster Flight But Longer Distance?

 Quoting Starlionblue (Reply 22):From what I've seen, approach mode will auto-activate

Careful : it's the Approach Phase that will be automatically activated, provided you are in NAV mode ( the" activation will occur because of the deceleration point ( a pseudo wpt ) which is computed by the system to take you at the FAF at the S speed. ( the system is programmed for a *decelerated final approach* ).

 Quoting Starlionblue (Reply 22): if you haven't activated it "by hand" in the FMS

This is not recommended : it's too late for FMGS / MCDU manipulations at that stage... just confirm your flight plan is sequenced ( in case of a go-around, mainly) or get the next flown-over wpt via a *Direct to*. It's much better / safer to complete the approach on HDG / ALT HOLD until Glide slope interception... and keep both pilots heads up.

The above will leave you with nothing in terms of final approach : It's only after you've selected *LS* - for "landing system"- that you get the ILS Loc and GS scales... and only after you've pressed the *APPR* P/B that your aircraft is ready for the approach : LOC and GS blue ==> Loc * / GS * ==> Loc and GS green + all the annunciations on your cat capability...etc...
(BTW, that's the moment you'd select the second AP on for an autoland... it's now normal procedure on many airlines to do so, even if you plan a manual cat I ILS : just disconnect the APs when taking over manually ( that explains the double cavalry charge, one for each AP)
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Pihero
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### RE: Faster Flight But Longer Distance?

 Quoting speedbird128 (Reply 23):Thanks, but that's the *how*. Not typically *when*.

Anwered in my first § above.

 Quoting speedbird128 (Reply 23): If ATC says reduce to minim clean, that would be the green dot?

That's normal practice, yes, as Green Dot speed is below the ICAO 210 kt clean init phase at a GW
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Starlionblue
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### RE: Faster Flight But Longer Distance?

 Quoting speedbird128 (Reply 23):Does sound easy, and obviously the green dot varies on airframe weight at the time, so is not a static value.

The rule of thumb I've learned for the 320 is that green dot is twice the weight in tons plus 80 when flying under 20000 feet, and for every 1000 feet above 20000 it increases by 1 knot.

 Quoting Pihero (Reply 24): Quoting Starlionblue (Reply 22): From what I've seen, approach mode will auto-activate Careful : it's the Approach Phase that will be automatically activated, provided you are in NAV mode ( the" activation will occur because of the deceleration point ( a pseudo wpt ) which is computed by the system to take you at the FAF at the S speed. ( the system is programmed for a *decelerated final approach* ).

Ah thanks. Now I get the nomenclature difference.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

Pihero
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### RE: Faster Flight But Longer Distance?

 Quoting Starlionblue (Reply 26):The rule of thumb I've learned for the 320 is that green dot is twice the weight in tons plus 80 when flying under 20000 feet, and for every 1000 feet above 20000 it increases by 1 knot.

The last § of my post # 17 was rudely interrupted. : It should read " Green Dot speed is below the ICAO 210 kt clean init phase at a GW of less than 62.5 tonnes. The FMGS computes GD thus :
GD = (GW x 2) + 85 kt... add 1kt per thousand feet above FL 200..."

Hence in my example GD = (62.5 x 2) + 85 = 125 + 85 = 210 kt

So it's more than a rule of thumb.
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Starlionblue
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### RE: Faster Flight But Longer Distance?

 Quoting Pihero (Reply 27): Quoting Starlionblue (Reply 26): The rule of thumb I've learned for the 320 is that green dot is twice the weight in tons plus 80 when flying under 20000 feet, and for every 1000 feet above 20000 it increases by 1 knot. The last § of my post # 17 was rudely interrupted. : It should read " Green Dot speed is below the ICAO 210 kt clean init phase at a GW of less than 62.5 tonnes. The FMGS computes GD thus : GD = (GW x 2) + 85 kt... add 1kt per thousand feet above FL 200..." Hence in my example GD = (62.5 x 2) + 85 = 125 + 85 = 210 kt So it's more than a rule of thumb.

It is so cool when reality conincides with theory.

On a side note, as much as I detested studying for the ICAO Principles of Flight exam both times I had to take it, I love the subject matter itself.

[Edited 2015-07-22 01:18:25]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

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