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GentFromAlaska
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Redeye Flights And Fuel Consumption

Sun Feb 24, 2008 8:49 pm

In this day and age of exuberant Jet-A fuel cost. I'm attempting to validate what I suspect might be an urban legend as it relates to fuel consumption on red-eye flights when compared to flying the same route on the identical aircraft, with identical engines at other times of the day. I suspect I'm going to need the opinion of a rocket scientist, literally.

I realize there are a allot of variables including airspeed, winds aloft, location of the jet stream, direction of flight and several others I'm sure.

I've been told commercial jet airlines the size of the 737 767, 777, A318, A319 A320 burn less fuel flying red-eyes than they would flying the identical route at other times of the day. Part of it makes sense considering nighttime atmospheric cooling. Part of me wants to think the temp at 35000-39000, whether it be midday or midnight would not make that much difference as cold is cold at that altitude.

In this day and age of expensive fuel it seems more and more airlines are choosing to operate red-eye flights where the duration of flight is in the four or five hour range. I was always under the impression red-eyes exist because its keeps a aircraft flying when it would otherwise be parked RON somewhere, or it allows a an aircraft to meet a bank of flights departing from a hub in the early morning hours as was my case on a 767 from HNL to LAX. which had me arriving at me arriving in LAX about 0515-ish connecting to a 0730 departure to the east-coast.

I was told a flight operating at zero dark thirty burns less fuel when the temperature falls 15-20 or more degrees overnight from the days high temperature when compared to a like flight departing at mid day. The fact that we were flying across the pond might make it a little cooler. Twenty degrees is roughly the day/night variation of temps in Honolulu for most of the year.

Thanks in Advance.
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jkudall
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RE: Redeye Flights And Fuel Consumption

Sun Feb 24, 2008 8:58 pm

A lot of fuel can be saved by aircraft getting more direct routings from ATC simply because airspace is less congested at night. There is also less ATC vectoring needed at night because again, the airspace is less congested (Unless it is an airport like MEM which is busy late at night). Taxi times are less during non-peak times at night, thus saving even more fuel.

So simply put, aircraft get more shortcuts at night than they do in the day.

[Edited 2008-02-24 13:02:26]
 
thegeek
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RE: Redeye Flights And Fuel Consumption

Mon Feb 25, 2008 1:29 am

I'd have thought that while cool air would aid climb, you'd loose the benefit in the cruise, due to having to push through more dense air. Perhaps at high altitude temps don't vary as much as on the ground.
 
PPVRA
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RE: Redeye Flights And Fuel Consumption

Mon Feb 25, 2008 2:12 am



Quoting GentFromAlaska (Thread starter):
In this day and age of expensive fuel it seems more and more airlines are choosing to operate red-eye flights where the duration of flight is in the four or five hour range. I was always under the impression red-eyes exist because its keeps a aircraft flying when it would otherwise be parked RON somewhere, or it allows a an aircraft to meet a bank of flights departing from a hub in the early morning hours as was my case on a 767 from HNL to LAX. which had me arriving at me arriving in LAX about 0515-ish connecting to a 0730 departure to the east-coast.

Daytime flights waste a lot of time for the pax that could be used for work or whatever else. It also makes sense to put a plane on a longer flight at night instead of your usual 1-2 hours quick flight. . . sleep for maybe an hour and then wake up, that's just a total waste. Three hours isn't so bad - especially if you have 4 time zones to get used to.
"If goods do not cross borders, soldiers will" - Frederic Bastiat
 
ba97
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RE: Redeye Flights And Fuel Consumption

Mon Feb 25, 2008 2:20 am

On overnight flights, for example across North America, do the planes fly slower since arriving early does no benefit if they can not land? I guess the same over an ocean would apply.
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Tornado82
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RE: Redeye Flights And Fuel Consumption

Mon Feb 25, 2008 2:24 am



Quoting Ba97 (Reply 4):
On overnight flights, for example across North America, do the planes fly slower since arriving early does no benefit if they can not land?

Most major airports in the US are 24/7 operations... for the most part (yeah there are exceptions so the nit pickers can stay away) there are fewer airports with "curfews" than counterparts in Europe, etc.
 
roseflyer
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RE: Redeye Flights And Fuel Consumption

Mon Feb 25, 2008 3:18 am

I don't think temperatures makes the big difference. Temperatures at over 30,000ft don't seem to vary much over a day. I think the big difference in addition to more direct routings is also more altitude flexibility. At night, variations in altitute are a bit easier than during the day. Being able to do a more direct climb to cruising altitude and avoiding an early descent will save fuel.

Quoting Ba97 (Reply 4):
On overnight flights, for example across North America, do the planes fly slower since arriving early does no benefit if they can not land? I guess the same over an ocean would apply.

I would assume that planes are flying at the same speed regardless of time nowadays. Unless there are significant delays to be made up, I don't think airlines would want to fly faster during the day.
If you have never designed an airplane part before, let the real designers do the work!
 
MTSUATC
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RE: Redeye Flights And Fuel Consumption

Fri Feb 29, 2008 3:03 am

Fly on United and listen to channel 9 on a red eye. Most of the controllers will advise the pilots to "resume own navigation." That same flight during the day will have more vectors to waypoints and intersections. So as others have said the more direct routings will save fuel.
 
vxg
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RE: Redeye Flights And Fuel Consumption

Mon Mar 10, 2008 4:05 am

"Resume own navigation" simply means 'fly the route you were cleared for earlier'. If the route consists of complex airways, waypoints, etc. then you're still obligated to fly it. However when the controller says "United XXX fly direct Dulles" then you're saving gas.

There are a few times where I've heard pilots decline the direct routing, even at night. My guess is that its perhaps to avoid weather or maybe avoid arriving too early for their landing slot. Anyone care to speculate?

VxG
 
lowrider
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RE: Redeye Flights And Fuel Consumption

Mon Mar 10, 2008 4:30 am



Quoting Vxg (Reply 8):
There are a few times where I've heard pilots decline the direct routing, even at night. My guess is that its perhaps to avoid weather or maybe avoid arriving too early for their landing slot. Anyone care to speculate?

Sometimes you need the burnoff to get below max landing weight. You may be advised not to arrive before XX time because of parking availability. Sometimes the kids need glasses and shoes.
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Mir
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RE: Redeye Flights And Fuel Consumption

Mon Mar 10, 2008 5:57 am



Quoting Vxg (Reply 8):
However when the controller says "United XXX fly direct Dulles" then you're saving gas.

I've heard a JetBlue red-eye flight out of PDX to JFK cleared to Wilkes-Barre, PA (the starting point for the STAR) shortly after departure. That's a nice shortcut.

Quoting Thegeek (Reply 2):
I'd have thought that while cool air would aid climb, you'd loose the benefit in the cruise, due to having to push through more dense air.

That probably has a little effect, but it's more than balanced out by the fact that jet engines love the cold.

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
thegeek
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RE: Redeye Flights And Fuel Consumption

Mon Mar 10, 2008 11:37 pm



Quoting Mir (Reply 10):
That probably has a little effect, but it's more than balanced out by the fact that jet engines love the cold.

Isn't the advantage of the cold that it reduces pressure on turbine inlet temp? I surmise, that at cruise throttle settings, you don't have to worry about that, so the jet engines don't really care about the cold in this part of the trip.
 
tdscanuck
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RE: Redeye Flights And Fuel Consumption

Tue Mar 11, 2008 3:38 am



Quoting Thegeek (Reply 11):


Quoting Mir (Reply 10):
That probably has a little effect, but it's more than balanced out by the fact that jet engines love the cold.

Isn't the advantage of the cold that it reduces pressure on turbine inlet temp?

It's not the pressure, its the density and temperature (directly related). Higher density means less delta-V required for the same thrust = the engine not working as hard. Lower temperature goes right through to increased EGT margin, although at cruise that's not so much of an issue.

Tom.
 
thegeek
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RE: Redeye Flights And Fuel Consumption

Tue Mar 11, 2008 6:51 am

I knew I phrased that poorly. When I said pressure, I meant on EGT margin.

Does that mean higher mass flow at lower temps? I can see how the engines would like that.
 
xv408
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RE: Redeye Flights And Fuel Consumption

Tue Mar 11, 2008 1:06 pm



Quoting Thegeek (Reply 13):
Does that mean higher mass flow at lower temps? I can see how the engines would like that.

Yes, this is right. On a test stand, pulling max rpm, you can get the other side of Max Take-off thrust on Continuous settings, and even then have some TGT to spare. that is how the engine manufacturers get their best thrust values in the "my engine is more powerful than yours" race - nice cold day, high pressure atmosphere, etc. It's like pumping treacle.
 
dw747400
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RE: Redeye Flights And Fuel Consumption

Tue Mar 11, 2008 2:43 pm



Quoting Vxg (Reply 8):
There are a few times where I've heard pilots decline the direct routing, even at night. My guess is that its perhaps to avoid weather or maybe avoid arriving too early for their landing slot. Anyone care to speculate?

Weather/turbulence avoidance is probably the big one. Keep in mind, due to variations in winds aloft, a direct course is not always best. It doesn't happen all that often that a nice dogleg save you time in fuel, but I've seen it. About a week ago I was working a flight that cut 15 minutes off a flight from TPA to IAH by going up and over the panhandle rather than battling headwinds in the gulf.
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Boston92
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RE: Redeye Flights And Fuel Consumption

Tue Mar 11, 2008 3:17 pm

Think of it this way:

The aircraft would be saving a considerable amount of fuel by flying at night due to much less traffic and much faster taxi times. Waiting for 2 hours at JFK to depart at peak time would use a lot more fuel than leaving at an off peak hour.
 
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LAXintl
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RE: Redeye Flights And Fuel Consumption

Tue Mar 11, 2008 7:16 pm

Quoting Vxg (Reply 8):
There are a few times where I've heard pilots decline the direct routing, even at night. My guess is that its perhaps to avoid weather or maybe avoid arriving too early for their landing slot. Anyone care to speculate?

Direct routings DO NOT always save fuel, in fact, typically the flight planning system will pick a route to gain a greater tailwind, via the jetstream. Sometimes, taking a direct routing, will actually leave the jetstream, thus lowering the groundspeed, thus taking more time/fuel.

that is why, many wise airmen will decline a long direct routing.

Rgrds

[Edited 2008-03-11 12:16:51]
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LHRBFSTrident
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RE: Redeye Flights And Fuel Consumption

Wed Mar 12, 2008 5:48 pm



Quoting Laxintl (Reply 17):
Sometimes, taking a direct routing, will actually leave the jetstream, thus lowering the groundspeed, thus taking more time/fuel.

that is why, many wise airmen will decline a long direct routing.

I suppose this is why the SQ and TG ULH routes to Asia from the USA sometimes fly eastbound and sometimes fly westbound on the same sector, despite significant difference in the distance flown..?
 
Mir
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RE: Redeye Flights And Fuel Consumption

Thu Mar 13, 2008 3:34 am



Quoting Thegeek (Reply 11):
Isn't the advantage of the cold that it reduces pressure on turbine inlet temp?

I've been told that it's because when you heat cold air as opposed to warm air, you're raising its temperature by a larger percentage, which gets you more thrust for the same power setting (or the same thrust for a lower power setting - either way it's good).

Quoting LHRBFSTrident (Reply 18):
I suppose this is why the SQ and TG ULH routes to Asia from the USA sometimes fly eastbound and sometimes fly westbound on the same sector, despite significant difference in the distance flown..?

That's exactly why.

-Mir
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Viscount724
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RE: Redeye Flights And Fuel Consumption

Thu Mar 13, 2008 3:49 am



Quoting LHRBFSTrident (Reply 18):
I suppose this is why the SQ and TG ULH routes to Asia from the USA sometimes fly eastbound and sometimes fly westbound on the same sector, despite significant difference in the distance flown..?

And I think they sometimes fly almost due northbound using a routing that passes fairly close to the North Pole. I think that routing is farly common on JFK/EWR/YYZ-HKG flights where the great circle mileage passes almost over the North Pole and then down across Russia and China. In the other direction I think the more usual North Pacific routing via Japan and Alaska is more common to benefit from usual tailwinds.
 
thegeek
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RE: Redeye Flights And Fuel Consumption

Thu Mar 13, 2008 7:48 am



Quoting Mir (Reply 19):
I've been told that it's because when you heat cold air as opposed to warm air, you're raising its temperature by a larger percentage, which gets you more thrust for the same power setting (or the same thrust for a lower power setting - either way it's good).

FWIW that's not quite right. When you compress colder air, you get less temperature rise and do less work for a given pressure ratio, or for a given amount of work, you get a higher pressure ratio. I think that's what you're thinking of.
 
BAE146QT
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RE: Redeye Flights And Fuel Consumption

Thu Mar 13, 2008 6:23 pm



Quoting RoseFlyer (Reply 6):
I don't think temperatures makes the big difference. Temperatures at over 30,000ft don't seem to vary much over a day

Maybe not, but what about on takeoff and initial climb, where the temperature variance can be quite high, and where you're burning a huge quantity of fuel?

Quoting Thegeek (Reply 21):
FWIW that's not quite right. When you compress colder air, you get less temperature rise and do less work for a given pressure ratio, or for a given amount of work, you get a higher pressure ratio. I think that's what you're thinking of.

I thought cold=better because it needs less compression - its mass is higher in the same volume that gets sucked in.

Is that one of those explanations given to the layman ("Not quite right, but it'll do"), or are we talking about the same equation from different sides of the equals sign?
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tdscanuck
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RE: Redeye Flights And Fuel Consumption

Fri Mar 14, 2008 12:23 am



Quoting BAe146QT (Reply 22):

Is that one of those explanations given to the layman ("Not quite right, but it'll do"), or are we talking about the same equation from different sides of the equals sign?

The latter. For a given thrust, you need less pressure ratio with denser air, so with colder air the engine doesn't have to spin as fast and doesn't work as hard. Flipped around, for a given engine speed (pressure ratio) colder (denser) air will generate more thrust.

Tom.
 
BAE146QT
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RE: Redeye Flights And Fuel Consumption

Fri Mar 14, 2008 7:51 pm



Quoting Tdscanuck (Reply 23):
The latter.

Cheers Tom - many thanks.
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thegeek
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RE: Redeye Flights And Fuel Consumption

Sat Mar 15, 2008 3:31 am



Quoting BAe146QT (Reply 22):
Maybe not, but what about on takeoff and initial climb, where the temperature variance can be quite high, and where you're burning a huge quantity of fuel?

In this case you will have the potential to save a fair bit of fuel, although I'm not sure of the exact %. Climb with reduced thrust never helps fuel burn, but assuming that you don't do that, you will get into the thinner, colder air faster reduce fuel burn, not to mention all the factors from above which will only aid SFC on climb.

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