GentFromAlaska From United States, joined Feb 2005, 597 posts, RR: 1 Posted (1 year 9 months 1 week 2 days 16 hours ago) and read 3305 times:
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.
Man can be taken from Alaska. Alaska can never be taken from the man
Jkudall From United States, joined Mar 2004, 560 posts, RR: 3 Reply 1, posted (1 year 9 months 1 week 2 days 16 hours ago) and read 3300 times:
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]
"It takes a big man to cry, but it takes a bigger man to laugh at that man."
Thegeek From Australia, joined Nov 2007, 890 posts, RR: 0 Reply 2, posted (1 year 9 months 1 week 2 days 12 hours ago) and read 3197 times:
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 From Brazil, joined Nov 2004, 6078 posts, RR: 48 Reply 3, posted (1 year 9 months 1 week 2 days 11 hours ago) and read 3180 times:
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 From Canada, joined Apr 2004, 367 posts, RR: 0 Reply 4, posted (1 year 9 months 1 week 2 days 11 hours ago) and read 3174 times:
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.
there is economy class, business class, first class...then Concorde..pure class
Tornado82 From , joined Dec 1969, posts, RR: Reply 5, posted (1 year 9 months 1 week 2 days 11 hours ago) and read 3172 times:
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 From United States, joined Feb 2004, 5627 posts, RR: 27 Reply 6, posted (1 year 9 months 1 week 2 days 10 hours ago) and read 3144 times:
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.
My job is to make it so your flight is not delayed. Come fly the friendly skies!
MTSUATC From United States, joined Feb 2008, 120 posts, RR: 0 Reply 7, posted (1 year 9 months 5 days 10 hours ago) and read 2784 times:
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 From United States, joined May 2004, 95 posts, RR: 0 Reply 8, posted (1 year 8 months 3 weeks 4 days 9 hours ago) and read 2518 times:
"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?
Lowrider From United States, joined Jun 2004, 2711 posts, RR: 12 Reply 9, posted (1 year 8 months 3 weeks 4 days 9 hours ago) and read 2511 times:
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.
Mir From United States, joined Jan 2004, 13135 posts, RR: 65 Reply 10, posted (1 year 8 months 3 weeks 4 days 7 hours ago) and read 2491 times:
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.
Thegeek From Australia, joined Nov 2007, 890 posts, RR: 0 Reply 11, posted (1 year 8 months 3 weeks 3 days 13 hours ago) and read 2394 times:
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.
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.
Xv408 From United Kingdom, joined Nov 2006, 52 posts, RR: 0 Reply 14, posted (1 year 8 months 3 weeks 3 days ago) and read 2304 times:
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 From United States, joined Aug 2001, 977 posts, RR: 1 Reply 15, posted (1 year 8 months 3 weeks 2 days 22 hours ago) and read 2285 times:
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.
Boston92 From United States, joined Aug 2006, 3342 posts, RR: 8 Reply 16, posted (1 year 8 months 3 weeks 2 days 22 hours ago) and read 2271 times:
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.
"Why does a slight tax increase cost you $200 and a substantial tax cut save you 30 cents?"
Laxintl From United States, joined May 2000, 12140 posts, RR: 22 Reply 17, posted (1 year 8 months 3 weeks 2 days 18 hours ago) and read 2240 times:
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]
From the desert to the sea, to all of Southern California
LHRBFSTrident From United Kingdom (Northern Ireland), joined Nov 2006, 519 posts, RR: 0 Reply 18, posted (1 year 8 months 3 weeks 1 day 19 hours ago) and read 2137 times:
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..?
Next up: BD LHR-DUB, AA DUB-ORD-DFW-LAX - Platinum Challenge complete!
Mir From United States, joined Jan 2004, 13135 posts, RR: 65 Reply 19, posted (1 year 8 months 3 weeks 1 day 10 hours ago) and read 2075 times:
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..?
Viscount724 From Switzerland, joined Oct 2006, 10807 posts, RR: 9 Reply 20, posted (1 year 8 months 3 weeks 1 day 9 hours ago) and read 2071 times:
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 From Australia, joined Nov 2007, 890 posts, RR: 0 Reply 21, posted (1 year 8 months 3 weeks 1 day 5 hours ago) and read 2051 times:
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 From United Kingdom, joined Sep 2006, 969 posts, RR: 0 Reply 22, posted (1 year 8 months 3 weeks 19 hours ago) and read 1998 times:
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?
Tdscanuck From Canada, joined Jan 2006, 5677 posts, RR: 56 Reply 23, posted (1 year 8 months 3 weeks 13 hours ago) and read 1968 times:
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.
Thegeek From Australia, joined Nov 2007, 890 posts, RR: 0 Reply 25, posted (1 year 8 months 2 weeks 6 days 10 hours ago) and read 1867 times:
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.
Sponsor Message: 
From United States, joined Feb 2005, 597 posts, 
From Australia, joined Nov 2007, 890 posts, 
From Brazil, joined Nov 2004, 6078 posts, 
From Canada, joined Apr 2004, 367 posts, 
From , joined Dec 1969, posts, 
From United Kingdom, joined Nov 2006, 52 posts, .gif){kind=small}
From United Kingdom (Northern Ireland), joined Nov 2006, 519 posts, 
From Switzerland, joined Oct 2006, 10807 posts, 
Sponsor Message: 
Printer friendly format
