Ikramerica From United States of America, joined May 2005, 21029 posts, RR: 60 Reply 1, posted (5 years 3 weeks 2 days 13 hours ago) and read 4305 times:
At what point does flying slower not save fuel?
I've always wondered why airlines don't fly slower on red-eyes. And flying at a "cruise ship" speed from East to West would also allow for red-eyes in the other direction, leave NY at midnight arrive LA at 5AM. The alternative is RON so the airline has a plane for the early transcon heading east, so in this case it would not hurt utilization.
Of all the things to worry about... the Wookie has no pants.
RJdxer From , joined Dec 1969, posts, RR: Reply 3, posted (5 years 3 weeks 2 days 13 hours ago) and read 4255 times:
Quoting Ikramerica (Reply 1): At what point does flying slower not save fuel?
If you're flying into a strong headwind. Slowing down only prolongs the agony. It really depends a lot on the weather, type of aircraft, cruise altitude, and to some extent the departure and arrival cities. If you are leaving NY the extra time you lose waiting to take off has to be made up somewhere. If you are arriving in NY and get sent into a hold, slowing down to get there only adds additonal minutes of flying time. So slowing down helps out sometimes, especially if you have a good tailwind, but not always.
I liked the reference to how Southwest has slowed down, they must make up the lost time during taxi!
LAXintl From United States of America, joined exactly 13 years ago today! , 22059 posts, RR: 51 Reply 4, posted (5 years 3 weeks 2 days 13 hours ago) and read 4233 times:
There is something quite simple here called Cost Index at play here.
Simply put, the cost of time vs. cost of fuel
At some point the cost of time become higher then the cost of fuel so there is no point in flying even slower to eek out additional fuel savings, as the total cost curve heads upwards.
The fuel portion of the equation is pretty obvious to most folks, however the cost of time includes host of marginal additional cost factors which include labor, maintenance and its accruals, ownership(some engines/planes/parts are leased or paid for based on hours of use), plus issues related to network(or hub) connectivity, fleet usage needs (if an airline flew every single flight 10mins slower, over thousands of daily flights it would now need XX additional planes to cover the schedule), and so forth...
So as the cost of fuel goes up, its relation to the cost of time rises. As fuel prices decline, then the cost of time will take greater importance.
[Edited 2008-05-01 18:32:10]
From the desert to the sea, to all of Southern California
ER757 From Cayman Islands, joined May 2005, 2228 posts, RR: 8 Reply 5, posted (5 years 3 weeks 2 days 12 hours ago) and read 4140 times:
Quoting LAXintl (Reply 4): At some point the cost of time become higher then the cost of fuel
That's a good point, but based on the amounts of money supposedly saved by the relatively small amount of additional time added to the flights, I think it's a long way from reaching this threshold. Of course, that assumes the savings reported in the article are hard and verifiable numbers.
WESTERN737800 From United States of America, joined Feb 2008, 684 posts, RR: 2 Reply 6, posted (5 years 3 weeks 2 days 12 hours ago) and read 4115 times:
A company I used to work for is paying pilots $5 for every tailwind they can find while only charging them $10 for headwinds.
OPNLguy From , joined Dec 1969, posts, RR: Reply 7, posted (5 years 3 weeks 2 days 12 hours ago) and read 4090 times:
Quoting RJdxer (Reply 3): I liked the reference to how Southwest has slowed down, they must make up the lost time during taxi!
Ouch!
Seriously, we do utilize best wind routings quite a bit, and depending on the jet stream du jour, can save time fuel/time westbound, even at this time of the year.
LAXintl From United States of America, joined exactly 13 years ago today! , 22059 posts, RR: 51 Reply 8, posted (5 years 3 weeks 2 days 12 hours ago) and read 4090 times:
Quoting ER757 (Reply 5): I think it's a long way from reaching this threshold.
You forget they have been applying this cost index principle for a long time -- all they are doing now is tweaking the speed down slightly more as the cost of fuel rises in relative terms compared to the cost of time.
Its very much a fine balancing act, as the article states;
Quote: Airlines must strike a delicate balance, seeking an aircraft's "sweet spot" on fuel use without slowing down so much that other costs, and flight delays, rise, Mann said: "Everything's a tradeoff."
For any pilots on a.net flying modern large glass cockpit jets, the Cost Index is actually a value entered in the FMC which tells the aircraft to operate according a variable speed schedule.
From the desert to the sea, to all of Southern California
Pilotpip From United States of America, joined Sep 2003, 3084 posts, RR: 12 Reply 9, posted (5 years 3 weeks 1 day 22 hours ago) and read 3824 times:
Quoting LAXintl (Reply 8):
For any pilots on a.net flying modern large glass cockpit jets, the Cost Index is actually a value entered in the FMC which tells the aircraft to operate according a variable speed schedule.
We don't do that on the 170. We're just filing at .74 where we used to do .78 on all legs. This saves about 300pph and only costs us about 5-10 minutes over a two hour leg. Most big airports are slowing you early anyway.
Another way to really save some fuel is to get to top of climb and cruise power ASAP. Of course, this isn't always possible coming out of big airports where they have issues with other traffic.
Flipdewaf From United Kingdom, joined Jul 2006, 1525 posts, RR: 1 Reply 10, posted (5 years 3 weeks 1 day 22 hours ago) and read 3811 times:
Quoting Futurecaptain (Reply 2): Quoting Ikramerica (Reply 1):
At what point does flying slower not save fuel?
When you fly slower than the minimum drag speed. Slower than L/D max.
While there is less drag at L/Dmax and so you use less fuel you are actually flying slower so you might use 20% less fuel per hour but it may take you 30% longer to get there so you just actually use more fuel flying at L/Dmax. The way to get where you are going for the least fuel possible is to fly at a speed where ML/D is a peak and fly there.
LAXintl From United States of America, joined exactly 13 years ago today! , 22059 posts, RR: 51 Reply 13, posted (5 years 3 weeks 1 day 16 hours ago) and read 3674 times:
IAHFLYR From United States of America, joined Jun 2005, 4723 posts, RR: 25 Reply 14, posted (5 years 2 weeks 6 days 20 hours ago) and read 3514 times:
Quoting RJdxer (Reply 3): I liked the reference to how Southwest has slowed down,
Funny thing is the opinion that Southwest is the fastest in the sky, or used to be....from what I witness they would have been about 3rd or 4th on my list for airlines that fly fast......I think they got the title from always wanting to cancel the speed on a STAR at the corner post, but even then most of the time they'd be within the 270-280 KIAS range, not all that fast!!!
But as OPNLguy can attest Southwest did put out a memo to the air traffic organization about 4 months ago saying they'd be adjusting their speeds some during certain phases of flight.
Any views shared are strictly my own and do not a represent those of any former employer.
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