747400sp From United States of America, joined Aug 2003, 3183 posts, RR: 3 Posted (6 years 15 hours ago) and read 7890 times:
The 727 was a pretty fast plane compared to the DC-9/MD-80, 737 and A320. It had a cruising speed of mach 0.84, it faster the a 707 and MD-11 mach 0.83 and the same speed as a 747-100/200 and L-1011. But lately there speed been reduce to mach 0.78 around the same as a MD-80, 737 NG and A320. Why have they reduce the 727 speed so much?
When the 727 starting flying at 0.84M, fuel was $0.10 per gallon.
Slowing down to 0.80M or so saves a lot of kerosene.
Another factor is that it was routine in the mid '60s to fly at Mmo. If you overspeed slightly and set of the overspeed warning horn, it was not big deal.
Today, the FBW systems start pulling back the throttle or pitching up if you get within 0.005M of Mmo.
And if you do exceed Mmo and the FDR or datalink tattles on you, the Flight Ops department will launch an investigation of your piloting skills.
So much for the days of leather jacket/white scarf aviators.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
Access-Air From United States of America, joined Sep 2000, 1938 posts, RR: 22 Reply 5, posted (6 years 11 hours ago) and read 7744 times:
Id say that more than likely that it was just to save fuel.....
If you look at old airline schedules and you compare the flight times say back in the 70s with what they are today, some of the flighst can be up to 20 mins faster than today...But then it prolly also accounts for the extra air traffic congestion and that I beleive at one point the FAA was fining airlines for late flights so to counteract the late flights they would pad their schedules so they had some wiggle room in case of ATC delays or whatever....
CosmicCruiser From United States of America, joined Feb 2005, 2045 posts, RR: 18 Reply 7, posted (5 years 12 months 4 days 18 hours ago) and read 7553 times:
Quoting Liedetectors (Reply 6): a hush kit can kill the performance of an engine.
I don't think that has anything to do with cruise mach. We had hush kits on ours at FEDEX and we flew the same cruise always. The -200 could go very, very fast! LOL More than likely it's fuel savings.
Access-Air From United States of America, joined Sep 2000, 1938 posts, RR: 22 Reply 8, posted (5 years 12 months 4 days 16 hours ago) and read 7517 times:
I wonder if it also has to do with the fact the 727 has a relatively short wing span for its size...It also seems to be pretty thin as well...
Technically speaking the 727-200 (153 feet) is one foot longer than the longest 707-300 series(152 feet) ....however I believe the wingspan is only like 108 feet long on both the 727-100 and 200....vs like 146 feet on a very wide chorde 707 wing.
I have personally watched a United 727-100 rocket off of a 4700 foot runway in Moline headed for Denver and it only used maybe half of it....It was awesome...I audio tape recorded it too, however I wish I had video camera back then....grrrrrrrrrrr!!!!
CosmicCruiser From United States of America, joined Feb 2005, 2045 posts, RR: 18 Reply 10, posted (5 years 12 months 4 days 15 hours ago) and read 7485 times:
Economy cruise calculated by the FMS based on input CI, Cost Index. We use CI 200 which usually results in about .832-.835M. I can edit the speed to whatever( not to exceed high speed buffet) but from .850 or greater you can definitely see an increase in fuel burn. The 727s have a given "econ" cruise still about .820-.830 (best I remember) that they use but our policy is if you're running late and may miss the sort ..go fast.
Greasespot From Canada, joined Apr 2004, 3015 posts, RR: 26 Reply 11, posted (5 years 12 months 4 days 6 hours ago) and read 7361 times:
Quoting Liedetectors (Reply 6): Many 727s have hushkits on them. a hush kit can kill the performance of an engine.
Hush kits do not affect the performance of the engines.
Most B727's are freighters and have a cargo door. Because of the door they are speed restricted. I cannot remember what the Not to exceed speed is... Going from memory it is 350knots ( i think)...
Plus as said earlier it is cheaper to fly slower than fast.
GS
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
CosmicCruiser From United States of America, joined Feb 2005, 2045 posts, RR: 18 Reply 12, posted (5 years 12 months 4 days 4 hours ago) and read 7321 times:
Quoting Greasespot (Reply 11): Because of the door they are speed restricted.
It's been about 10yrs. since I last flew a 727 but I don't think there was any speed restriction because of the cargo door. Just checked the 727 manual and it says 350kts @ S.L. 370kts/.880m @ 25,000'. The only speed restriction associated with the cargo door is 250 kts. with a main cargo unlocked indication.
Greasespot From Canada, joined Apr 2004, 3015 posts, RR: 26 Reply 14, posted (5 years 12 months 3 days 12 hours ago) and read 7142 times:
Quoting CosmicCruiser (Reply 12): It's been about 10yrs. since I last flew a 727 but I don't think there was any speed restriction because of the cargo door.
Maybe it is the ones modified with the AEI doors. I know our -200s have mode B (if that is the faster mode) on the ASI locked out.
GS
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
Corey07850 From United States of America, joined Feb 2004, 2500 posts, RR: 5 Reply 15, posted (5 years 12 months 3 days 10 hours ago) and read 7113 times:
IAHFLYR From United States of America, joined Jun 2005, 4136 posts, RR: 28 Reply 16, posted (5 years 12 months 1 day 13 hours ago) and read 6954 times:
CosmicCruiser From United States of America, joined Feb 2005, 2045 posts, RR: 18 Reply 17, posted (5 years 12 months 1 day 9 hours ago) and read 6877 times:
Quoting IAHFLYR (Reply 16): WOW an FMS in a B727......lovely, any glass panels?
I was responding to an earlier post that said the MD-11 did .83 and I posted that .830 is just econ cruise and we can go faster if we wish. The 727 is the same in that the co. flight plans them at about .82-.83 but they can and do go faster especially if they're running late.
If you want to cruise at .76/.78/.84/.90 go for it.
But if your following other traffic thats at .78 then ATC will have issues with you and tell you to slow up/speed up or vector you out of sequence. That would burn more fuel than going at .78 - along with the crowd.
.78 probably gives the best distance traveled against time with the best fuel burn.(NextGen engines aside.) Its like traveling along the highway at 70MPH compared to 80MPH. You'll get there faster but you'll burn more fuel.
KD
Liedetectors From United States of America, joined Jul 2005, 356 posts, RR: 0 Reply 20, posted (5 years 12 months 8 hours ago) and read 6706 times:
Quoting TheSunseeker (Reply 19):
I thought Trump, for example, has the 727 because its FAST!
his 727 will be faster then most because he doesnt carry the passenger load that a conventional 727 would. he probably carries 10 pax on that thing at any time.
DC3CV3407AC727 From United States of America, joined Dec 2003, 314 posts, RR: 3 Reply 21, posted (5 years 11 months 4 weeks 21 hours ago) and read 6594 times:
its all about the fuel,we cruise at Long Range Cruise settings (LRC),that put us at .78-.80 mach for cruise,but we are fully capable of any speed up to our limiting mach of .88 if the situation,or mission demands it.
the rumble of round engines is like music to me,likewise the thunder of thr JT8D
Miamiair From , joined Dec 1969, posts, RR: Reply 22, posted (5 years 11 months 4 weeks 19 hours ago) and read 6582 times:
As airplanes age, they accumulate weight from mods, repairs, dirt, etc. which can contribute to slower speeds. External repairs, peeling paint, surface rougness will all increase drag.
EssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2 Reply 23, posted (5 years 11 months 3 weeks 4 days 12 hours ago) and read 6430 times:
Why are the airlines losing money??? A 727 can't increase fares, but it does burn a lot of gas to go fast...
Oryx From Germany, joined Nov 2005, 126 posts, RR: 0 Reply 24, posted (5 years 11 months 3 weeks 3 days 19 hours ago) and read 6374 times:
Quoting Greasespot (Reply 11): Hush kits do not affect the performance of the engines.
Some do.
It's allways a challenge to get the noise down without increasing fuel consumtion or decreasing performance. Even more so when modifying an already existing design.
SlamClick From United States of America, joined Nov 2003, 10039 posts, RR: 75 Reply 25, posted (5 years 11 months 3 weeks 3 days 16 hours ago) and read 6693 times:
It should be mentioned also that it is rather difficult to get around another plane flying the same route, even if you are M.06 or M.08 faster. That is only 35-45 knots at typical cruise temperatures.
So if you are in-trail spaced 20nm behind another plane and need to be 20nm ahead it will take a full hour at that higher cruise, plus whatever time is added being maneuvered left/right or up/down to get to that point. So if you departed Las Vegas headed for Chicago behind a .76 plane it is entirely possible that you would never get by, factoring in time to climb and time to descend.
Traffic often slows us all down to Citation speeds.
Happiness is not seeing another trite Ste. Maarten photo all week long.
Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 26, posted (5 years 11 months 3 weeks 3 days 14 hours ago) and read 6676 times:
Quoting SlamClick (Reply 25): Traffic often slows us all down to Citation speeds.
And we all know those Citations are sometimes struck by birds from behind.
[Edited 2006-06-06 19:38:36]
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
SlamClick From United States of America, joined Nov 2003, 10039 posts, RR: 75 Reply 28, posted (5 years 11 months 3 weeks 3 days 13 hours ago) and read 6799 times:
Quoting Miamiair (Reply 27): Are you implying that the Citation has a calendar instead of an air speed indicator?
Oh no! It has an airspeed indicator. It is made by Smiths and reads in furlongs-per-fortnight.
Happiness is not seeing another trite Ste. Maarten photo all week long.
Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 29, posted (5 years 11 months 3 weeks 3 days 10 hours ago) and read 6750 times:
Quoting SlamClick (Reply 28): Quoting Miamiair (Reply 27):
Are you implying that the Citation has a calendar instead of an air speed indicator?
Oh no! It has an airspeed indicator. It is made by Smiths and reads in furlongs-per-fortnight.
Maybe it would help if it could take off from a conveyor belt...
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
hmmmm... From Canada, joined May 1999, 2087 posts, RR: 7 Reply 30, posted (3 months 2 weeks 4 days 11 hours ago) and read 4178 times:
Keep in mind that the 727 was just a faster plane than today's planes, so it has to slow down to stay in the line. A plane's cruising speed is determined by its wing. And those first and second generation jets had faster wings than what replacement them. For example, the 727 had a 32 degree sweep, whereas the A320 or a 757, planes typically used as replacements, use only a 25 degree sweep. The fastest plane in the sky is the 747, with a 37.5 degree sweep, the sharpest angle of any commercial airliner.
A good example of this in action is the flight times of the '70s and '80s vs today. When I used to fly down to TPA from YYZ, in the old 727 days, it was always a 2:25 minute flight in the schedule. Now it is a 2:45 minute flight in the schedule using the A320.
Strangely, as the decades have progressed, air travel has, to a good extent, gotten slower.
An optimist robs himself of the joy of being pleasantly surprised
HaveBlue From United States of America, joined Jan 2004, 1981 posts, RR: 1 Reply 31, posted (3 months 2 weeks 4 days 11 hours ago) and read 4108 times:
Damn you hmmmmm, you resurrected a 6 year old thread! I saw a reply by SlamClick, one of the most esteemed members and one of my favorites, and got all excited thinking he was back on the forums. That elation was short lived. Wish Slam was back!
Max Q From United States of America, joined May 2001, 2615 posts, RR: 19 Reply 32, posted (3 months 2 weeks 4 days 9 hours ago) and read 4038 times:
You don't always save fuel by slowing down, in the B727 you could save fuel by flying .83 to .84 mach into a headwind. Spending less time in the air means you burn less fuel overall.
Quoting Greasespot (Reply 14):
Maybe it is the ones modified with the AEI doors. I know our -200s have mode B (if that is the faster mode) on the ASI locked out.
It has nothing to do with the cargo door.
The A / B mode adjustable VMO is based on the Zero fuel weight. While some operators choose to 'hard wire' to the slower 'B' mode at CAL we generally operated in the A mode unless our ZFW was above a certain number (don't remember it) in which case we had to select B.
A mode VMO was 380 knots transitioning to an MMO of .92.
She was fast.
Quoting hmmmm... (Reply 30): For example, the 727 had a 32 degree sweep
35 Degrees actually
[Edited 2012-02-07 00:00:03]
The best contribution to safety is a competent Pilot.
Damn you hmmmmm, you resurrected a 6 year old thread! I saw a reply by SlamClick, one of the most esteemed members and one of my favorites, and got all excited thinking he was back on the forums. That elation was short lived. Wish Slam was back!
I wish several members were back !
The best contribution to safety is a competent Pilot.
clydenairways From Ireland, joined Jan 2007, 682 posts, RR: 0 Reply 34, posted (3 months 2 weeks 4 days 7 hours ago) and read 3923 times:
Its all about money.
Most airlines these days have fairly strict fuel economy SOP's, so the pilots are told what cruise speed to fly at.
This speed will be variable depending on weight/temp/Alt etc but in general it means flying slower. Particularly on old guzzlers like the 727.
clydenairways From Ireland, joined Jan 2007, 682 posts, RR: 0 Reply 37, posted (3 months 2 weeks 3 days 18 hours ago) and read 3432 times:
Quoting Max Q (Reply 36): Don't know where you get that information. At my Airline the cruise speed is left up to the Pilots based on the situation.
Most Airlines i know in Europe have SOP's for Fuel Economy these days, and line pilots are expected to follow them, unless there is a valid reason not to.
For example, if a particular company doesn't want you to speed up if running late, then you don't.
Max Q From United States of America, joined May 2001, 2615 posts, RR: 19 Reply 38, posted (3 months 2 weeks 3 days 17 hours ago) and read 3365 times:
Quoting clydenairways (Reply 37):
Most Airlines i know in Europe have SOP's for Fuel Economy these days, and line pilots are expected to follow them, unless there is a valid reason not to.
For example, if a particular company doesn't want you to speed up if running late, then you don't.
Makes no sense to me, taking the discretion away from the Pilots to operate as needed is shooting yourself in the foot.
The best contribution to safety is a competent Pilot.
vikkyvik From United States of America, joined Jul 2003, 6510 posts, RR: 29 Reply 39, posted (3 months 2 weeks 3 days 16 hours ago) and read 3337 times:
Quoting HaveBlue (Reply 31): Damn you hmmmmm, you resurrected a 6 year old thread! I saw a reply by SlamClick, one of the most esteemed members and one of my favorites, and got all excited thinking he was back on the forums.
He does pop up from time to time. Have to be paying attention to catch him, though.
Quoting Max Q (Reply 38): Makes no sense to me, taking the discretion away from the Pilots to operate as needed is shooting yourself in the foot.
Isn't what he's describing basically Cost Index? You enter a CI, and the FMS pops out a cruise Mach, based on whatever considerations? And aren't CIs generally mandated by the company?
The spirit of Massachusetts is the spirit of America!
Viscount724 From Switzerland, joined Oct 2006, 18464 posts, RR: 17 Reply 40, posted (3 months 2 weeks 3 days 16 hours ago) and read 3345 times:
Quoting Access-Air (Reply 8): Technically speaking the 727-200 (153 feet) is one foot longer than the longest 707-300 series(152 feet)
Actually 3 inches longer. 727-200 153 ft. 2 in. 707-320 152 ft. 11 in. However the 707-320 fuselage is 9 ft. longer than the 727-200 fuselage. The 727's overall length disparity vs. the 707 is due to the T-tail configuration.
Quoting HaveBlue (Reply 31): you resurrected a 6 year old thread!
How do people even find a 6-year-old thread to resurrect it?
Max Q From United States of America, joined May 2001, 2615 posts, RR: 19 Reply 42, posted (3 months 2 weeks 3 days 9 hours ago) and read 3118 times:
Quoting vikkyvik (Reply 39):
Isn't what he's describing basically Cost Index? You enter a CI, and the FMS pops out a cruise Mach, based on whatever considerations? And aren't CIs generally mandated by the company?
Not where I work, the Company tells us to use whatever cost index is necessary to provide the best result.
If we are running late we will go as fast as we can to make up time by using a higher cost index, if we are on time, or
operating a long range flight at the limits of the Aircraft's performance we will use a lower one (down to 0 if necessary)
Priorities are:
Safety
Providing the smoothest ride possible.
On time
Fuel saving.
It is up to the individual Flight crew's, led by the Captain to prioritize and manage these goals.
What you are describing is a very autocratic, inflexible system that does not make the best use of your most vital resource, the Professional Pilots entrusted with passengers lives and Aircraft worth hundreds of millions of dollars
The best contribution to safety is a competent Pilot.
B777LRF From Luxembourg, joined Nov 2008, 719 posts, RR: 3 Reply 43, posted (3 months 2 weeks 3 days 8 hours ago) and read 3108 times:
To summarise:
The 727 is still as fast as it ever was. The only thing that's been lost is the airlines willingness to spend money flying fast, unless it's operationally required.
From receips and radials over straight pipes to big fans - been there, done that, got the hearing defects to prove
Fabo From Slovakia, joined Aug 2005, 826 posts, RR: 1 Reply 44, posted (3 months 2 weeks 3 days 7 hours ago) and read 3070 times:
Quoting Max Q (Reply 42): Not where I work, the Company tells us to use whatever cost index is necessary to provide the best result.
It is the best result part that varies. IIRC I read in some (passenger) airlines manual that going fast to shave off few minutes is advisable, but only to a certain point. Makes sense to me, if you are going to go through the costs of rebooking, compensations etc. anyway, might as well save a couple hundred on fuel. Granted I do not know what penalty, if any, would be applied if the captain elected to go Mmo anyway.
On an unrelated note, it has been said that it is good for fuel economy to go faster than normaln in a case of headwinds, I wonder if FMCs calculations versus CI do factor this in? If you can input the route winds into the FMC, will it calculate higher ECON CRZ speed in headwind compared to tailwind, or is that up to the crew?
France 1789, Eastern Europe 1989, Northern Africa 2011
glen From Switzerland, joined Jun 2005, 170 posts, RR: 2 Reply 45, posted (3 months 2 weeks 3 days 7 hours ago) and read 3052 times:
Quoting Fabo (Reply 44): On an unrelated note, it has been said that it is good for fuel economy to go faster than normaln in a case of headwinds
It is not a good thing for fuel economy to go faster in case of headwinds, but it is good for the overall-costs of a flight. CI is the relation between Time related costs and Fuel Costs. In a headwind situation time related costs increase as flight time gets longer. So up the a certain point it makes sense to increase fuel costs (flying faster), to keep time related costs down. How much is different for every airline (and therefore also the optimum CI), as it depends from the airline structure (e.g. maintenance contract with a lot of fix prices versus contracts with a lot of time depending costs....) and fuel prices.
Quoting Fabo (Reply 44): If you can input the route winds into the FMC, will it calculate higher ECON CRZ speed in headwind compared to tailwind
Yes, it will. Even if you don't put the winds into the FMS, it will calculate and fly a higher speed in actual headwind conditions and lower speeds in tailwind conditions.
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
On an unrelated note, it has been said that it is good for fuel economy to go faster than normaln in a case of headwinds, I wonder if FMCs calculations versus CI do factor this in? If you can input the route winds into the FMC, will it calculate higher ECON CRZ speed in headwind compared to tailwind, or is that up to the crew?
The FMC will increase Econ cruise mach with an increase in headwind.
Quoting glen (Reply 45):
It is not a good thing for fuel economy to go faster in case of headwinds
I disagree, while increasing speed into a headwind may seem counterintuitive my experience has shown the resulting shorter flight time burns less fuel (less time spent flying into the headwind) and more than compensates for the short term increase in fuel burn.
This is exactly why the FMC in Econ will increase cruise mach with an increase in headwind, it is not just time related costs.
The best contribution to safety is a competent Pilot.
clydenairways From Ireland, joined Jan 2007, 682 posts, RR: 0 Reply 47, posted (3 months 2 weeks 3 days 6 hours ago) and read 3038 times:
Quoting Max Q (Reply 38): Makes no sense to me, taking the discretion away from the Pilots to operate as needed is shooting yourself in the foot.
I never said that. I think you are misunderstanding me.
Quoting Max Q (Reply 42):
If we are running late we will go as fast as we can to make up time by using a higher cost index,
If that is your company SOP then fine, but if it's not and you still decide to do it anyway then your not operating in occordance with your SOP's.
For some airlines, fuel is so expensive they never want you to speed up, even if you are 2 hours late.
Quoting Max Q (Reply 42):
What you are describing is a very autocratic
I would hardly describe that as autocratic, some LCC's maintain and publish league tables of the most/least fuel efficient pilots, now thats autocratic!
Quoting vikkyvik (Reply 39): Isn't what he's describing basically Cost Index? You enter a CI, and the FMS pops out a cruise Mach, based on whatever considerations? And aren't CIs generally mandated by the company?
That's what i'm taking about, but we don't have an FMS cost index, just data cards of speeds for ALT/WT/ISA combinations.
We also have a company profile for climb and descent, unless required for terrain clearance or ATC requirements.
It is not a good thing for fuel economy to go faster in case of headwinds
I disagree, while increasing speed into a headwind may seem counterintuitive my experience has shown the resulting shorter flight time burns less fuel (less time spent flying into the headwind) and more than compensates for the short term increase in fuel burn.
That's what glen said in the second half of the sentence you are quoting.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
glen From Switzerland, joined Jun 2005, 170 posts, RR: 2 Reply 49, posted (3 months 2 weeks 3 days 6 hours ago) and read 3034 times:
Quoting Max Q (Reply 46):
I disagree, while increasing speed into a headwind may seem counterintuitive my experience has shown the resulting shorter flight time burns less fuel (less time spent flying into the headwind) and more than compensates for the short term increase in fuel burn.
Even if you spend less time in the headwind it is always the same ESAD (equivalent still air distance) you have to travel, independent of your speed and that is what defines the fuel consumption for a given route, not the time in the air. Best for fuel consumption would always be speed for max. range (which corresponds to CI=0).
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
tdscanuck From Canada, joined Jan 2006, 11037 posts, RR: 72 Reply 50, posted (3 months 2 weeks 3 days 3 hours ago) and read 2983 times:
Quoting bhill (Reply 35):
Curious, if properly maintained per manufacturer specs and FAA regulations should age ever be a contributing factor with regards to MM numbers?
It shouldn't make a difference. As airplanes age they get heavier and the engines get thirstier so it might take you a little more fuel to hit the same speed but the speed capability should always be there.
Quoting Max Q (Reply 46): This is exactly why the FMC in Econ will increase cruise mach with an increase in headwind, it is not just time related costs.
Exactly...the FMC is trying to minimize total cost (fuel/distance based costs *and* time-based costs). But it has no idea what fuel costs today or the pay structure of the airline it's working for. The Cost Index is a (very rough) proxy to tell the FMC the relative importance that the airline places on fuel and time based costs; based on that the FMC will attempt to minimize total cost for the trip.
Quoting glen (Reply 49): Best for fuel consumption would always be speed for max. range (which corresponds to CI=0).
Yes, but airlines rarely go for minimum fuel consumption because it's too slow and uneconomic for reasons unrelated to fuel burn.
clydenairways From Ireland, joined Jan 2007, 682 posts, RR: 0 Reply 51, posted (3 months 2 weeks 3 days 3 hours ago) and read 2975 times:
Quoting tdscanuck (Reply 50): Yes, but airlines rarely go for minimum fuel consumption because it's too slow and uneconomic for reasons unrelated to fuel burn.
Your right, most don't. But there are a few things a company can do to change this, if their type of operation allows it.
Negotiate a change in some of your overhaul contracts to cycle based rather than time based.
Crew pay based sector based rather than hours.
tb727 From United States of America, joined Jun 2005, 1140 posts, RR: 4 Reply 55, posted (3 months 2 weeks 2 days 13 hours ago) and read 2567 times:
Quoting brons2 (Reply 54): Is the 727 subject to mach tuck at all?
Not to my knowledge. I don't think the aerodynamics would really allow it. The only aircraft I have ever flown were mach tuck was a concern was the Learjet 20/30 series. The design of the 727 and just the wing itself blows my mind every time I look at it because it was all done before computers ruled.
Fabo From Slovakia, joined Aug 2005, 826 posts, RR: 1 Reply 57, posted (3 months 2 weeks 1 day 23 hours ago) and read 2409 times:
Quoting glen (Reply 49): Even if you spend less time in the headwind it is always the same ESAD (equivalent still air distance) you have to travel, independent of your speed and that is what defines the fuel consumption for a given route, not the time in the air. Best for fuel consumption would always be speed for max. range (which corresponds to CI=0).
I disagree, ESAD will not be the same. Compare the examples:
You are flying a 200nm ground distance track, directly against a steady, 100kt headwind.
If you were to fly with a TAS of 200kts, your groundspeed would be 100kts, trip would take 2 hours, and ESAD would be 400nm.
If you were to fly with a TAS of 300kts, your groundspeed would be 200kts, trip would take 1 hour, and ESAD would be 300nm.
Disregarding time-related costs, if the fuel flow at 300kts was less then twice the 200kts FF, it would be more economical to fly at 300kts than 200kts. Time-related costs move this even more in favour of higher airspeed.
On the opposite way, 200kts will then consume about a third less fuel than 300kts, given 1:2 fuel flow figures.
(fuel constumption per mile assuming 100pph/200pph FF would then be 1/2 and 2/3 pounds per mile in still air, 1 pound per mile in HW case for both speeds, and 1/3 or 1/2 pound of fuel per mile in TW case)
France 1789, Eastern Europe 1989, Northern Africa 2011
You are right, a while after I wrote this it also came to my mind, that ESAD is depending on actual speed.
But I'm still sure, you won't save fuel by flying faster. The increase in fuel consumption will always be greater than the gain in time. Why? With the increase of speed, your drag increases exponentially, while the gain in time is only proportionally. In your example you gain time by factor 2, but (assuming simplified a quadratic increase) your drag and therefore your fuel consumption by factor 2.25.
I'm sure there are people here who can prove this with actual performance data, but I would be very surprised (but not unhappy ) to learn, that a increase in speed in a headwind situation could actually save fuel.
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
Fabo From Slovakia, joined Aug 2005, 826 posts, RR: 1 Reply 59, posted (3 months 2 weeks 16 hours ago) and read 2028 times:
Quoting glen (Reply 58): In your example you gain time by factor 2, but (assuming simplified a quadratic increase) your drag and therefore your fuel consumption by factor 2.25.
Yes, but it is also true jet engines work more efficiently at higher RPM, so they might infact generate 2.25 thrust with 2 times as much fuel, of course it might be the case that they will not.
I say it is possible to save fuel by going faster, but in a limited number of cases.
France 1789, Eastern Europe 1989, Northern Africa 2011
Max Q From United States of America, joined May 2001, 2615 posts, RR: 19 Reply 60, posted (3 months 2 weeks 13 hours ago) and read 2003 times:
Quoting glen (Reply 58):
I'm sure there are people here who can prove this with actual performance data, but I would be very surprised (but not unhappy ) to learn, that a increase in speed in a headwind situation could actually save fuel.
Well, I can tell you that in over 25 years flying jets that flying faster (within reason) into a headwind will save you fuel.
Theories are just that !
The best contribution to safety is a competent Pilot.
tb727 From United States of America, joined Jun 2005, 1140 posts, RR: 4 Reply 61, posted (3 months 2 weeks 10 hours ago) and read 1963 times:
Quoting Max Q (Reply 60): Well, I can tell you that in over 25 years flying jets that flying faster (within reason) into a headwind will save you fuel.
Theories are just that !
Ditto! I push them up into a headwind and pull them back(reluctantly ) with a tailwind.
Well, I have also 20 years on jets, but never on a such a lady like a 727 or similar - all aircraft I flew were always FMS-equipped. I was never paying too much attention about the speed computation and was therefore always thinking the only reason for the increase in headwind-situations was only to minimise costs acc. CI.
Back at home I went now a bit deeper in to the books (thank you guys) and found indeed a graph, which shows that in headwind, a increase in speed also increases range (i.e reduces fuel consumption).
For my defence: In addition to the CI, the FMS takes this fact into account as well. So as far i understand, a further increase above the already higher speed calculated by the FMS, would have again a negative effect on fuel consumption.
[Edited 2012-02-11 05:00:24]
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
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