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Taxi Fuel Burn Experiment  
User currently offlineTotalCruise From UK - England, joined Mar 2010, 5 posts, RR: 0
Posted (4 years 5 months 1 week 5 days 17 hours ago) and read 5109 times:

Hello!

This is my first post so lets hope its a good one!

Can anyone tell me this something....for the majority of airliner types out there - does the aircraft require more fuel burn to start the aircraft rolling from stationary? I am trying to work out that for 2 aircraft travelling on the same taxi way - if one aircraft is forced to stop and start several times, but the other rolls to the departure point without stopping - would the aircraft that starts and stops several times in a queue burn more fuel? Lets assume for this debate that both aircraft cover the same distance on the taxi way and are on the taxiway for roughly the same amount of time. The only difference is that one aircraft will taxi at normal speed and then queue. The other aircraft will taxi much slower but will not need to queue. Which one burns more fuel?????


Thanks everyone!

20 replies: All unread, jump to last
 
User currently offlinesandyb123 From UK - Scotland, joined Oct 2007, 1096 posts, RR: 0
Reply 1, posted (4 years 5 months 1 week 5 days 17 hours ago) and read 5090 times:
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Welcome to A.net Totalcruise!

Quoting TotalCruise (Thread starter):
would the aircraft that starts and stops several times in a queue burn more fuel?

It's hard to say exactly but you can assume that start stop is more fuel intensive that a constant role. Same in your car. Basic physics states it takes more power / fuel to start an aircraft from stationary than to keep a rolling aircraft rolling.

Sandyb123



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User currently offlinecpd From Australia, joined Jun 2008, 4879 posts, RR: 38
Reply 2, posted (4 years 5 months 1 week 5 days 17 hours ago) and read 5042 times:

I think one of the strategies is to keep going steadily, rather than starting and stopping all the time. That will also be easier on the brakes too.

User currently offlinepnwtraveler From Canada, joined Jun 2007, 2235 posts, RR: 12
Reply 3, posted (4 years 5 months 1 week 5 days 17 hours ago) and read 5017 times:

If you listen to the aircraft on the taxiways you can hear a marked difference in sound between an aircraft that is on a steady taxi right to the threshhold or even onto the runway, versus one that holds and starts and stops. In the first case the aircraft keeps the forward momentum rolling and doesn't need to rev the engines unless it does so with one particular engine to aid in turning. When an aircraft comes to a complete stop all forward momentum is lost once it stops. You can clearly hear that in order to get moving again the throttles have to be pushed a fair ways forward to get the aircraft rolling again. For this reason as well some airlines are trying to lessen the reverse of the engines on arrival to aid in slowing the aircraft.

It is similar to hyper driving (some also call it hypermiling) a car that saves considerable gas. You anticipate a light changing and take your foot off the gas and coast as long as possible before applying the brakes. As well if you are approaching a light that is about to turn green you are far better to keep the vehicle rolling in a slow coast than drive up to the light quickly and apply your brakes coming to a complete stop. In hyper driving you obviously have to react to traffic around you, but if you accelerate more slowly, brake as little as possible, keep the car rolling instead of a complete stop, and use reasonable throttle responses your gas mileage improves dramatically.


User currently offlineFlyDeltaJets87 From , joined Dec 1969, posts, RR:
Reply 4, posted (4 years 5 months 1 week 5 days 17 hours ago) and read 5009 times:

Yes, it takes more fuel to overcome the initial friction (the static coefficient of friction is always higher than the dynamic coefficient). And it also takes more fuel to get the plane up to speed than it does to keep it moving at the same speed. (And just as in your car if you shifted to neutral while driving and allowed the car to coast, the plane will do the same. You can get the plane up to the speed you want and then idle the power and it will coast for a while).

If you would like a further explanation, I can go into acceleration, momentum exchange, etc.   


User currently offlinecatiii From United States of America, joined Mar 2008, 3029 posts, RR: 4
Reply 5, posted (4 years 5 months 1 week 5 days 17 hours ago) and read 4912 times:

I know that, at USAir, they encourage rolling take-offs whenever possible and say they use less fuel. Ideally you'd contact the tower before the hold short line and be issued a teakeoff clearance without ever having to hold short, taxi onto the runway, and be off.

User currently offlineTotalCruise From UK - England, joined Mar 2010, 5 posts, RR: 0
Reply 6, posted (4 years 5 months 1 week 5 days 15 hours ago) and read 4704 times:

Okay thats excellent. I was in a meeting with an Airline Techinical Manager last week ( I wont say which one) who claimed that there is a negligable difference in fuelburn for getting their A330 and A340 fleet to move from stationary. My basic physics knowldege for static and dynamic friction coefficient tell me that tbere MUST be more power required to get the aircraft to move. I think the next question is what sort of fuel burn delta might an aircraft suffer from needing to stop and start a couple of times on the taxiway? I dont expect this one to be answered - but i think we all agree that the ops manager i spoke to last week may have been incorrect  

User currently offlineMir From United States of America, joined Jan 2004, 21558 posts, RR: 55
Reply 7, posted (4 years 5 months 1 week 5 days 15 hours ago) and read 4591 times:

Quoting TotalCruise (Reply 6):
i think we all agree that the ops manager i spoke to last week may have been incorrect

Not so fast, there. It does take more power to overcome static friction as opposed to rolling friction, but if the idle thrust of the engines is enough to do that, then you can taxi around all day, stopping and starting to your heart's content, without ever moving the throttles above idle, and you can do so without any difference in fuel burn (save for the fact that an aircraft that stops frequently on the taxiway will have a longer taxi time-wise, and that will result in more fuel being burned). Some aircraft are easier to move than others. Some aircraft will start rolling at idle thrust, but only at lighter weights. Some won't start rolling unless you bump the throttles a bit regardless of how much you weigh. And some will start rolling at idle power even at MTOW.

So it really depends, and the person you spoke to may not have been wrong.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlinePackcheer From United States of America, joined Nov 2008, 327 posts, RR: 0
Reply 8, posted (4 years 5 months 1 week 5 days 12 hours ago) and read 4265 times:

Just as a second thought to

Quoting TotalCruise (Reply 6):
but i think we all agree that the ops manager i spoke to last week may have been incorrect

The A340 has 4 engines, the A330 is a twin. With the smaller engines on the A340 and there being 4 of them, a very small adjustment may be needed to move the aircraft.

With the A330 being a twin, a larger amount of thrust needed (out of two engines) to move the aircraft.

It is possible that the negligible difference is due to

The small adjustment on the A340 x 4 engines = larger adjustment needed on the A330 x 2 engines



Things that fly, Girls and Planes...
User currently offlineGLEN From Switzerland, joined Jun 2005, 221 posts, RR: 2
Reply 9, posted (4 years 5 months 1 week 5 days 6 hours ago) and read 4043 times:

Quoting Packcheer (Reply 8):
The A340 has 4 engines, the A330 is a twin. With the smaller engines on the A340 and there being 4 of them, a very small adjustment may be needed to move the aircraft.

With the A330 being a twin, a larger amount of thrust needed (out of two engines) to move the aircraft.


The whole thing is just the opposite way:
The A340 has four engines but each of them has less thrust. Comparing the A340-300 to the A330-200 for example, they have the same amount of thrust - the A340 with 4, the A330 with 2 engines (at least max. available thrust - probably similar at idle; maybe even a bit more thurst for the A330).
On the other hand, at MTOW the A340 is much heavier (275t vs. 230t).With a heavy A340 you need quite a bit of thrust out of idle to start moving while a A330 will most of the time start taxiing at idle thrust by its own (even close to MTOW: quite slowly, but it will on a level taxiway, ).
Once the planes are moving, the A330 needs quite some breaking in order not to get too fast, while the 340 won't accelerate too much.

Needless to say, that on a light A340 in the weight range of a A330 you will have similar behaviour for the two aircraft.



"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 10, posted (4 years 5 months 1 week 5 days 4 hours ago) and read 4012 times:

For whatever my 2 cents are worth I think the big issue is just standing in the queue. At our hubs the ramp tower will initiate gate holds to prevent the queue to get too long just for that reason. To taxi to a point whether you stop/start or roll may not be that big of a deal. Now throw in a 1, 2, or 3 eng taxi and it gets more complex. Our co. believes a taxi w/ 1 eng shutdown is USUALLY more efficient and the stats are available however I can tell in a MD-10 it takes more power to get rolling than it does in the -11 with 1 shutdown, relatively speaking.

User currently offlinecobra27 From Slovenia, joined May 2001, 1011 posts, RR: 0
Reply 11, posted (4 years 5 months 1 week 5 days 4 hours ago) and read 3996 times:

I think taxiing aircraft is wasteful in fuel consumption and engine life, and also bad for environment (not only because it uses fuel, but also because around that 5% fuel that doesn't get burned at low rpm, ends up in airport environment) . I don't see the economics. When everybody is complaining about expensive fuel, nobody seems to use somesort of tugs or something
Also foreign object damage can also be an issue.
You turn up engines, warm them up and away you go all in a few minutes


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 12, posted (4 years 5 months 1 week 5 days 2 hours ago) and read 3938 times:

Quoting cobra27 (Reply 11):
When everybody is complaining about expensive fuel, nobody seems to use somesort of tugs or something



Well you have to be reasonable here. Consider at our main hub there's about 200 flights that arrive in a 2 30 hr window and depart again in about the same window. Now try tugging all these jets to 2 or 3 runways and using the most dist runway will be about 3 mi. ! Let's say you do this then depending on weather conditions warm up time will vary and you can't configure the jet untill all engs are started. This is about the time any malfunctions will show up so you now delay everyone behind you or TAXI away to maint.

Quoting cobra27 (Reply 11):
You turn up engines, warm them up and away you go all in a few minutes

See above


User currently offlinemusang From United Kingdom, joined Apr 2001, 864 posts, RR: 7
Reply 13, posted (4 years 5 months 1 week 5 days ago) and read 3883 times:

Good Day All.

Quoting cobra27 (Reply 11):
nobody seems to use somesort of tugs or something

Virgin looked at this at Gatwick a couple of years ago, and I'm not sure whether they actually tried it - I certainly never saw any and its my base. But the other issues, apart from tying up loads of tugs, were (a) the logistics of driving the tugs back to the apron area, conflicting with taxiing aircraft, and (b) the unacceptable stress on the nose legs.

I can't give a source for those reasons, but it seemed reasonable at the time, and I'm sure VS weren't the only carrier or airport to investigate this over the years.

Regards - musang


User currently offlineTotalCruise From UK - England, joined Mar 2010, 5 posts, RR: 0
Reply 14, posted (4 years 5 months 1 week 4 days 20 hours ago) and read 3780 times:

Quoting Mir (Reply 7):
Not so fast, there. It does take more power to overcome static friction as opposed to rolling friction, but if the idle thrust of the engines is enough to do that, then you can taxi around all day

With one of the green initiatives being a single engine taxi (for airbus aircraft currently - boeing startup procedures i beleve do not lend itself well to this) - then i would have thought that any active taxi manangement system to minimise the aircraft start/stop instances would be welcome. Couldnt aircraft start to absorb delay on the taxi way rather than wait in queue at the various hold points?


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 15, posted (4 years 5 months 1 week 4 days 19 hours ago) and read 3760 times:

Quoting TotalCruise (Reply 14):
Couldnt aircraft start to absorb delay on the taxi way rather than wait in queue at the various hold points?

yes start with a gate hold as traffic backs up. We also use a delayed eng start on all models in the fleet.


User currently offlineKELPkid From United States of America, joined Nov 2005, 6370 posts, RR: 3
Reply 16, posted (4 years 5 months 1 week 3 days 19 hours ago) and read 3495 times:

Quoting pnwtraveler (Reply 3):
It is similar to hyper driving (some also call it hypermiling) a car that saves considerable gas. You anticipate a light changing and take your foot off the gas and coast as long as possible before applying the brakes.

That is at best a gamble, and if you loose the gamble too often (have to apply the brakes in your car quickly many times), you will end up replacing your brake pads, brake shoes, and rotors (and probably have to have your rotors turned a few times due to warping them-brake rotors don't like repeated sudden stops) in a much shorter interval than if you just drove normally   



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineKELPkid From United States of America, joined Nov 2005, 6370 posts, RR: 3
Reply 17, posted (4 years 5 months 1 week 3 days 18 hours ago) and read 3479 times:

Quoting TotalCruise (Reply 14):
With one of the green initiatives being a single engine taxi (for airbus aircraft currently - boeing startup procedures i beleve do not lend itself well to this) -

And which Boeing aircraft would that be? I have been on more than one 737 flight where we taxied out single engine...it becomes obvious when they start up the second engine on the taxiway  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineRJ111 From , joined Dec 1969, posts, RR:
Reply 18, posted (4 years 5 months 1 week 3 days 3 hours ago) and read 3364 times:

Sometimes on a grass surface it can take nearly takeoff power to get a C152 moving - if you're in a divot.

User currently offlinePGNCS From United States of America, joined Apr 2007, 2821 posts, RR: 45
Reply 19, posted (4 years 5 months 5 days ago) and read 3046 times:

Quoting TotalCruise (Reply 14):
With one of the green initiatives being a single engine taxi (for airbus aircraft currently - boeing startup procedures i beleve do not lend itself well to this)

I'm no Boeing cheerleader, but every Boeing twin I have flown (as well as the 727) is perfectly capable of taxiing out (or in) on a single engine. There are reasons why this may not be a great idea (slippery ramp, heavy weight, sitting on asphalt, tight turns to make, etc.) but it is very routine on not only Airbus, but Douglas (and McD) as well as Boeing twins.

Quoting Mir (Reply 7):
Not so fast, there. It does take more power to overcome static friction as opposed to rolling friction, but if the idle thrust of the engines is enough to do that, then you can taxi around all day, stopping and starting to your heart's content, without ever moving the throttles above idle, and you can do so without any difference in fuel burn (save for the fact that an aircraft that stops frequently on the taxiway will have a longer taxi time-wise, and that will result in more fuel being burned). Some aircraft are easier to move than others. Some aircraft will start rolling at idle thrust, but only at lighter weights. Some won't start rolling unless you bump the throttles a bit regardless of how much you weigh. And some will start rolling at idle power even at MTOW.

The reality is that most large jets (possibly excepting a couple if extremely lightly loaded) need greater (often much greater) than idle power to start rolling. Starting and stopping frequently also has brake wear and brake temperature implications.


User currently offlinepilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 20, posted (4 years 5 months 4 days 21 hours ago) and read 2972 times:

Many airlines have extensive fuel management procedures when on the ground. We single engine taxi when conditions warrant and have a set of basic rules on when it's best to shut down the APU, do a crossbleed start, or shut down the engines and leave the APU running.

Something as simple as shutting off pack 1 as long as the front office is comfortable will save 50pph. Doesn't make much of a difference when one aircraft does it for 20 minutes, but when you have 20 aircraft doing it 6 times a day the cost savings can quickly add up to a couple million dollars over the course of a year.

Perhaps more important to me, that fuel is still sitting in the wings if we have to do a go around or have to get around some wx or hold. Fuel in the tanks=options.



DMI
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