totalcruise
Posts: 5
Joined: Sun Mar 14, 2010 10:37 pm

Taxi Fuel Burn Experiment

Sun Mar 14, 2010 10:50 pm

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!
 
sandyb123
Posts: 744
Joined: Mon Oct 15, 2007 9:29 pm

RE: Taxi Fuel Burn Experiment

Sun Mar 14, 2010 10:55 pm

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
Member of the mile high club
 
User avatar
cpd
Posts: 4574
Joined: Sat Jun 28, 2008 4:46 am

RE: Taxi Fuel Burn Experiment

Sun Mar 14, 2010 11:03 pm

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.
 
pnwtraveler
Posts: 1067
Joined: Wed Jun 13, 2007 2:12 am

RE: Taxi Fuel Burn Experiment

Sun Mar 14, 2010 11:07 pm

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.
 
FlyDeltaJets87
Posts: 4479
Joined: Fri Aug 11, 2006 3:51 am

RE: Taxi Fuel Burn Experiment

Sun Mar 14, 2010 11:09 pm

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.   
"Let's Roll"- Todd Beamer, United Airlines Flight 93, Sept. 11, 2001
 
catiii
Posts: 2391
Joined: Mon Mar 31, 2008 1:18 am

RE: Taxi Fuel Burn Experiment

Sun Mar 14, 2010 11:26 pm

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.
 
totalcruise
Posts: 5
Joined: Sun Mar 14, 2010 10:37 pm

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 12:35 am

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  
 
Mir
Posts: 19093
Joined: Mon Jan 05, 2004 3:55 am

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 12:58 am

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
 
packcheer
Posts: 134
Joined: Thu Nov 06, 2008 8:28 pm

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 3:34 am

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 avatar
glen
Posts: 262
Joined: Thu Jun 30, 2005 4:43 pm

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 10:16 am

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
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 11:59 am

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.
 
cobra27
Posts: 939
Joined: Wed May 23, 2001 6:57 pm

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 12:17 pm

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
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 2:09 pm

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
 
musang
Posts: 788
Joined: Sun Apr 08, 2001 4:11 am

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 4:24 pm

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
 
totalcruise
Posts: 5
Joined: Sun Mar 14, 2010 10:37 pm

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 8:23 pm

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?
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: Taxi Fuel Burn Experiment

Mon Mar 15, 2010 8:45 pm

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.
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: Taxi Fuel Burn Experiment

Tue Mar 16, 2010 8:48 pm

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 :-)
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: Taxi Fuel Burn Experiment

Tue Mar 16, 2010 10:08 pm

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 :-)
 
Rj111
Posts: 3007
Joined: Wed Sep 08, 2004 9:02 am

RE: Taxi Fuel Burn Experiment

Wed Mar 17, 2010 1:25 pm

Sometimes on a grass surface it can take nearly takeoff power to get a C152 moving - if you're in a divot.
 
PGNCS
Posts: 2249
Joined: Sun Apr 15, 2007 5:07 am

RE: Taxi Fuel Burn Experiment

Mon Mar 22, 2010 3:37 pm

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.
 
pilotpip
Posts: 2820
Joined: Fri Sep 19, 2003 3:26 pm

RE: Taxi Fuel Burn Experiment

Mon Mar 22, 2010 7:19 pm

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

Who is online

Users browsing this forum: No registered users and 17 guests