CaptainJon
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How Do Planes Taxi Exactly?

Sat Apr 21, 2007 3:22 pm

This maybe a dumb question, but I've always wondered how does a plane taxi? I mean, what causes it to move on the ground? For example, on a jet, do the turbofans cause the aircraft to move? Or do they simply generating electricity like how an electrical generating turbine would and the plane moves forward?

I imagine props are similar?

What confuses me is, when the plane is sitting parked, I often hear the engines revving up, to a rather powerful level that the parking breaks seem it shouldn't keep that plane parked. (Usually this seems to happen after pushback and the captain is going through the pre-flight check list).

Okay I hope I didn't confuse anyone, because I think I confused myself.
 
Tristarsteve
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RE: How Do Planes Taxi Exactly?

Sat Apr 21, 2007 6:48 pm

Yes, at present all aircraft use engine thrust to taxi. The parking brakes will keep an aircraft stationary up to nearly take off power.
A system of electric motor taxying has been developed, and you will see it on new aircraft soon. Delta has signed up for a system on their new B737s.
 
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Starlionblue
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RE: How Do Planes Taxi Exactly?

Sat Apr 21, 2007 10:56 pm

Quoting CaptainJon (Thread starter):

What confuses me is, when the plane is sitting parked, I often hear the engines revving up, to a rather powerful level that the parking breaks seem it shouldn't keep that plane parked.

The brakes (note spelling) can hold the aircraft even if the engines are almost at full thrust. They are viciously powerful.
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SP90
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RE: How Do Planes Taxi Exactly?

Sat Apr 21, 2007 11:10 pm

Quoting TristarSteve (Reply 1):
A system of electric motor taxying has been developed, and you will see it on new aircraft soon. Delta has signed up for a system on their new B737s

Indeed. Check out this prototype by Boeing.

767 Nose Wheel Electric Drive

I wonder how well it will work in wet/icy conditions. Would they still require a tug for pushback and engine power to taxi? Maybe an advance traction control would limit wheel slippage enough for it to work no matter the surface conditions.
 
InnocuousFox
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RE: How Do Planes Taxi Exactly?

Sat Apr 21, 2007 11:45 pm

In a manner of speaking, taxiing is a failed attempt at taking off. Big grin
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pizzaandplanes
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 12:47 am

So, what brakes do pilots use on landing- parking or emergency brakes?

Or is mostly thrust reversers?
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Marquis
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 1:51 am

Quoting Pizzaandplanes (Reply 5):
So, what brakes do pilots use on landing- parking or emergency brakes?

Or is mostly thrust reversers?

They use neither of the first two ones you mentioned for landing. They simply apply the wheel brakes by pushing the upper tip of their rudder pedals, very similar to that in car. They can also select a certain degree of constant deceleration via the auto brakes switch. The A380 for example incorporates an automated system, by which you can preselect a certain turn-off from the runway and the system will apply brake force according to the pilot's desired exit.

Thrust reversers by contrast only assist the wheel brakes in order to lower their wear. Some jet engined aircraft are not even equipped with thrust reversers as a safe landing roll can be perfectly conducted with wheel brakes only.

[Edited 2007-04-21 18:56:33]
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Goldenshield
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 2:15 am

Quoting Marquis (Reply 6):
Thrust reversers by contrast only assist the wheel brakes in order to lower their wear. Some jet engined aircraft are not even equipped with thrust reversers as a safe landing roll can be perfectly conducted with wheel brakes only.

To add to this: when an aircraft is decelerating, thrust reversers assist the brakes in slowing the aircraft, with the brakes being the primary way to slow the aircraft down.

An analogy to this is that of a car with a manual transmission: when you are approaching a stop light, you would use the brakes to slow you down, but if you incorporate compression braking (downshifting the transmission) into the equation, you can either: 1) save your brakes a lot of wear, by reducing the pressure and/or how long they are used, or 2) increase your deceleration rate by using compression braking on top of normal braking.
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GQfluffy
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 2:46 am

Quoting Goldenshield (Reply 7):
An analogy to this is that of a car with a manual transmission: when you are approaching a stop light, you would use the brakes to slow you down, but if you incorporate compression braking (downshifting the transmission) into the equation, you can either: 1) save your brakes a lot of wear, by reducing the pressure and/or how long they are used, or 2) increase your deceleration rate by using compression braking on top of normal braking.

And 3) Eat alot of gas. I did this for a while in my Taurus SHO. After a month or so of comparing gas milage, I was averaging 3-4 mpg less each tank... YMMV
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CaptainJon
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 2:55 am

Quoting Starlionblue (Reply 2):
The brakes (note spelling) can hold the aircraft even if the engines are almost at full thrust. They are viciously powerful.

Oops! But thanks for the answer. And to everyone else. I learned something new!
 
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Starlionblue
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 3:11 am

Quoting GQfluffy (Reply 8):
And 3) Eat alot of gas. I did this for a while in my Taurus SHO. After a month or so of comparing gas milage, I was averaging 3-4 mpg less each tank... YMMV

I don't understand what happened to you here. On pretty much any modern car, the electronics will cut fuel flow to idle when using engine braking. So even if you are engine braking your gas consumption at that point is the same as idle.

Oh wait. You have a Taurus (sorry couldn't resist)

[Edited 2007-04-21 20:11:27]
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 3:12 am

Quoting Goldenshield (Reply 7):
...with the brakes being the primary way to slow the aircraft down.

For performance calculations, yes, however at least in practice on the CRJ, brakes are usually only engaged as reverse thrust is set to idle (below 75 knots). On the Dash it was similar- we slowed via beta, and used wheel braking briefly right before making the turnoff. Both were as per company procedures.
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Goldenshield
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 3:21 am

Quoting Doug_Or (Reply 11):
For performance calculations, yes, however at least in practice on the CRJ, brakes are usually only engaged as reverse thrust is set to idle (below 75 knots).

There are always exceptions to the rule. I remember reading something about that in regards to loss of airflow to the rudder on aircraft of that configuration; however, is that a -200, or -7/900 we're talking about? At tight airports (BUR, SNA,) I would figure that policy could lead to possible 'issues.'
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BAE146QT
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 3:58 am

Quoting Starlionblue (Reply 10):
I don't understand what happened to you here. On pretty much any modern car, the electronics will cut fuel flow to idle when using engine braking.

Yes indeed. Note also that in the case of idle, the fuel consumption will look like < 0. Of course that's physically impossible. However;

Quoting Starlionblue (Reply 10):
So even if you are engine braking your gas consumption at that point is the same as idle.

Yes sir.

Also, most injection engines will pre-charge the rail, and return it to the tank if it's not used. Hence the less-than-zero usage. Really GOOD engines, (like two of mine) will look at your driving style and precharge the rail accordingly. If you trick it, it does the return in bulk. This can be really odd when you see a good 1/4 tank disappear then come back again...
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 4:04 am

Quoting Goldenshield (Reply 7):
An analogy to this is that of a car with a manual transmission: when you are approaching a stop light, you would use the brakes to slow you down, but if you incorporate compression braking (downshifting the transmission) into the equation,

Hehe... now we will see signs along the runway that say "no Jake Braking"!

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BAE146QT
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 5:42 am

Quoting InnocuousFox (Reply 14):
Hehe... now we will see signs along the runway that say "no Jake Braking"!

LOL Well the wheels aren't (directly) braked by the engines, so I guess the equivalent would be "No reverse thrust".

Isn't that the case already? ISTR that Palm 90 reversed from the gates under her own power and I have read here that some aircraft with rear-mounted engines still do so...
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Tristarsteve
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 6:35 am

Quoting InnocuousFox (Reply 14):
that say "no Jake Braking"!

OK I give up. Whats a jake when you use the brakes?
 
InnocuousFox
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 6:58 am

Jake Braking is the technique mentioned before of downshifting to use the high engine revs to slow down a truck. It makes quite a racket so many communities prohibit it at night... or entirely. I was making a joke on the analogy in Reply 7.
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VC-10
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 8:09 am

Quoting Starlionblue (Reply 2):
The brakes (note spelling) can hold the aircraft even if the engines are almost at full thrust.

Not on the A320 series. If you are at full power on those and you do not want to move E.g. maint grnd run, you have to use the toe brakes to apply full braking pressue to prevent you from moving
 
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Starlionblue
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 8:37 am

Quoting VC-10 (Reply 18):
Not on the A320 series. If you are at full power on those and you do not want to move E.g. maint grnd run, you have to use the toe brakes to apply full braking pressue to prevent you from moving

Isn't that what I said?  Wink

Quoting Starlionblue (Reply 2):
The brakes (note spelling) can hold the aircraft even if the engines are almost at full thrust.

Yes I know the original poster was talking parking brakes, but I just said brakes.
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doug_or
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 8:48 am

Quoting Goldenshield (Reply 12):
There are always exceptions to the rule. I remember reading something about that in regards to loss of airflow to the rudder on aircraft of that configuration; however, is that a -200, or -7/900 we're talking about? At tight airports (BUR, SNA,) I would figure that policy could lead to possible 'issues.'

It is -200 through -900, but the policy is to stop the airplane on the runway, not applying brakes untill below 80 knots unless neccesary. At most airports this works fine, but obviously there are runways were brakeing shortly after touchdown is neccesary.
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speedracer1407
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 9:07 am

Quoting InnocuousFox (Reply 17):
Jake Braking is the technique mentioned before of downshifting to use the high engine revs to slow down a truck.

I dont' think it's a technique so much as it's a specific mechanism fitted to many road-going diesel-powered trucks/busses. "Jake" is just a brand (Jacob) of compression brake, which releases compressed air from combustion chamber after the compression stroke so that it doesn't return any energy to the un-fueled power stroke. This is the noisy part, and doesn't necessarily involve high revs. Regular engine braking in a diesel (not very effective) or gas engine is simply a technique, and isnt' noisy.

O
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CaptainJon
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 9:14 am

Quoting Starlionblue (Reply 19):
Yes I know the original poster was talking parking brakes, but I just said brakes.

Yes I was referring to parking brakes, not break wind. I know the difference, I was rather sleepy when I typed it up so when I proofread it, I read it wrong.

 
Tristarsteve
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 5:31 pm

Quoting Starlionblue (Reply 10):
On pretty much any modern car, the electronics will cut fuel flow to idle when using engine braking.

I was under the impression that, in a fuel injected car, when you take your foot off the accelerator to slow down, the injection system cuts the fuel to zero and you are running on air.
This is why it is fuel efficient to leave the car in gear while going down hill. You use less fuel than if you put it into neutral as then you are idling, and using fuel.
Is this correct?
 
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Starlionblue
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RE: How Do Planes Taxi Exactly?

Sun Apr 22, 2007 9:31 pm

Quoting TristarSteve (Reply 23):
I was under the impression that, in a fuel injected car, when you take your foot off the accelerator to slow down, the injection system cuts the fuel to zero and you are running on air.
This is why it is fuel efficient to leave the car in gear while going down hill. You use less fuel than if you put it into neutral as then you are idling, and using fuel.
Is this correct?

Wichever the case (idle or zero) GQFluffy's car seems to be malfunctioning. Then again, it is an American car so I wouldn't expect any less.  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
brettbrett21
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 12:09 am

Do jets taxi on idle, aside from power-up to get going?
What about props?
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InnocuousFox
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 3:12 am

I would suspect that it is very similar to a car. As you mentioned, there is an initial powerup to get going. After that, the only relevant Newtonian law is that the continued thrust needs to match the wind resistance and the ground/wheel resistance. That is, the momentum of the plane keeps it going for the most part. They can throttle back down to an idle-like setting to keep it rolling.
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 4:19 am

Turboprops also taxi on idle. They need an initial power up to get them going just like jets do, but once they get going their momentum keeps them moving and they generally tend to accelerate, except in strong headwinds (on gound idle those propellers act like large metal discs that will slow you down very effectively). Interesting thing is you can use beta (reverse idle) to keep them from picking up too much speed. Doesn't burn any more fuel than normal ground idle, and it has pretty much the same effect as 'riding the brakes' (but in a good way).
 
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 5:37 am

I've watched Ryanair 737-800's at Luton come off of the runway and then taxi in at what looks like around 30-40 knots and then apply reverse idle to slow as they came to turns, they sure are in a hurry!
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Starlionblue
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 10:03 am

Quoting InnocuousFox (Reply 26):
I would suspect that it is very similar to a car. As you mentioned, there is an initial powerup to get going. After that, the only relevant Newtonian law is that the continued thrust needs to match the wind resistance and the ground/wheel resistance. That is, the momentum of the plane keeps it going for the most part. They can throttle back down to an idle-like setting to keep it rolling.

Correct. A lot of pilots also complain about idle being a bit too much thrust for taxi. I was talking with a pair of CRJ pilots who complained about this. They used to idle reverse one of the engines until maintenance told them to stop doing that. So now they just heat the brakes.
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Someone83
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 6:26 pm

Quoting SilverComet (Reply 27):
Turboprops also taxi on idle. They need an initial power up to get them going just like jets do, but once they get going their momentum keeps them moving and they generally tend to accelerate, except in strong headwinds (on gound idle those propellers act like large metal discs that will slow you down very effectively). Interesting thing is you can use beta (reverse idle) to keep them from picking up too much speed. Doesn't burn any more fuel than normal ground idle, and it has pretty much the same effect as 'riding the brakes' (but in a good way).

And why do several airline have in their Op. manual to only taxi using one engine with turboprops to save fuel? If they only where running at idle would that be necessary
 
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Starlionblue
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 8:03 pm

Quoting Someone83 (Reply 30):

And why do several airline have in their Op. manual to only taxi using one engine with turboprops to save fuel? If they only where running at idle would that be necessary

Yes. Consumption is still lowered significantly by turning the engine off.
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SilverComet
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 8:37 pm

Quoting Someone83 (Reply 30):

Every little bit helps. At some airports taxi time can take up as much as one hour, with congestion and all. Even at idle the engines are still burning something, and if you had to choose between burning that something and half that something, which would you prefer? Plus it makes sense since ground idle on two engines is still too much thrust for taxiing anyway.

Once with a colleague we were wondering how much of a difference those 'little bits' actually make. My company currently operates 11 aircraft (a mix of ATR's, A319's and A343's and soon some A332's). Imagine on every leg the pilots managed to cut down on one minute of flight time. Here goes:

1 minute saved per leg
x 75 kgs of fuel saved per minute (mixed fleet average)
x 0.35 USD per kg of JET A1 (European average)
x 40 legs per day (approx)
x 365 days in a year (approx  Wink)

You do the math.
 
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HAWK21M
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 11:16 pm

Quoting SilverComet (Reply 32):
At some airports taxi time can take up as much as one hour

Which are these Airports.
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SilverComet
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RE: How Do Planes Taxi Exactly?

Mon Apr 23, 2007 11:57 pm

Quoting HAWK21M (Reply 33):
Which are these Airports

LHR, for one.

It has happened before that one tonne of fuel be burnt on ground during the taxi leading up to departure, due to congestion at LHR. Others like CDG or FRA also come to mind, and I can easily imagine other large airports in the US having the same issues like ATL, ORD or LAX, although I never been there.
 
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Starlionblue
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RE: How Do Planes Taxi Exactly?

Tue Apr 24, 2007 12:17 am

Quoting HAWK21M (Reply 33):
Quoting SilverComet (Reply 32):
At some airports taxi time can take up as much as one hour

Which are these Airports.

Has happened to me numerous times at JFK.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
InnocuousFox
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RE: How Do Planes Taxi Exactly?

Tue Apr 24, 2007 12:53 am

Quoting HAWK21M (Reply 33):
Which are these Airports.

STL was horrible back in the TWA days.


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twal1011727
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RE: How Do Planes Taxi Exactly?

Thu Apr 26, 2007 5:51 am

Quoting Goldenshield (Reply 7):
An analogy to this is that of a car with a manual transmission: when you are approaching a stop light, you would use the brakes to slow you down, but if you incorporate compression braking (downshifting the transmission) into the equation, you can either: 1) save your brakes a lot of wear, by reducing the pressure and/or how long they are used,

I would much rather use brakes on my car than downshifting (maunal or automatic) under normal conditions.
The brakes are alot cheaper to replace than a clutch or burned up automatic tranny(especially front wheel drives)

KD
 
bond007
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RE: How Do Planes Taxi Exactly?

Thu Apr 26, 2007 6:18 am

Quoting TWAL1011727 (Reply 37):
I would much rather use brakes on my car than downshifting (maunal or automatic) under normal conditions.
The brakes are alot cheaper to replace than a clutch or burned up automatic tranny(especially front wheel drives)

Similarly, thrust reverser maintenance costs more than the savings to the brakes.

Jimbo
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Goldenshield
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RE: How Do Planes Taxi Exactly?

Thu Apr 26, 2007 7:36 am

Quoting TWAL1011727 (Reply 37):
The brakes are alot cheaper to replace than a clutch or burned up automatic tranny(especially front wheel drives)

No disrespect, but if one downshifts a stick properly, he will only go through a few brake pads before he has the clutch plate replaced, and not nearly as many times as an automatic has theirs replaced. Hell, my truck is 11 years old and has 160,000 miles on it, and the clutch was replaced last year for the first time, and it had moderate wear on it for as much stop-and-go driving I do. I've also only had to place 1 set of front brakes, with the rear brakes still factory, with quite a few miles left on them.
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Starlionblue
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RE: How Do Planes Taxi Exactly?

Thu Apr 26, 2007 9:24 am

Quoting TWAL1011727 (Reply 37):
I would much rather use brakes on my car than downshifting (maunal or automatic) under normal conditions.
The brakes are alot cheaper to replace than a clutch or burned up automatic tranny(especially front wheel drives)

On older cars, you also run the risk of brake fade if you brake a lot. I'd rather combine brakes and engine braking. It's not as if the clutch on a manual transmission will see much more wear with engine braking.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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HAWK21M
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RE: How Do Planes Taxi Exactly?

Thu Apr 26, 2007 3:49 pm

Quoting SilverComet (Reply 34):
It has happened before that one tonne of fuel be burnt on ground during the taxi leading up to departure, due to congestion at LHR



Quoting Starlionblue (Reply 35):
Has happened to me numerous times at JFK.



Quoting InnocuousFox (Reply 36):
STL was horrible back in the TWA days

Considering the Taxi time utilised & the unnecessary fuel burn.That would be a big hit to Airline profits.Why wasn't spacing provided.

Is it still continuing or are there improvements.

rgds
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Scooter01
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RE: How Do Planes Taxi Exactly?

Thu Apr 26, 2007 4:52 pm

Quoting Starlionblue (Reply 40):
On older cars, you also run the risk of brake fade if you brake a lot.

That's why you should have the brakefluid changed once a year. -No matter the age of the car-

Scooter
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