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Faro
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Physics Of The Push-Back

Sat Dec 25, 2010 2:43 pm

Does a push-back truck have to be heavier than the aircraft it is moving? In general, what are the weight/co-efficient of friction/power criteria required in the push-back equation?

Faro
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oly720man
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RE: Physics Of The Push-Back

Sat Dec 25, 2010 3:08 pm

Quoting faro (Thread starter):
Does a push-back truck have to be heavier than the aircraft it is moving?

You're not pushing against the weight of the aircraft, just against the friction.

A tug, eg here

http://www.littlebigtugcompany.co.uk/specs.htm

weighs about 72 tonnes which is less than half the weight of the fuel in a B747 and 15-20% of the take-off weight.

What the tug can pull, through the towbar, depends on the gear (and the towbar). In lowest gear it's around 30 tonnes for the above example.

http://www.littlebigtugcompany.co.uk/perform.htm
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Faro
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RE: Physics Of The Push-Back

Sat Dec 25, 2010 3:32 pm

Quoting oly720man (Reply 1):
You're not pushing against the weight of the aircraft, just against the friction.
Quoting oly720man (Reply 1):
What the tug can pull, through the towbar, depends on the gear (and the towbar). In lowest gear it's around 30 tonnes for the above example.

So with a high enough gearing ratio, the weight of the tug is immaterial?

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oly720man
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RE: Physics Of The Push-Back

Sat Dec 25, 2010 4:21 pm

Quoting faro (Reply 2):
So with a high enough gearing ratio, the weight of the tug is immaterial?

The tug has to have a certain weight otherwise it won't generate enough traction force to move anything.
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Viscount724
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RE: Physics Of The Push-Back

Sat Dec 25, 2010 7:29 pm

Asume the physics are somewhat similar to railway locomotives and the weight of the train they can pull. A single locomotive can pull dozens of loaded freight cars, each of which can sometimes weigh about half as much as the locomotive. And airport tow tractors don't have to cope with hills.
 
474218
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RE: Physics Of The Push-Back

Sat Dec 25, 2010 9:49 pm

Quoting oly720man (Reply 3):
The tug has to have a certain weight otherwise it won't generate enough traction force to move anything.


"Correct". When we had an SR-71 divert to McCoy AFB in about 1968, we borrowed a small tug to move the aircraft in and out of the hanger for engine runs. The little tug was fine out on the apron but as soon as it was on the smooth hangar floor the tires would just spin (no traction) and we had to manually push the aircraft in and out of the hangar.
 
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Scooter01
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RE: Physics Of The Push-Back

Sun Dec 26, 2010 12:49 am

Quoting 474218 (Reply 5):
we had to manually push the aircraft in and out of the hangar.

-and I bet "we" weighed a lot less than the aircraft....

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FlyASAGuy2005
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RE: Physics Of The Push-Back

Sun Dec 26, 2010 5:12 am

Look at RJs. Many airlines actually use the LEKTRO (almost exclusively used in ATL on C & D). Very small piece of equipment that doesn't weigh anywhere near the CR2 or 7. Only issue is during ice..
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HAWK21M
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RE: Physics Of The Push-Back

Sun Dec 26, 2010 6:00 am

Surface condition & Taper create issues with pushbacks of lighter capacity.
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litz
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RE: Physics Of The Push-Back

Tue Dec 28, 2010 9:34 pm

In the railroad world, we call this "tractive effort", which is essentially how much traction each wheel can exhert before slipping (spinning).

That's why a single 225,000 lb locomotive can pull dozens of loaded freight cars across flat Kansas ... but soon as you get into the hills, it can only pull 5-10.

Of course, railroad engines can drop sand under the wheels for more traction ...

I can't imagine that being a good idea on an airport tarmac ...  

Airplane tugs aren't much different ... you have two forces to balance ... one is the friction inherent in the airplane's wheels, and the other is the "grip" of the tug's wheels on the ground. As long as the "pull" of the tug exceeds the friction prior to the tug's wheels losing their "grip", the airplane will roll.

Once moving, inertia will tend to want to keep it moving, so you have to remember to take that into effect when trying to stop. Otherwise, it'll just drag you along.

There is a reason it takes a mile and a half to stop a moving freight train.
 
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DocLightning
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RE: Physics Of The Push-Back

Tue Dec 28, 2010 10:23 pm

Quoting HAWK21M (Reply 8):
Surface condition & Taper create issues with pushbacks of lighter capacity.

Why? Wouldn't it cause trouble with heavier capacity?
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fr8mech
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RE: Physics Of The Push-Back

Tue Dec 28, 2010 11:41 pm

It seems a lot of folks are missing, what I always thought, was the bigger issue when it came to tug weight: stopping the aircraft.

It really isn't that difficult to get an aircraft moving. You see charity aircraft pulls. I've seen half a dozen guys get a B757 moving by pushing the gear. Aircraft are designed, among other things, to roll as easily as possible on the ground so as the engines have to expend minimal power to get the aircraft moving.

The mass of a moving airplane, even at 2 or 3 mph, presents an issue for stopping. That's where the weight of the tug comes in.
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Faro
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RE: Physics Of The Push-Back

Wed Dec 29, 2010 12:16 am

Quoting fr8mech (Reply 11):
The mass of a moving airplane, even at 2 or 3 mph, presents an issue for stopping. That's where the weight of the tug comes in.

Thanx for the insight, didn't think of that!

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RE: Physics Of The Push-Back

Wed Dec 29, 2010 4:02 am

Quoting FlyASAGuy2005 (Reply 7):
Look at RJs. Many airlines actually use the LEKTRO (almost exclusively used in ATL on C & D). Very small piece of equipment that doesn't weigh anywhere near the CR2 or 7.

Towbarless tugs like the LEKTRO scoops the nosegear off the ground. Therefore the weight of the airplane's nose is pushing down on the tug helping the tug's traction.
 
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RE: Physics Of The Push-Back

Wed Dec 29, 2010 12:51 pm

Any tug will pull a plane. Stopping it is the problem
 
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RE: Physics Of The Push-Back

Wed Dec 29, 2010 2:09 pm

Quoting fr8mech (Reply 11):
The mass of a moving airplane, even at 2 or 3 mph, presents an issue for stopping. That's where the weight of the tug comes in.

Well, not really, or at least not directly. It takes slightly total energy to stop the plane than it does to get it moving (slightly less because of wheel friction on the airplane side). The issue with stopping is how it's done: by brake conversion to heat at the tug wheels. There may be insufficient heat-sinking on a small tug, and you need good anti-lock brakes to keep the wheels rolling. (Otherwise, you're doing the braking at the tire-ground interface, which is a definite fail.) If you could arrange for the tug motor to exert the same force (in reverse) while stopping as it did getting the plane moving, and if the tug tires had the same traction in either direction, there would be no problem.

A large tug will have larger brakes that can sink more heat, and more tire-ground surface which makes controlling the process a lot easier.
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RE: Physics Of The Push-Back

Wed Dec 29, 2010 2:15 pm

Quoting FlyASAGuy2005 (Reply 7):
Look at RJs. Many airlines actually use the LEKTRO (almost exclusively used in ATL on C & D). Very small piece of equipment that doesn't weigh anywhere near the CR2 or 7. Only issue is during ice..

I'd imagine it helps that those tugs bear the weight normally on the nose gear?

Quoting b78710 (Reply 14):
Any tug will pull a plane. Stopping it is the problem

True. I've seen bag tugs push mainline jets in a pinch.
 
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Aesma
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RE: Physics Of The Push-Back

Wed Dec 29, 2010 2:15 pm

With an electric tug you should be able to use the engines for braking (and charging back the batteries). Electric trains do that, too (without the batteries).
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RE: Physics Of The Push-Back

Wed Dec 29, 2010 8:42 pm

Quoting PITingres (Reply 15):

I'll disagree here a little bit. The heavier the tug, the more friction it exerts. Irrespective of how you are braking, if the tug is unable to keep its wheels 'glued' to the pavement, the aircraft will push it around.
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RE: Physics Of The Push-Back

Wed Dec 29, 2010 9:59 pm

Quoting fr8mech (Reply 18):
I'll disagree here a little bit. The heavier the tug, the more friction it exerts. Irrespective of how you are braking, if the tug is unable to keep its wheels 'glued' to the pavement, the aircraft will push it around.

Absolutely. But, it works both ways; there's nothing special about braking that affects the tug/ground traction. (If not all tug wheels are driven, or if there's an uneven fore/aft weight on the tug, that would change the pushing vs braking picture at least somewhat.)

What DOES matter is that if the tug tires spin a bit while getting the plane moving, nobody cares as long as it's not excessive and the plane is being accelerated. If the tires grab or slide during braking, you DO care, since you don't necessarily have distance to fritter away. The force diagrams are pretty much the same, just reversed, but the scenario and boundary conditions are different.
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RE: Physics Of The Push-Back

Thu Dec 30, 2010 9:09 am

Quoting b78710 (Reply 14):
Any tug will pull a plane. Stopping it is the problem

Tugs & Towbars have load limits too.

Quoting DocLightning (Reply 10):

Why? Wouldn't it cause trouble with heavier capacity?

To a lesser extend as Momemtum produced would be more than a lighter tug.

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hal9213
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RE: Physics Of The Push-Back

Thu Dec 30, 2010 1:47 pm

Quoting faro (Thread starter):
Does a push-back truck have to be heavier than the aircraft it is moving?

No offense meant, but I found the thought of a 600T truck pushing a fully loaded A380 very amusing 
 
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RE: Physics Of The Push-Back

Thu Dec 30, 2010 2:47 pm

Quoting oly720man (Reply 1):
Quoting faro (Thread starter):
Does a push-back truck have to be heavier than the aircraft it is moving?

You're not pushing against the weight of the aircraft, just against the friction.

Correct. Another analogy would be a 200 lb person pushing a 2,000 lb car. It is fairly easy to do. It doesn't take 2,000 lb of people (10 guys) to move a 2,000 lb car.
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RE: Physics Of The Push-Back

Thu Dec 30, 2010 6:26 pm

Quoting hal9213 (Reply 21):
Quoting faro (Thread starter):
Does a push-back truck have to be heavier than the aircraft it is moving?

No offense meant, but I found the thought of a 600T truck pushing a fully loaded A380 very amusing 

No offense taken  ; in retrospect, I realise it were a rather naïve question. Simply, the thing was always nagging me; I guess I should have thought a little more before posting but all the contributions were nonetheless quite illuminating. Thanx to everybody.

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hal9213
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RE: Physics Of The Push-Back

Fri Dec 31, 2010 1:01 pm

Quoting faro (Reply 23):
No offense taken ; in retrospect, I realise it were a rather naïve question.

Hehe. I think it was good. However naive a question, usually as professionals add more insight to the topic, it can get very interesting. (As to the physics, actual models, website of the manufacturer, etc.)
 
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RE: Physics Of The Push-Back

Fri Jan 07, 2011 7:03 am

I can't believe that nobody has mentioned the keyword "coefficient of static friction" -- this is the key here. This is 9th-grade / 1st-year "O"-Level physics, people.  http://en.wikipedia.org/wiki/Friction

Quoting faro (Reply 2):
So with a high enough gearing ratio, the weight of the tug is immaterial?

No.  

When the plane is pushed back, there are two opposing forces at work: the force of the tug on the plane, and the opposing force of the plane on the tug. The tug has to overcome the force that the plane exerts on the tug.

As has been mentioned, the force that the plane exerts on the tug is due to the friction in the plane's wheels, etc., and planes are carefully engineered to minimize this force by putting in low-friction bearings, etc.

The force that the tug exerts on the plane is limited by the amount of traction that the tug can get through its wheels, which is limited by the static friction the wheels can achieve on the surface. It is equal to the weight of the tug times the coefficient of static friction between the tug's wheels and the surface it is on. A slippery surface (smooth hangar floor, ice, wet pavement, ...) has a lower coefficient of static friction than a "grippier" surface.

So: suppose the plane exerts a force of 5,000N (totally picking that out of the air). Then the tug has to be able to exert at least that much force to move the plane.

If the tug weighs 1000kg (which works out to about 10,000N), then if the coefficient of static friction between the wheels and the surface is 0.5 or greater, then the tug will be able to exert 5000N (10k * 0.5) of force on the plane, and it will be able to move the plane.

If the product of the coefficient of static friction times the tug's weight is less than 5000N in this example, then the tug will never be able to move the plane -- the tug's wheels will slip before the tug can get the plane moving.
 
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RE: Physics Of The Push-Back

Fri Jan 07, 2011 9:48 am

Quoting faro (Thread starter):
Does a push-back truck
have to be heavier than the aircraft it is moving?

No. Otherwise this would be a heavy dude.
http://www.youtube.com/watch?v=tls-Jli6eQE&feature=player_embedded

Essentialy the static friction coefficient of the tires of the tug needs to be high enough to ensure the following relation:

Ffriction,truck = cstat,truck * mtruck * g > cstat,plane * mplane * g + Ffriction,hubs = Ffriction,plane = Fplane>truck;

essentially the force the plane exerts onto the truck is the sum of the static friction of the plane's wheels on the ground (relatively small) plus the friction of the wheels turning around the hubs (also relatively small). This is why the trucks always have those huge tires with comparably low pressure and a deep profile, it increases the static friction coefficient.
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RE: Physics Of The Push-Back

Sun Mar 10, 2013 10:29 am

Quoting oly720man (Reply 1):
You're not pushing against the weight of the aircraft, just against the friction

Very well said.

Somehow out here the towbar types are preffered to the TBL ones.....notice LG damage can be less on the latter in case of an error.
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RE: Physics Of The Push-Back

Sun Mar 10, 2013 2:00 pm

Quoting HAWK21M (Reply 27):
Somehow out here the towbar types are preffered to the TBL ones.....notice LG damage can be less on the latter in case of an error.

You probably don't get a lot of snow and ice. 
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Scooter01
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RE: Physics Of The Push-Back

Sun Mar 10, 2013 2:50 pm

Quoting HAWK21M (Reply 27):
Very well said.

Where do you find these old threads?

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jman40
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RE: Physics Of The Push-Back

Mon Mar 11, 2013 2:10 am

What if the tug is on a conveyor belt?  
 
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RE: Physics Of The Push-Back

Mon Mar 11, 2013 2:33 am

Quoting Scooter01 (Reply 29):
Where do you find these old threads?

A huge number of pages down the line after spending a huge amount of time while bored?  
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bristolflyer
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RE: Physics Of The Push-Back

Mon Mar 11, 2013 2:35 am

Check out these videos...

VW Toureg towing a 747:

http://www.youtube.com/watch?v=MOG90IQEbhI

Toyota Tundra towing the Space Shuttle:

http://www.youtube.com/watch?v=Wd-NsF8IGq8

In round numbers the Tundra is about 5,200 lbs and the Space Shuttle/trailer is about 300,000 lbs.
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m1m2
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RE: Physics Of The Push-Back

Mon Mar 11, 2013 7:51 am

I think the most important reason to have a little heavier tug would be to get the whole thing stopped after you had it moving. Not a problem if you move it really slow, but you would have to be careful.
 
T prop
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RE: Physics Of The Push-Back

Mon Mar 11, 2013 8:18 am

On smaller aircraft you can have to big of a tug. I remember reading an incident report where a ground crew drove off with only the nose gear off a Dash 8 when they didn't notice the captain give the brakes applied signal. Apparently the normal tug had broken down so they grabbed the next available one that had just finished pushing back another airplane. The last airplane that tug pushed back? - a DC-10.   
 
m1m2
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RE: Physics Of The Push-Back

Mon Mar 11, 2013 8:25 am

Yeah, those big tugs wouldn't know the Dash 8 was there, or in this case a part of the Dash 8! I know an empty Dash doesn't offer much resistance unless it's a Q4, then you can notice it with a smaller tug.
 
vikkyvik
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RE: Physics Of The Push-Back

Mon Mar 11, 2013 4:43 pm

Quoting m1m2 (Reply 33):
I think the most important reason to have a little heavier tug would be to get the whole thing stopped after you had it moving. Not a problem if you move it really slow, but you would have to be careful.

It certainly helps, but as stated earlier, the physics of stopping an airplane aren't really any different than starting it moving. You do have less friction in the bearings (kinetic vs. static), and more importantly, you don't want to waste time/distance stopping an airplane that's already moving.  

Out of curiosity, do the pilots ever apply any braking to help slow the airplane during pushback? Is this inadvisable due to the possibility of tilting backwards?
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jetmech
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RE: Physics Of The Push-Back

Mon Mar 11, 2013 5:51 pm

Quoting faro (Thread starter):
In general, what are the weight/co-efficient of friction/power criteria required in the push-back equation?

According to page 78 and page 2 (section 5-8-0) respectively of the following;

http://www.boeing.com/commercial/airports/acaps/747_8.pdf
http://www.airbus.com/fileadmin/medi...ata/AC/Airbus-AC_A380_20121101.pdf

The draw bar pull for the 747-8 and A380-800 at MTOW on level ground with no engines running is approximately 17.5 tonnes and 25 tonnes respectively. The graphs are a little tricky to use, but I think I have it correct. Both documents give the tug mass required for various coefficients of friction.

Regards, JetMech

[Edited 2013-03-11 10:54:11]
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jetmatt777
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RE: Physics Of The Push-Back

Wed Mar 13, 2013 1:11 pm

Quoting vikkyvik (Reply 36):
Out of curiosity, do the pilots ever apply any braking to help slow the airplane during pushback? Is this inadvisable due to the possibility of tilting backwards?

No. The pushback is rated for the a/c it is pushing and is capable of stopping the a/c under it's own power.

It would be hard to coordinate brake application between the driver and the pilot. If the pilot applied more power than the pushback driver, the airplane would stop very quickly, and the pushback would continue pushing the airplane (or even ram into the towbar enough to cause damage). When slowing down, the pushback needs to have all of the leverage in the stopping process. The tension is in the pin on the pushback hitch, the airplanes momentum on the hitch is dragging the pushback during the braking process, but the pushback holds the leverage in slowing everything down.
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rwessel
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RE: Physics Of The Push-Back

Thu Mar 14, 2013 4:14 am

Quoting jetmatt777 (Reply 38):
It would be hard to coordinate brake application between the driver and the pilot. If the pilot applied more power than the pushback driver, the airplane would stop very quickly, and the pushback would continue pushing the airplane (or even ram into the towbar enough to cause damage).

Only if the difference in braking is so large that the deceleration of the aircraft from the aircraft's brakes exceeded the deceleration of the tug along from the tugs brakes. The obvious place that would bite is if the aircraft applied brakes and the tug did not, but even a small amount of braking on the tug (ignoring traction issues), should decelerate just the tug very quickly (big tires, big brakes).
 
FlyDeltaJets
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RE: Physics Of The Push-Back

Sun Mar 17, 2013 5:14 am

Quoting vikkyvik (Reply 36):
Out of curiosity, do the pilots ever apply any braking to help slow the airplane during pushback? Is this inadvisable due to the possibility of tilting backwards?

In an emergency where the driver is not able to stop then the crew would be asked to apply the brakes. I personally experienced this while towing a SA plane a while back. The towbar disconnected from the aircraft while towing and I had to have the crew apply the brakes.
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RE: Physics Of The Push-Back

Sun Mar 17, 2013 6:01 am

Quoting FlyDeltaJets (Reply 40):
The towbar disconnected from the aircraft while towing and I had to have the crew apply the brakes.

True.....under conditions that the towbar is NOT connected to the pushback truck, can the Aircraft brakes be used to prevent a runaway aircraft situation.
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