Pushback tug

[multi_engined]

*I wasn't quite sure where to post this question, so if it is in the incorrect forum please move it to the correct*

I have always wondered but never got a real answer to this...

How does such a small pushback tug have the power or force to push back a 747-400? They are tiny, a 747 is huge... you would think that it wouldn't be able to move it considering the weight difference also..

How is it physically done? I think it is amazing!

Thank you for any replies!

[18-Wheeler]

Nah it's not that hard.
I'll make a guess here - A big jet makes about 1,000 pounds of thrust at idle. So that's a total of 4,000 pounds.
A 747 will creep away at idle power at most weights with the brakes off.

So for a tug to move a 747 it'd need to make at least 4,000 lbs push - We'll make that the minimum.

If you make the tyres say 4' in diameter then the you need a torque of 8,000 ft-lbs at the axles. If you have a large diesel of say 400 ft-lbs (that's a small one) then with only 20:1 reduction gearing you'll get that torque. Well there's losses but that's close enough.
So there you go - With only a small engine it's possible to push a very large aeroplane around. Not quickly, but you get the idea I hope.

[multi_engined]

Thank you for the reply!

However when the aircraft is pushed back (say 747) the engines would be producing zero thrust wouldn't they? I thought the engine initiation start sequence is commenced at the end of pushback, normally the earliest you hear the engines beginning to spool is when the tug is turning the aircraft onto the straight taxyway markings?.. So if this was true the tug would require a lot less torque to move such a large airliner?..

[Hold West]

Thank you for the reply!

However when the aircraft is pushed back (say 747) the engines would be producing zero thrust wouldn't they? I thought the engine initiation start sequence is commenced at the end of pushback, normally the earliest you hear the engines beginning to spool is when the tug is turning the aircraft onto the straight taxyway markings?.. So if this was true the tug would require a lot less torque to move such a large airliner?..

Multi, the bit with the engines was just to show you how he guesstimated the force required to move the aircraft. The tug generates the force required to move the aircraft back, and the low gearing of the tug tranlates the engine torque into the power equired. That's why the tug and aircraft will be moving slowly, yet the tug's engine is turning relatively high rpms.

[18-Wheeler]

Multi, the bit with the engines was just to show you how he guesstimated the force required to move the aircraft. The tug generates the force required to move the aircraft back, and the low gearing of the tug tranlates the engine torque into the power equired. That's why the tug and aircraft will be moving slowly, yet the tug's engine is turning relatively high rpms.


Yep, I was just trying to make an educated guess as to how much force is needed, and then from that how much torque.
Also note that with the small tugs, we cannot start more than two engines (this varies though) as the tug is not strong enough to push the aeroplane back against the idle thrust.
I believe the big tugs are in the order of 70 tonnes odd.

[Bolty McBolt]

The posts on this thread have described with the right gear ratios its easy to move a large aircraft. I have seen a 747 nudged back to its correct parking position after over shooting its mark on taxi in with a baggage tug BUT the aircraft brakes were used to halt its movement.

As my physics teacher would say, "give me a big enough lever and I can move the world"

Your large aircraft "tow bar" style tug weighs in the vicinity of 50 - 60 tons with one or two turbo super charged diesel engines powering it.

The brakes are massive, usually borrowed from large earth moving/mining equipment.

I haven't had it confirmed yet but there is talk of a 80 ton tug required for the A380 to provide the stopping power for an max weight push back....

the weight requirement becomes redundant with the nose lift type tug :ok:

[PAXboy]

I wonder if the perception of the 744 as heavy is heightened by the fact that it is large? Yes, it is heavy of course, but it looks heavier than it is!!! Because the wings cover such a width, then it makes the whole machine look that much bigger and, by extension, heavier. There are places in it's interior that are just empty space. Although less so in 'Y', of course. :p

[Aloon]

A tug that can push an a/c doesn't neccisarily mean it can tow it.... I know it's not the question in hand... but slightly relevant...

There are minnima on weight of tugs due to stopping disstances, the ability to handle the weight of the a/c under braking.. Though not with nose gear lift up tugs...... Towbars have shear pins to protect against over stressing the nose gear, but this can cause more hazzard to ground crew when it does shear!

Engines running can make a hell of a difference! Especially on a slight incline... Wet weather also...

A heavy yet rolling object is relatively easy to push..... Push a car on the flat... easy! Uphill and you're buggered! You get the idea!

[Intruder]

Besides raw power or torque, the tug requires simple weight to ensure enough traction between the tires and the ground. Besides the big diesel engine, you'll likely find a lot of lead ballast under the bonnet (and boot).