Towing aircraft on airfields

[A A Gruntpuddock]

As someone interested but with no involvement in aviation, I was thinking of collisions whilst taxying, aircraft getting onto the wrong runway, fuel wasted in getting to & from the runway, etc.

Why not have the planes towed out all the way and just start engines a few minutes before takeoff & shut down after leaving the runway? Power for radios, lighting and so forth could be provided via a link from the towing vehicle.

Teams of dedicated tugs could be equipped with GPS so that both they and controllers know exactly where they are.

Possibility of errors would be less anyway because, unlike pilots, the drivers would be familiar with the layout and procedures of only one airport as well as any recent changes thereof.

A second crew member could be watching the wings to ensure that they miss parked aircraft, vehicles, etc.

I expect that accidents and fuel costs would both be minimised, especially in poor visibility.

This is not happening so where am I going wrong?

[HEATHROW DIRECTOR]

I'm afraid you are a little late as that idea has been thought of before. The chaos which would ensue at a busy airport would not be justifiable. ATC know where aircraft and vehicles are, either by looking out of the window or by using ground rada; they do not need GPS.

There are sometimes not enough tugs to do the job intended for them and to have sufficient vehicles to service movements at a busy airfield would be very costly.

[joy ride]

There is talk, and even a test last year, of an airliner's wheels driven by electric motors for taxiing. Obvious drawback: weight of motors and extra battery and charging equipment, but "might" one day be practicable.

[Mister.E]

As an example after a lengthy tow to a position just short of the runway go to start and a start valve fails or an aircraft that has the APU inop and would require an airstart or two on some types so would involve a lot of equipment at a remote point.

[N707ZS]

Who pays for the tug and fuel and the two extra wages of the driver and observer.

[A A Gruntpuddock]

OK, thanks for all the replies.

I saw a shot of an airport and a plane was being towed well away from the buildings.

Just wondered why it wasn't more prevalent.

[Crazy Voyager]

Also enginges need a period of running before you can take off, this is sometimes an issue since a lot of airlines today taxi single engined and they sometimes misstime their enginge start. The result is that your number 1 at the hold might not be ready to depart and if you don't have the option of moving other traffic via intersections or other taxiways you're stuck until number 1 can get going.

[Phileas Fogg]

Aircraft will often taxi in, after landing, with engine(s) shut down to save on fuel but taxying out before starting/testing/warming up engines is not really advisable.

[HEATHROW DIRECTOR]

<<I saw a shot of an airport and a plane was being towed well away from the buildings.>>

At busy airfields that happens a great deal. Maintenance areas are often some distance from the parking gates so aircraft are towed between the two when maintenance is required or to free stands for other aircraft.

[Una Due Tfc]

You wouldn't be able to taxi very fast either. Some tugs weigh several tonnes, they are snails. Also stopping can be a bit of an issue. Tow bars snapping is a very common occurance. It's extremely bad for the nose gear to tow the aircraft with a full load

[Cough]

Phileas - Yet it IS routinely done!

[Phileas Fogg]

It's extremely bad for the nose gear to tow the aircraft with a full load

Genuine question,

So if towing a fully laden aircraft in a forward direction is damaging to the airframe then surely pushing a fully laden aircraft back off stand is similarly as damaging, is it not?

[Donkey497]

Nope, the internal aircraft structure around the nose wheel is set up to absorb the shock loads of landing where the nose leg is pushed backwards from the friction of the initially stationary tyre on the runway. As the wheel starts to rotate until it is up to speed, the force on the nose leg will decrease. At this initial contact the force on the nose leg is in the same direction as that of the act of pushback.


Taking the other condition of towing, the force on the leg is in entirely the opposite direction and instead of trying to force the leg towards the tail, you end up trying to tear it out through the nose of the aircraft and there just isn't the same amount of structure forward of the nosewheel to distribute the same magnitude of forces, hence when aircraft are towed forwards, they tend to be as empty as possible, distances are kept to a minimum and speed is kept low.

[HEATHROW DIRECTOR]

Donkey497. I accept what you say as I am not an engineer. However, when I worked at Heathrow BA would frequently tow full aircraft to remote holding areas whilst they waited for their departure slot and to leave the gate open for arriving traffic. Whether it still goes on I do not know.

[Burnie5204]

Yeah, short term from one side of the apron to another infrequently (for each individual airframe) at one airport it wouldnt be an issue.

Towing every aircraft to and from the runway and back with full loads at every airport puts that strain on the system for a lot longer and so the fatigue builds up faster.


Its like how aircraft components are lifed in hours. If you only fly one short domestic sector it'll take a lot longer to get to the service interval than if the aircraft flies a transatlantic sector.

[HEATHROW DIRECTOR]

Burnie... BA used to tow them from the stands to rwy 23 to hold.

[Burnie5204]

You do surprise me.

Though I suppose if they're their aircraft they can do what they want. They've obviously decided to accept the increased fatiguing by long distance towing fully loaded aircraft.

[awblain]

Aircraft are indeed designed to be pushed back by the nose wheel, and not pulled along by it.

Having said that, very large aircraft are picked up by the nose wheel and shuffled around Frankfurt all the time; even when empty, they're not that light.

If there was a big commercial incentive, then a "lift the main gear" tug could be used to trundle about to and from the runway. However, it's not likely to be worth the bother unless there's a very long taxi or queue. With fuel consumption on the ground no more than 10% of that in the air, it takes a lot of time to pay for a new class of tugs. Also, if there's a problem starting, then you're stuck miles from the gate in a line of aircraft in need of help.

[PAXboy]

A A Gruntpuddock If you search the forums on variations of the topic, you will find various threads. If it was commercially viable - it would have happened.

[Out Of Trim]

Some Tugs Waugh several tonnes...

Yes Indeed, some can weigh around 70 tonnes! And the burn a lot of fuel..

[SloppyJoe]

picked up by the nose wheel and shuffled around Frankfurt all the time; even when empty, they're not that light.

If there was a big commercial incentive, then a "lift the main gear" tug

Lifting a nose wheel is not the same as lifting a main wheel. Due to the centre of gravity almost over the main wheels it would mean having to lift almost the whole weight of the aircraft, about 233,000kg for an A330. Lifting the nose is a a couple of 1000kgs I would have thought.

[A A Gruntpuddock]

I would have thought that the forces on the nosewheel when towed are caused by wind, rolling resistance of the tyres and inertia.

Although the rolling resistance will increase with the weight and number of wheels, inertia potentially creates the greatest force.

If acceleration/ braking is regulated then the inertia loads could be controlled to within safe limits, so the effect of aircraft weight is diminished?

Or have I got this wrong as well?

[joy ride]

The greater the plane's mass the greater the inertia, then as you say, rolling resistance (tyres), friction (bearings and ground surface), gradient, wind etc. all add to the task. A very smooth and steady acceleration from standstill to taxi speed will certainly reduce strain whatever is being towed.