Towing to runway

[jabird]

Not so long ago, VS trialled a scheme whereby aircraft were towed to the runway, with engines being started at t-x (x=time needed to get going). I believe the trial was at EGKK.

Given that one stat quoted for a return for passenger carrying airships is that they could "fly to New York on the same amount of fuel a 747 needs to taxi to the runway (# of seats not quoted), is there not a clear potential for fuel saving using this technique?

Assuming the tow-truck cost £100 per hour and is used for an extra 15 mins, would this not equate to one seat's worth of PSC at EGLL?

Now I would never want to denigrate the job done by the many pilots on this forum, but until the take-off roll starts, are you not just driving a very large, very heavy and thirsty but slow bus?

So what were the reasons for it being scrapped? Did it cause complications in the take-off queue? Savings less than expected, or a safety issue?

[HEATHROW DIRECTOR]

Having worked at a major airport I was suprised when this idea was first mentioned. To have many aircraft being towed around an airfield could cause major problems and have a bearing on the tactical decisions of the Tower controllers organising the departure sequence. E.g. Some aircraft may have to wait for a few minutes at the holding point whilst others can be cleared on to the runway immediately. Exactly what each aircraft may have to do cannot be easily determined until they are close to the runway. And that's just the start!

[RB311]

About 3 weeks ago I heard an interview on the radio with a head engineer at Easyjet who was explaining a new scheme to fit electric motors to the main gear wheels and power them by the APU. Thus the aircraft would electically motor itself to the holding point, then fire up the engines and head off.

Not sure how this would work on slippery surfaces, or how you would start the engines remotely without ground supervision. But I am sure they've worked that all out.

[WHBM]

Originally Posted by RB311

a new scheme to fit electric motors to the main gear wheels and power them by the APU.

Anyone who thinks the APU could generate enough current to power electric motors to move a fully loaded aircraft at taxy speed needs to go back to GCSE Physics class. Look at the size (and weight and complexity) of engine, alternator, heavy cabling and motors that a diesel railway locomotive needs, for maybe 2-3 times the load but with much less rolling resistance on steel rails.

A bit like motors to spin up the wheels prior to touchdown. That one comes around about once every 20 years or so.

[118.70]

How about a fuel-cell ?

DLR Press Portal - Press Releases - DLR at the Singapore Airshow

Aren't jet engines used for taxiing incredibly inefficient ? And isn't poor fuel burning at low power the cause of the jet-engine smell around airports ?

[No RYR for me]

Have a look at this video:

VIDEO: L-3 and Lufthansa get moving with e-taxi demonstrator

[Invicta DC4]

I seem to recall this subject being discussed before when Virgin first proposed it.

The issue I understand was that it may cut down on engine life if the time from start up to TO power is relatively short. Also, less time to identify any potential issues with the engines, but as I said, that's how I recall it.

[redED]

or how you would start the engines remotely without ground supervision

Single engine taxi resulting in starting an engine remotely without ground supervision is very common.

[nigel osborne]

Jaibird,

Re towing to runway.

I remember one of the reasons it was put to bed was that it caused big delays.The taxing speed was low and caused big queues of planes and delays.

I would imagine the pilots then have to start up on the runway and do their checks there causing some more delays.

They are trialing electric motors on planes for a while now,however problem up to now has been increased weight and strain on undercarriage.

Sure this will be the way forward in the future though.

Nigel

[PAXboy]

I have seen this subject debated several times in various forums here. The main points against are (as I recall):

• If the tug is not available when the a/c is ready for push - the airport gets the blame. If they miss their slot?
• The numbers of tugs needed for peak time would be very high - because you have to allow for the long return trip to the terminal area to collect the next. Thus you would have to have many tugs sitting around in low periods.
• How does the tug get back to the terimal? taxiways are one way and unless there is an easy route back, a new tug track would have to be laid and some places would not have space for that.
• So the tug gets the a/c to the start up bay: Should it wait until the start is complete, or just go? If the start cannot be completed, how do you get the a/c back to the terminal? It would have to go onto the runway and off at the first exit. Failed starts are few these days but the busier the field, the greater number of problems in a year. Anyone fancy a failed start blocking one of LHRs two?

The next big discussion point has been: Wear and tear on the a/c.

• The nose wheel is designed to handle the weight to the tug or to hold the weight on a lift up tug. Manufacturers have said (I read) that they are not sure about the extra strain on regular towing, much more than the towing for maintenance. So the nose gear might have to be strengthened = weight.
• The idea of electric motors (donkey engine) attached to the nose gear was (I read) tried but the weight of the required powerful motor meant that it was not cost effective to take it with you. So you have to have folks disconnecting and collecting the donkey motors at the start point and returning them to the terminal.
• So - what speed limits would there be for towing or the donkey engine?
• Will you have enough of the right types of donkey motors for the number of departures and types you have that day?

You would just have to start in the normal way if:

• No tug available
• No tug staff available
• No donkey available
• No staff available to mount/remove donkey

Accordingly, depending on which way you look at it, this method could work better for small regional fields with not many rotations and small delays not a problem. Or, the big hubs, because there are more movements and so the potential savings are great. But, of course, the potential delays are greater.

[CabinCrewe]

Imagine all the engine faults detected at startup, but whilst sitting at the threshold on the main runway....
The E-taxi demo would seem to knock any theories of non workable physics.

[jabird]

Thanks for the diverse replies. Fairly conclusive then. I'm sure it was well discussed at the time, but it is hard enough to search for specific airlines or airports, picking a general term like this which probably went under a different title not so easy. If anyone has links I'd always be keen to read.

Thanks.

J.

[MidlandDeltic]

Anyone who thinks the APU could generate enough current to power electric motors to move a fully loaded aircraft at taxy speed needs to go back to GCSE Physics class. Look at the size (and weight and complexity) of engine, alternator, heavy cabling and motors that a diesel railway locomotive needs, for maybe 2-3 times the load but with much less rolling resistance on steel rails.

A diesel locomotive has to take up to 3000 tonnes at speeds of 60mph+, or maybe 400 tonnes at 100mph. What does a 737 weigh - 70 tonnes? A class 153 diesel railcar (capable of doing 75mph) weighs 40 odd tonnes and has a small, bus type engine producing 285hp.

Much of the complicated electronics in locomotives are required due to the needs of maximising power at rail over a large speed range - which would not apply in the scenario. In any event, I believe Airbus have been doing trials of this with an A320, which rather proves the point.

MD

[Peter47]

Towing might make sense for wide bodies. The tug driver could also be qualified as an engineer (standard pracitice for some US airlines I believe). Crew costs are the same whatever the size of a/c. You could tow with the APU running or without but the latter would require removing air hoses & power lines so would be rather a hassle. The a/c would still be under the command of the flightcrew. There could be additional work for ATC unless extra roads were built - airports with close parrallels might require underpasses or additional ground controllers. (What would happen at FRA?)

Boeing (I'm not certain about Airbus) are not happy about stresses on nosegears. New a/c could come with strengthened gear as standard. The cost of strengthening and recertifying existing nosegear would doubtless be prohibitive.

I reckon that the savings would be worthwhile at airports with long taxis such as JFK, FRA & LHR.

Electric towing makes more sense for lighter aircraft. You might need to fit a larger APU but that could probbaly be justified. An article in Flight this week quotes savings of 2 - 4%. This would obviously vary by route. For a sector such as LHR - AMS it is possible to spend as long taxiing as flying so the savings are obvious. I've flown between JFK & DCA where the published sector time was three times the actual flight time. Much of this is down to very long taxi times at JFK.

The flight article notes that the nose wheel only carries c10% of the mass of an a/c. This raises the question of why not fit motors to the main gear - most cars are rear wheel drive, or are there more serious technical issues with this?