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I've been looking at some proposals to try to reduce the time that an airliner's main engines are run at idle or near idle when taxiing to and from runways--a situation that uses non-trivial amounts of fuel and greatly increases local emissions.

I've seen proposals for fleets of tugs and onboard wheel motors, but I wonder if there might not be a simpler way. Why not use a continuously moving cable system, such as that used by San Francisco cable cars? Simply run a cable in a trench in the center of a taxiway, and have a means of attaching to and detaching from this cable as a way to tow aircraft down a taxiway and to the end of the runway. I don't know the details, but I do know cable car designers figured it out more than a century ago, so surely it can be done.

Similarly, one might also utilize a roundhouse system to rotate aircraft to the proper position to pick up the cable. Again, roundhouses have been used with trains for well over a hundred years, and locomotives are quite heavy. Clearly one would need ways to slow the cable speed at termini, but this is done on high-speed ski lifts. Some means of "clutching" to buffer a sudden application of force would also be in order--but again, think of the cable cars, or steam catapults/deceleration cables on aircraft carriers. It's been done. Surely it can be done in this context.

What's not to like? The benefits of reduced fuel consumption and emissions are obvious, and it beats a flotilla of tugs, by eliminating empty return trips, logistical hassle, and large numbers of staff. Also likely beats an onboard motor system, on weight, cost, and complexity. There are tons of reasons to do it.

Seems obvious to me. What are the arguments against it?

Jamiec
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lapsedwondering
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    Not for taxiing, but there are similar systems in place for launching planes from aircraft carriers. These are obviously much lighter aircraft, not jetliners, and it's not a continuously moving cable so much as a steam (or in one case electromagnetically) driven shuttle plate embedded in the deck of the carrier. But it is planes being dragged by a hook underneath, in this case fast enough to launch off a short deck. – Darrel Hoffman Jul 20 '20 at 14:41
  • In addition to what Darrel mentioned about aircraft carrier fixed-wing aircraft, recreational and commercial gliders as well as recreational and commercial drones are sometimes launched via a cable system. – End Anti-Semitic Hate Jul 20 '20 at 17:39
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    related: https://aviation.stackexchange.com/q/24479/3159 – BowlOfRed Jul 20 '20 at 19:03
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    Generally, cable cars are very simple systems. They are not complex networks, they are usually in concept a big oval. Airports are not, generally, big ovals - they can have immense amounts of crisscrossing taxiways. Yes, you can have "cars" switch over between two circuits (see: multi-section cable cars), but at a simple X intersection, you have 12 possible paths an airplane might take and you'd need cables for all of those. This is a mechanical nightmare. –  Jul 20 '20 at 21:09
  • If a tug breaks, another will take its place. What happens if your system breaks? Will all planes ground to a halt? The current system is very simple and flexible. Not to mention once you've committed to a system, it's very expensive to replace. – Mast Jul 21 '20 at 06:42
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    Related: https://aviation.stackexchange.com/q/9101/1626 – gerrit Jul 21 '20 at 10:24
  • I was in a museum in the Netherlands in 2017 and saw an exhibition called I think "100 years of Schiphol" on the KLM airport. One of the exhibits was about sustainability efforts and included a video about a hypothetical system for landing planes which did not involve wheels. Instead, as a plane neared the runway, its speed was matched by a sort of "sled" on the ground running on a kind of rail. The video showed the plane somehow docking to this sled to complete its landing. I did some web searches, but unfortunately could not this video or any mention of such a system online. – Michael Jul 21 '20 at 18:04
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    @Michael That was luchtvaartmuseum Aviodrome with 100 jaar Schiphol. I don't think the media you're looking for are on-line. – Mast Jul 21 '20 at 20:21
  • Cable cars are some of the least energy efficient forms of transportation. Less than 2% of the energy used to power the system propels the vehicle. The rest is lost to moving the cable, turning the pulleys. – gwally Jul 22 '20 at 12:01

5 Answers5

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Way to go on thinking outside of the box. Actually, it might work. Although the logistics of it are above my pay-grade. Maybe, some smart group out there could make it work.

But, your question doesn’t ask for ways to make it works. It specifically asked for reasons it might not works. So, here it goes:

  1. cable car feasibility over all does not have a great track record. If we were to make a Ben Franklin style decision graph of cable car systems that worked versus those that proved ineffective, Wikipedia lists a lot more defunct systems than it does working systems. All of which run on tracks.
  2. If you get more than two runways at the same airport, the taxiway system become increasingly complex. An aircraft would need an alternate means of locomotion when switching from one runway/taxiway to another. The use of one continuous loop cable would be rendered impossible.
  3. As Ron Beyer has pointed out, departing aircraft would still need time to warm up their engines prior to take off. Currently, that time is spent taxiing. Arriving aircraft would still need to power climate control systems. It would be a cheaper and more efficient use of time and resources to design a system that eliminates wait time between start up and takeoff. Keeping the traffic flow moving at all times without having to retrofit airports or entire airline fleets would benefit all.
  4. Most cable car systems operate with cable cars of lighter weight and similar mass to each other compared to commercial aircraft. How would airports be able to account for the sheer mass differences between a single pilot C208 after a long cross country versus an A380 taking off at MTOW.
  5. Power failure.
  6. Power in night time IMC with four or five airliners about to break out at minimums for GPS approaches on parallel runways.
  7. In the case of a constantly moving cable, an aircraft the size of an A320 could cause a lot of damage if it were not able to unhook itself from the cable at the appropriate time,
  8. According to Wikipedia, the efficiency of a large cable car system is questionable. “Approximately 95% of the tractive effort in the San Francisco system is expended in simply moving the four cables at 9.5 miles per hour.“
Dean F.
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  • Thanks for the good input! OK, so I am definitely liking the scheme a lot less now. I am not a pilot, but it seems there is some thought that maybe 5 min of warmup is sufficient, rather than 25-40. If so, what about this? https://www.taxibot-international.com/ . Looks like it's been built and is in limited use. Doesn't the APU provide sufficient power for climate control, etc., such that main engines wouldn't be needed? – lapsedwondering Jul 20 '20 at 08:10
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    @lapsedwondering - I believe the APU does provide enough power for climate control. I am not sure what the cost or pollutions savings would be on running an APU vs an engine at idle. I would also check with someone more knowledgeable on engine warm up and cool down time on an airliner. Five minutes would be the absolute barest of minimums I would try for a piston driven GA aircraft. Generally, start-up, clearance delivery, avionics programming, taxi, run-up, and time at the runway hold short line totals about 15 minutes. 10 minutes at the quickest. – Dean F. Jul 20 '20 at 14:03
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    The minimum warm up time for a big jet engine is more like 20 minutes, There is also a minimum cool-down time running at idle. If you just stop a hot engine, the heat rises and thermal expansion bends the rotors because the top of the compressors and turbines are hotter than the bottom. That can be bad enough to make it impossible to restart the engine without exceeding the vibration limits. Some jet engines have small electric motors which keep the rotors turning slowly to avoid this, but it's an added complication and another potential failure condition. – alephzero Jul 20 '20 at 15:16
  • +1 Also, taking control of taxiing away from the pilots would mean they couldn't stop short as happened in this incident: http://www.avherald.com/h?article=4d8b926a&opt=0 – Dave Gremlin Jul 20 '20 at 16:14
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    Also, what happens when it snows? The cable has to run in a trench, which has to be narrow enough so that tires will not fall into it. Get a good snowstorm, and it fills up with snow, which likely turns to ice. At best it greatly increases the energy needed to move the cable; at worse it locks up the system and you have a heck of a job cleaning it out. (Note that per the Wikipedia link, the only still operating cable car system is in San Francisco, where it basically doesn't snow. It's also primarily a tourist attraction...) – jamesqf Jul 20 '20 at 16:22
  • @jamesqf - You are entirely right about the snow. Although, the San Fran system is not the last operating cable car. It is the last one that is operated manually for nostalgic purposes for tourist. I think San Fran also had a unique situation with the hilly terrain back when horsepower was limited to as little as one – Dean F. Jul 20 '20 at 16:28
  • @jamesqf if we're retrofitting planes with hook spots, we might as well retrofit them to have four wheel assemblies instead of three. Although I do admit it adds weight... – John Dvorak Jul 20 '20 at 19:18
  • Point 4 is fairly easy: Use a synchronous machine as a drive, it'll self-regulate to the electrical input frequency divided by number of pole pairs. As long as it's beefy enough, it'll just have a higher power draw when faced with the A380, and it won't shoot the Cessna into space. (And, frankly, with a VVVF converter and some basic regulating circuitry, even on an asynchronous machine, this is not a tough challenge. Freight train engines can work with our without several thousand tons of load, too.) –  Jul 20 '20 at 21:13
  • @John Dvorak: We don't have to think about just the commercial airplanes, but about all the maintenance vehicles running around. Not to mention all the small GA planes that want to use the airport, too. And military: quite a number of fighter jets use my local commercial field. – jamesqf Jul 20 '20 at 21:55
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    "cable car feasibility over all does not have a great track record." --> I suppose no pun intended? – chux - Reinstate Monica Jul 21 '20 at 02:26
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    @DeanF. the modern age hasn’t provided anything that can handle those hills. Adhesion-rail streetcars are right out. Trolley buses can’t quite climb the grades either, although maybe they now can with flux vector AC drives. They’ve had Twin Buses, diesels with 2 engines. It’s not an easy problem. – Harper - Reinstate Monica Jul 21 '20 at 02:32
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    @chux-ReinstateMonica - maybe. – Dean F. Jul 21 '20 at 03:40
  • That #8 is what kills it. A tug car would be at least 30-50% efficient and wouldn't be limited to pre-set routes either. – Perkins Jul 21 '20 at 21:30
  • @Harper - Reinstate Monica: Actually there are a number of systems that can handle steeper grades. Per Google, the steepest grade in the SF cable car system is a mere 21%. Cog railway systems (common in Switzerland) regularly handle more than that. The steepest does a 45% grade: https://en.wikipedia.org/wiki/Pilatus_Railway – jamesqf Jul 22 '20 at 04:29
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Practically this would not work due to the sheer size of the cable you would need. A fully loaded A380 is 1.25 million pounds. A B787 is .5 million pounds and a B737 is about 150,000 pounds. On a busy day at a large airport you may have 20 airplanes or more in line, assuming a mix of airplanes you'd have somewhere around 5 million pounds of airplanes to tow around. I don't have the figures, but the cable would have to be very thick to handle that load. Thick cables are heavy to begin with, and you'd have miles and miles of it to move, which takes a lot of energy. Curving large steel cables has to be done gradually, and needs substantial mechanisms, all of which would have to be maintained.

It would be expensive to build and maintain, and likely cost more energy to run than it would save.

GdD
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    “More energy to run than would save” that’s highly unlikely. The APU on an A320 could run San Francisco’s entire cable car system along with OAK’s cable car too. – Harper - Reinstate Monica Jul 20 '20 at 13:12
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    Figures: 5 million pounds to tow around with a rolling resistance of 0.015 would require around 330 kN. That's almost in the range of a 52 mm rope. Not completely out of the realm of possible solutions. – Arsenal Jul 20 '20 at 14:07
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    So about 2 inches @Arsenal? You'd have to over-spec it somewhat I would assume, in any case that's a chunky old cable! – GdD Jul 20 '20 at 14:23
  • @GdD I guess the hooking and unhooking would put quite a bit of extra stress to it, as that won't go very smoothly - not quite sure how that would be solved at all. – Arsenal Jul 20 '20 at 14:32
  • I'm not sure how that would work, frankly @Arsenal. There would have to be a way to put airplanes on it and take them off without stopping or nobody would get anywhere. They have mechanisms for chairlifts I suppose, so something like that I would guess. Having to pull a million pound airplane to taxi speed is no small force. – GdD Jul 20 '20 at 14:56
  • @Harper-ReinstateMonica Neither of those cable car systems haul 20 airliners at once – Asteroids With Wings Jul 20 '20 at 16:17
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    @AsteroidsWithWings Dude, they’re trains. They are a type of “heavy” aviators do not grasp. What’s more, they’re going up a frickin’ 60% grade, at max speed, because there’s only one speed lol. Tow force of an A380 on the flat vs a fully loaded Powell-Mason car at ruling grade... tough call. Good one for Mythbusters! It helps that while some cars are going up, others are going down. – Harper - Reinstate Monica Jul 21 '20 at 02:35
  • @Arsenal The size of the rope alone is definitely not the problem, I've handled electrical cables thicker than that. – Mast Jul 21 '20 at 07:01
  • I'm not saying it can't be done @Harper-ReinstateMonica, simply expensive and impractical for the purpose. – GdD Jul 21 '20 at 08:32
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    @GdD “Yeah, near every city, we’ll build an ultra-high-security area, that is itself the size of a city. This will house 2-1/2 mile long, 200’ wide strips of the thickest concrete ever laid. Machines the weight of a Big Boy loco (with turbines that burn fuel inside the turbine) will use these as a dragstrip to get to 150 mph in half their length, leap into the air itself, and travel halfway around the world with onboard fuel only, in, the, air, in hours. We will tow these leviathans to the end of the dragstrip with a cable system.” Hmm, that cable system sounds very impractical. – Harper - Reinstate Monica Jul 21 '20 at 14:29
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    I detect sarcasm captain! – GdD Jul 21 '20 at 18:27
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Cable cars, trains, and ski lifts essentially live their lives on the track or line. The repositioning and shuffling that must happen on an airport, especially pulling into and out of arrival/departure gates would make such limited track systems a nightmare. There would be a need for aircraft to move around under their own power or be towed by a tug, at least in non-movement areas, and if they already have their engines started or have a tow vehicle, it makes no sense to complicate the process by adding a track for only part of the journey. Additionally, there are plenty of airports where taxiing traffic may need to go both directions on a given strip of tarmac, yielding to traffic as needed. You would need 2 cable tracks going in opposite directions and some way to connect your aircraft to the right one.

There are certain select aviation applications for a cable tow. For example, certain airports with a lot of glider activity have a glider winch which pulls the unpowered aircraft down the runway and releases the cable upon takeoff. Nevertheless, underground cable tracks on a large, busy airport would be too limiting to be worth the monetary investment and time spent in procedural development.

Aaron Holmes
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  • Hmmm. Well, the grabbing and ungrabbing seems to have been worked out: https://www.streetcar.org/wheels-motion/cable-cars-work/#:~:text=Cable%20Cars%20have%20no%20engine%20or%20motor%20on%20the%20cars%20themselves.&text=There%2C%20powerful%20electric%20motors%20(originally,that%20slot%20between%20the%20tracks). – lapsedwondering Jul 20 '20 at 04:39
  • I’m not saying the grabbing and ungrabbing would be a problem. I’m saying, street cars have side-by-side tracks for going in opposite directions. Sometimes a single taxiway will need to accommodate centerline traffic in either direction. Don’t get me wrong, I think your idea is very clever. You just asked for arguments against it, so I gave it some thought. I’m glad you joined the site. Hope to hear more from you in the future. – Aaron Holmes Jul 20 '20 at 05:53
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    No worries at all--I was asking for arguments against, and thank you for the thoughtful response. Mostly throwing out an idea, not at all identifying with it. I do see the problems posed by engaging and disengaging a continuously running cable, and then there are further challenges of dismissing whatever hook or boom system one might not want to take along on a flight. And yes, huge initial costs and logistics problems. – lapsedwondering Jul 20 '20 at 08:01
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    @AaronHolmes that’s not a problem, just have 2 cables to grab. I believe a section of SF’s cable car does this. Or you could just reverse the cable drive; that’s what OAK’s cable car does. – Harper - Reinstate Monica Jul 20 '20 at 13:14
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As many answers point out, one large problem is the cable itself, and how much of it you'd need to pull around to let different planes move in different ways as needed. (And how big/heavy it would have to be).

A real proposal is a similar idea, but with electric power instead of cable towing. https://news.okstate.edu/articles/engineering-architecture-technology/2020/osus_new_product_development_centers_collaboration_with_aircraft_towing_systems_leads_to_prototype_test_launch_in_spring.html (@mitchute posted this link as a link-only answer).

ATS’ innovative aircraft transport system includes an electric-powered underground railway system.

“With a fully installed ATS system, a pilot can land on a runway and move onto the taxiway, where he or she can align the aircraft’s nose wheel into an ATS tow dolly,” Taylor said. “Once the nose wheel is secured, a pilot can turn off the main engines and ATS safely moves the aircraft using its channel system to the appropriate gate. The reverse process is used when pilots are ready to take off.

“Aircraft are simply automatically transported from the gate to the taxiway and then the runway. Pilots then turn on their engines and drive off the pull car, move into position and take off.”

So more like a subway (where trains draw electrical power from a 3rd rail), I think: there are fixed routes, but they're just rails with a mechanism for a powered "tow dolly" to get electrical power for itself and use it on the spot. So it can easily use sustainably-generated electrical power without needing onboard storage for it.

The tow dolly itself might have normal wheels, and only need the rails for power, and/or traction to pull a big plane, IDK.

Any specialized equipment for coupling / decoupling from different rails at intersections can go in this tow dolly, not on the plane itself. (Although you could do the same thing with tow dollies pulled by a cable, with a grabber motor powered by onboard batteries? Since you probably want some smarts and other motors in your tow dolly, electricity is an advantage over purely mechanical pulling)

While it might be feasible to build a tow-cable system, there are clearly better alternatives that would give a very similar result. i.e. I don't think there are many disadvantages to electric tow dollies that you wouldn't also have with cable towing. So there are probably no cases where cable tow would actually be the best choice.

Tow-cable systems waste a lot of power on cable friction. Building efficient high-power electric motors is a solved problem.

Other answers and comments have pointed out large potential problems, especially turbine cooldown time: you have to leave them on for some time anyway, or somehow have them spinning to move air. But not all even have an electric motor that could do that if supplied with power.

Peter Cordes
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    Wow--I LOVE OSU's solution! I've actually been inventing up a storm in my little pea brain since I asked this question, and I came up with something very similar--electrified rails, towbarless tug/locomotive, turntables. I found a neat conceptual video they came up with a few years ago, on YouTube: https://www.youtube.com/watch?v=AOu-vBZWqHw. It's a very exciting concept, and I would love to see these guys succeed.Rail is almost certainly the way to go, for traction, low rolling resistance, weight accommodation, staying, well, on track, durability. – lapsedwondering Jul 22 '20 at 15:20
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Look closely at the landing decks of the more modern Aegis / Arleigh Burke destroyers. There is a slotted trackway from the landing deck leading into each of the two helicopter hangars.

Harper - Reinstate Monica
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  • Note, too, that the deck of a destroyer has significantly fewer source/destination combinations for an aircraft to travel between and significantly fewer aircraft types to deal with, making this more practical. – FreeMan Jul 22 '20 at 14:37