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Comedian/podcaster/QI elf Dan Schreiber asked on Twitter, as proxy for his 4-year old

if it would ever be possible to hook a plane (like a 737) onto the back of another plane, like a train carriage, and then have both take off? I know the answer is no, but just wanted to double check with you, Twitter

and continued

I knew about gliders...Just wondering about the Boeings...in case some one comes in with an XKCD-style answer

For the purposes of focusing our question, I'd like to ask:

can a jet airliner TOW (not piggyback / eat) another airliner from takeoff to landing?

It is pre-noted that this is not a good idea, voids the warranty, etc. However, laying out considerations / difficulties / show stoppers, with particular attention to modern flavors of the 737, or noting which if any airliners would be preferable tow-ers or tow-ees, will be appreciated.

Erin Anne
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  • Particularly related answer here https://aviation.stackexchange.com/a/8952/3146 – Erin Anne Mar 21 '22 at 03:47
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    An F-102 or F-106 has been towed by a long cable attached to a C-141 in a NASA test project. – quiet flyer Mar 21 '22 at 04:26
  • Another case of you COULD but why would you want to? – Romeo_4808N Mar 21 '22 at 16:39
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    @CarloFelicione it was originally asked by a 4 year old! Little wonder that it isn't a practical idea! – Erin Anne Mar 21 '22 at 20:37
  • @Paul this question is not about piggyback. were any of the early shuttle tests tows? – Erin Anne Mar 23 '22 at 20:59
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    @quietflyer I was trying to exclude carrier aircraft that entirely contain the carried aircraft. In practice that's rare and typically the carried aircraft has to be partially reassembled after. Eat just seemed like a fun way to put it. – Erin Anne Mar 25 '22 at 19:50

3 Answers3

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enter image description herePic source

Yes it should be possible for an airliner to tow another airliner. In WW2 gliders with paratroopers on board were towed to their drop zones, the technology has been around for a while.

Another interesting story can be found in this link, about capt. Robbie Risner using his F-86 Sabre to push his wing mate's F-86 into friendly airspace during the Korean war. A fighter jet pushing another fighter jet.

Risner told Logan to shut down his engine, now almost out of fuel. Then he gently inserted the upper lip of his air intake into the tailpipe of Logan’s F-86....Miraculously, Risner nudged Joe Logan’s F-86 all the way to Cho Do, maintaining an airspeed of 190 knots and enough altitude to stay out of range of automatic weapons.

One airliner pulling another one would indeed take structural preparation. The wing/body intersection is the strong point of the plane, dimensioned to absorb the forces from the jet engines which will now be provided by the tow ropes. If the tow ropes are attached at this point pitch moments will be produced into both the towed (nose up) and the towing (nose down) aeroplane, which receives an automatic trim correction for the engine thrust.

Issues to tackle:

  1. Stability of the towee. The towed plane will need to deal with interference and stability issues, and will need to have its flight controls powered. The B737 does have (very heavy) manual backup modes for the elevators & ailerons, the secondary flight controls require electrical/hydraulic powering. From the APU or from a RAT (which the B737 does not have). The batteries provide 60 mins of backup power, but powered flight controls are essential.

    Longitudinal stability should be OK, since the plane is built to deal with the thrust of the turbofan engines. Directional stability must be improved by leading the tow rope through a support fixed at the node wheel structure. The rope won't always pull straight ahead like the engines do, and this degree of freedom needs to be eliminated as much as possible.

    From FAA site linked below

  2. Wake turbulence. The towed plane will need to evade wake turbulence from the towing plane, as depicted above. So the towed aeroplane can take off first, and remain above the vortex field - or fly below the wake field as in this video. As @Neil_UK mentions in a comment:

    Wake turbulence is not so much a problem, as a feature, for sail-plane towing. Once reasonably competent at straightforward tow-launches, the next exercise is to 'box the tug', that is, flying in, under, over, left and right of the tug plane's wake turbulence. – Neil_UK

    And on the strength of the wake field, from the FAA site

    Weight, speed, wingspan, and shape of the generating aircraft's wing all govern the strength of the vortex. The vortex characteristics of any given aircraft can also be changed by extension of flaps or other wing configuring devices. However, the vortex strength from an aircraft increases proportionately to an increase in operating weight or a decrease in aircraft speed. Since the turbulence from a “dirty” aircraft configuration hastens wake decay, the greatest vortex strength occurs when the generating aircraft is HEAVY, CLEAN, and SLOW.

    So the towing plane can reduce the wake vortex by eliminating the payload, extending its flaps, and speed up to max climb speed (point 3. below)

  3. Optimum range. The towing plane needs to provide thrust to overcome drag of two planes. The towed plane has its own wings and provides its own lift, so we'll only need to consider increased drag on the towing plane.

    In the Breguet range equation $\frac{V}{sfc} \cdot \frac{L}{D} \cdot log \frac{W_i}{W_f}$ the towed/towing plane combination has twice drag D, and the corresponding speed for maximum range will be lower than for a single plane.

enter image description here

One thing is clear: it can be done, because it has been demonstrated already: the Eclipse project mentioned in @ErinAnne's comment, a Starlifter towing a Delta Dart on a 1000 ft rope. This report mentions how the attachment point on the Dart was constructed at the nose, how the towing took place successfully (the Dart pilot took his hands off of the controls), and how the wake turbulence introduced some shakiness so stay either below or above the wake field.

Koyovis
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    excellent point on the introduced moments and I also really appreciate the link to your answer on the 737's flight controls – Erin Anne Mar 21 '22 at 05:39
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    at some point in its development the huge Me321 glider had to be towed by 3 Bf110 (!) – user721108 Mar 21 '22 at 08:38
  • FYI I expect your answer will be the accepted one but I'm going to activate the bounty once that's allowed so the acceptance will be delayed a bit by the 2-day ask-to-bounty time and then letting that stand for a bit longer – Erin Anne Mar 21 '22 at 20:45
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    There was also a case of an F-4 Phantom pushing another F-4 during Vietnam using the tailhook: https://en.wikipedia.org/wiki/Pardo%27s_Push – Davidw Mar 22 '22 at 18:57
  • I tried to track down more info about the Eclipse C-141A-towing-F-106 demo and, interestingly, in that the C-141 took off first and the F-106 stayed below. Haven't found an actual report yet talking about their wake turbulence conclusions, but https://www.f-106deltadart.com/eclipse.htm goes the deepest into the wake turbulence evaluation that I've seen. https://archive.org/details/NIX-EC97-44357-13 shows a good video of one takeoff – Erin Anne Mar 24 '22 at 23:46
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    @ErinAnne Indeed, the F-106 stays way below the wake turbulence field, which is possible as well. The F-106 needs more speed than the tow plane to get airborne, when it lifts off it does so at a high angle of attack - below the turbulence field is best for it. – Koyovis Mar 24 '22 at 23:59
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Here's a quick take that I will attempt to evolve into a better, more-XKCD-style answer over time:

I believe it's possible, but planning is required.

Trying to do this with fully-loaded planes isn't a good idea; you are more likely to succeed with reduced fuel (perhaps no fuel in the towed plane) and cargo.

This gives you excess thrust available in the towing plane, and every pound less of lift saves some fraction of a pound in towing force (I'm having trouble finding 737-specific numbers right now but Lift / Drag ratios of 15 to 20 aren't uncommon for an airliner and would change over the members of the 737 family).

The towing plane will have to generate both the thrust it needs and the thrust the towed plane would need, plus some additional from the towing bridle not being perfectly straight and taut. For 737s specifically this gets a little iffy just based on the ratio between maximum takeoff weight (MTOW) and empty weight; I don't see any model of the 737 with that ratio as high as 2.0. Perhaps you'd need to reduce the weight of the towed aircraft even more than "empty" (i.e. by removing engines).

There is every reason to believe that the towed aircraft could be rigged in such a way to receive the force, because the engine pylons and wing boxes already have to resist that force and more. Presumably we will be towing at speeds slower than cruising speed to buy ourselves less overall drag and better Lift / Drag ratios. Rigging the towing aircraft may be more complicated; my first guess would be attaching cables to the wing box at the wing roots, but that's a guess.

It will help if the towed aircraft can remain under control to maintain a good following attitude, use additional lift controls like slats and flaps during appropriate phases of the flight, retract and extend the landing gear, etc. It is not clear to me at this time if a towed 737 would have to have a running engine to do so, or if an APU would suffice, or if newer models (or other aircraft like a 787) could run their controls entirely from an electrical connection from the towing aircraft (which could end up being one of the costliest things to engineer in this whole proposal).

I still don't see any reason to believe that particularly long runways should be necessary. Though a 737+737 combo will likely not be able to operate out of all the runways a single 737 would, part of the point of long runways is to be able to abort takeoffs on the ground and we'll assume that among the operating controls that the towed aircraft has, it has working brakes.

Erin Anne
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    Having done a bit of towing in my time I can attest that the towed aircraft will typically have a tendency to ‘lock out’ in any direction, i.e. the pilot of the towed aircraft must constantly make corrections to avoid going catastrophically out of alignment. Whether a 737 is agile enough to be able to do that is a question in itself. – Frog Mar 21 '22 at 06:23
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    How long would the tow line be for the tower plane to avoid the towing plane’s wake turbulence? And is there a runway long enough to deal with that? – StephenS Mar 21 '22 at 13:26
  • What about stability? One would imagine towing would inherently be more stable, but is that the case? E.g., can it start to swing from side to side (or up/down) because it wasn't designed for towing? E.g., at different speeds. – Peter Mortensen Mar 21 '22 at 15:19
  • @StephenS Wake turbulence will be very bad and there is nothing that you can do to avoid it. This has been extensively researched, read about it here: https://en.wikipedia.org/wiki/Minimum_interval_takeoff You will need a towing cable of several kilometres which is obviously not feasible. – Criminal_Affair_At_SO Mar 21 '22 at 15:54
  • @StephenS that's where all those circular runway proposals come into play! – FreeMan Mar 21 '22 at 16:23
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    There are runways out there which are over 4km long which were intended as fallback landing sites for the Space Shuttle program. Perhaps one of those would serve the job! – Ruadhan2300 Mar 21 '22 at 16:36
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    @mmomtchev: "and there is nothing that you can do to avoid it" - Sure there is, just have the towee fly above and behind or directly behind the tower rather than below and behind it. – Vikki Mar 21 '22 at 19:46
  • @mmomtchev I don't understand why you think that ancient B-52s demonstrating that they could absolutely take off through multiple others' wake turbulence while also flying through dense black smoke means that for two aircraft you'd need not to do that and instead use a towing cable kilometers long. – Erin Anne Mar 21 '22 at 20:41
  • B-52s need to be spaced at least 9 seconds apart - at 140 knots this is 630m. Once in the air, you will surely be flying faster than this. I don't know if the 737 handles turbulence much better then the B-52. – Criminal_Affair_At_SO Mar 21 '22 at 20:48
  • takeoff and landing are the most critical wake-turbulence phases. In flight you have more options for both avoiding the turbulence and the ground. – Erin Anne Mar 21 '22 at 21:22
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    A fully-fueled airliner needs to be able to take off on a single engine if one fails during takeoff. This indicates that every (2-engine) airliner has at least twice as much thrust as necessary to take off, fully-fueled (if only marginally); therefore, a fully-fueled airliner with two functioning engines should have enough thrust to fly itself plus pulling another airliner behind it. – Skyler Mar 22 '22 at 20:05
  • @Skyler that's an interesting point. I guess it'd vary aircraft-to-aircraft, but presumably engine loss on takeoff puts the engines into some kind of emergency operating regime? Or is it common for takeoff thrust to be maximum available on a large airliner? I know I've seen at least a couple aircraft where TO isn't quite all the way to the stop. – Erin Anne Mar 23 '22 at 02:45
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    @Ruadhan2300 white sands is over 10km! – Tim Mar 23 '22 at 09:21
  • @Skyler And what happens if an engine fails during takeoff while towing? – StephenS Mar 25 '22 at 13:29
  • @ErinAnne Two engine TO thrust is often well below max to save fuel. Crews will calculate the ideal thrust based on weight and runway length. If an engine fails, you push the other throttle to max. And at some airports, there are special “escape” routes that must be used because on the normal routes you’d hit terrain with single-engine climb rates. – StephenS Mar 25 '22 at 13:34
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    @StephenS I was only looking at whether it's physically possible, not whether it's a good idea. – Skyler Mar 25 '22 at 13:40
  • @StephenS presumably if an engine fails during your towing takeoff you release the towed aircraft and then continue with your typical emergency procedures. But Skyler's right that's it's not supposed to be a good idea, per the stated caveats in the question. – Erin Anne Mar 25 '22 at 19:54
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It is surely possible in theory, but not that much in practice - at least not without some serious custom engineering.

One big problem is going to be find a point strong enough to attach the towing cable on both planes. Planes that are made to tow / be towed have special strong points.

Also do not forget that when towing, the towed plane usually releases and it lands on his own. This means that you will also need a cable release system and the towed plane will have to have everything required for autonomous flight and landing.

And as someone remarked in the comments, the wake turbulence is going to be extremely, probably prohibitively bad. It is already a problem for much smaller towing aircraft when launching gliders. However if the cable is long enough it should be possible to fly slightly above or slightly below the worst turbulence while keeping the tow angle close to zero.

Towing is very widely used for launching sail-planes and some heavier hang-gliding wings.

Criminal_Affair_At_SO
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    Are attachment points really that much of a problem? The engines (and by extension the wing spars) and the landing gear are made for those kind of forces. Maybe even the the tail (of the towing aircraft) and front landing gear (of the towed aircraft) would be strong enough. – Michael Mar 21 '22 at 16:35
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    When airplanes get towed on ground, they get towed by the front landing gear. But for take-off you will need extreme strength - equal to the engines thrust which for a 737 would be 7t to 10t depending on the model and the engines. The wing spars would probably be enough - but there are no such points exposed there on a stock airplane - you will need substantial modifications. – Criminal_Affair_At_SO Mar 21 '22 at 18:05
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    Ehm yeah, engineering. Engineers find the issues, and then fix them. – Koyovis Mar 22 '22 at 02:50
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    Wake turbulence is not so much a problem, as a feature, for sail-plane towing. Once reasonably competent at straightforward tow-launches, the next exercise is to 'box the tug', that is, flying in, under, over, left and right of the tug plane's wake turbulence. – Neil_UK Mar 22 '22 at 09:45
  • @mmomtchev: Maybe find a very long runway to allow takeoff with a lot less thrust. It would already need to be quite long for the engines of one airplane to accelerate the mass of two, so maybe imagine some very very long straight section of highway, or a custom built runway since this is purely hypothetical anyway. – Peter Cordes Mar 22 '22 at 17:44
  • Is 90 miles without a curve enough? Australia's Eyre Highway even has turnouts for smaller craft to land comfortably. But it might not be wide enough for an airliner. – Camille Goudeseune Mar 22 '22 at 21:16
  • If we're towing, why are we hauling the weight of fuel and engines on the towed aircraft? So, then we're building a whole new 737 size glider? Good luck getting that past the finance department. – WPNSGuy Mar 24 '22 at 23:02
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    the C-141 towing an F-106 mentioned elsewhere actually serves as an interesting example re: wake turbulence. The C-141 should easily be generating wake turbulence comparable to that of a 737 (MTOW much higher than any 737 in existence) and the much lighter F-106...doesn't even seem to bounce around https://archive.org/details/NIX-EM-0008-01 – Erin Anne Mar 24 '22 at 23:30