Circular Runways?

Multiple friends have sent me this BBC simulation of an airport with a banked circular runway: video.

One idea behind the circular runway is that it is supposedly always possible to land directly into the wind. No more crosswind landings, right? There are a few problems with this idea. First, the airplane’s touchdown point might be plus or minus 1000′ and, since the runway is constantly curving, therefore the heading at touchdown can’t be known precisely. The second problem is that when the wind is strong it also tends to vary in direction from moment to moment. So you could be perfectly set up to land into the wind and, five seconds later, the gusting 40-knot wind is coming from a 30-degree angle off the nose.

A slightly deeper problem is that “landing” is not “touchdown point.” The pilot who stops flying the moment the wheels touch is a… student pilot. The task is still “flying” until the plane slows down to about 60 knots, at which point the aerodynamic control surfaces become ineffective on a heavier plane. Only then does the task become “taxiing” (i.e., driving). With a proposed radius of 1.75 km and a landing roll of about 1 km the pilots would still be flying in a crosswind and it might be a lot more challenging than on a linear runway because the crosswind would be constantly varying.

One thing that might sort of work is the 3.5 km diameter.  A plane going 140 knots (final approach speed of faster airliners) needs a diameter of 6,000′ (1.8 km; source) to turn at a bank angle of 30 degrees. That’s less than 3.5 km so in theory this is possible. What about in practice? That’s where we get to the deepest problem with the idea: it forces pilots to conduct a destabilized approach.

The stabilized approach is the core of safe airliner landings and it is what we instructors try to teach, especially in heavier or faster personal airplanes. In the clouds, nothing changes below 1000′ (below 500′ in the clear). The flap setting, gear position, thrust (within reason), attitude (within reason), all stay constant. A conventional airliner with leading-edge slats can be flown pretty much hands-off all the way to touchdown (don’t try that with a Canadair Regional Jet, though!). You’re at 300′ above the runway and, despite having set everything up the way you thought it would work, it isn’t working? Instead of making radical adjustments in an attempt at a last-minute salvage you add power, retract some flaps, nose up, retract the gear, retract the rest of the flaps, and go around to try again.

What makes landing an airliner idiot-proof is that everything is perfectly set up about a minute in advance. Throw a circular banked runway into the mix and now landing requires heroic stick-and-rudder skills. The airplane was trimmed perfectly and flying itself down to the landing zone without even being on autopilot. Then, in the last 15′ of the flight it is time to put the airplane into a 30-degree bank and sync up with the runway circle? On the 4th leg of the day on the 3rd day of a trip after maybe 5 hours of sleep?

If we ignore all of the above and we assume that controllers can tell pilots where to go on the circle, I don’t understand the flow improvements. Airplanes ideally both land and take off into the wind. So the point of a typical departure is the same as the point of a typical landing? Can planes be packed tighter than in the current system where the departure point is about one mile ahead of the landing point?

The good news: it would work pretty well for helicopters! And JFK has a circular taxiway that is where I logged about half of my jet “flying” hours while waiting our turn during the “international push.”

Readers: What did I miss? Is the idea better than it seems at first glance?

 

20 Comments »

  1. toucan sam

    March 16, 2017 @ 11:17 pm

    1

    This is great!

  2. ChrisC

    March 17, 2017 @ 2:27 am

    2

    No.

  3. JP

    March 17, 2017 @ 3:06 am

    3

    I think we should ask these pilots If they would have liked to attempt a landing on a circular runway.

    http://avherald.com/h?article=4a5e80f3

  4. Andrea Matranga

    March 17, 2017 @ 4:01 am

    4

    Not to mention that berm holding up the outside edge of the runway would have its own very interesting airflow pattern in high wind conditions.

  5. Andrea Matranga

    March 17, 2017 @ 4:06 am

    5

    Maybe would work better for takeoffs than for landings.

  6. Jack D

    March 17, 2017 @ 8:57 am

    6

    On the other hand, if you could make it work it would end runway overrun accidents.

    You’ve mentioned all of the problems with landing on such a runway. What would be the disadvantages for takeoff?

  7. reha gur

    March 17, 2017 @ 12:33 pm

    7

    Real runways are always square fields. And yes, they have only grass on the surface.

  8. jack crossfire

    March 17, 2017 @ 1:36 pm

    8

    It only shows a commercial, but the point would not be wind direction but conserving real estate. As more of the world is forced to move to San Francisco in search of jobs, it’s going to take packing at least 5 airports between 4th st & 5th st to serve demand.

  9. Richard Anderson

    March 17, 2017 @ 1:40 pm

    9

    NASA experimented with the idea back in the 60’s using GM’S Arizona test track. I believe the largest aircraft used was a DC-6. Popular Science magazine had an article about the program.

  10. Tad Gallistel

    March 17, 2017 @ 3:30 pm

  11. Guy Widdowfield

    March 17, 2017 @ 6:02 pm

    11

    Only read this briefly just now. But as a private pilot licence holder (long elapsed now) some thoughts come to mind. It’s a great concept of always landing into a head wind (I’ve had some scary crosswind landings in my time) BUT how would approach lighting work? ILS approaches? VASI? And even just the simple “Piano Keys” to land somewhere close to if things go well?

  12. BruceH

    March 18, 2017 @ 3:17 am

    12

    To add to the points above…

    A supposed benefit is that planes always land into the wind, but also that noise can be evenly distributed. Well only if the wind obligingly moves around 15 degrees each day!

    And since this can’t be retro-fitted to existing airports, how many new airport sites are there that have a nice flat geography that allows uniform approach from all directions? You might get one in the whole world – and re-training pilots on a completely new landing approach scheme just for a single airport seems like a non-starter.

  13. Jan Krouwer

    March 18, 2017 @ 6:31 am

    13

    At airports with obstacles, how would instrument approaches work to be available for every direction?

  14. R o x

    March 18, 2017 @ 11:56 pm

    14

    dive into the study properly first before becoming dismissive in some blog based on a short video you’ve seen mr Harvard bro

  15. Maya

    March 19, 2017 @ 12:30 pm

    15

    This is thoroughly researched by Dutch engineers. As you all know Dutch civil engineering is miles ahead of American engineering. Those are not even able to build adequat levees
    Rest assured that this idea has extensive research funded with 20 million euros.

  16. John Crocker

    March 20, 2017 @ 8:55 am

    16

    This circular runway idea reminds me off a saying in the Air Force. Republic Aviation built the F-84 and F-105. We kidded Republic and said: “If you build a circular runway, we’ll build an aircraft that can barely takeoff on it.’

  17. Craig

    March 20, 2017 @ 10:01 am

    17

    I’m pretty sure this would also require a redesign of landing gear since you’re now tossing a lot of centrifugal forces against little sticks that are intended to run mostly in straight lines. Planes would get redesigned like NASCAR racers, with all their focus on making a single-handed turn all the time.

  18. philg

    March 20, 2017 @ 4:03 pm

    18

    Craig: If the runway is banked, at least at one groundspeed, wouldn’t it be possible for the turn to be a 1G turn such that the landing gear experienced the same loads as in a conventional landing? That does raise the question of what speed to choose when designing the runway diameter and bank angle!

  19. the other Donald

    March 22, 2017 @ 11:21 am

    19

    if prevailing wind is that variable, an asterisk shape like some old WWII training fields might work better, but would also require multiple approach aids, although maybe not if the last-seconds adjustments are OK. If the idea is to drive accident risk to zero, radically changing short final approach is a strange way to go.

    It’s a nutty idea, sorta like basing our national surface transportation on a grid of paved roads where everyone who can get a license is to hurtle toward each other head-on displaced by inches laterally. That works out to about 50K fatalities annually in USA.

  20. David Spellman

    March 23, 2017 @ 5:19 pm

    20

    Navy Lieutenant J.R. Conrey developed the idea for a circular runway in 1960. He designed a 300′ wide concentric tier of lanes stripped around a 32,000 foot circumference. The embankment supported speeds form 40mph at the base to 170mph at the top. He was killed before he could try it out. But the office of Naval Research authorized the project in 1964, and Commander Lloyd Smith used the GM Automotive Proving Grounds track outside of Phoenix to land and take off a T-28. Two other test pilots tried it and pronounced it routine and comfortable. “The physics handled everything.”In 1965, the team returned with an A-1 SkyRaider, a C-54 transport and an A-4 attack bomber, with lieutenant commander Skip Furlong in the A-4 and Lloyd Smith in the C-54. The final Navy report assigned the circular runway “a vital place in the future of aviation. The FAA was unenthusiastic, suggesting that the expense of constructing and maintaining the precisely banked, perfectly circular configuration and the complex tunnels underneath to provide access to a central terminal made the cost/benefit ratio a problem. See an article in Air&Space, Smithsonian’s magazine, from April/May 2004 and another in Time magazine in 1965.

    Worth noting that the whole concept was generated by Conrey after a right-angle crosswind hammered his plane on a Nebraska airstrip. The circular airstrip was designed to eliminate these issues…and did.

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