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?