Wednesday, February 04, 2009

Ramblin’ Wreck



My son is a freshman at Georgia Tech so, naturally, anything to do with Tech attracts my attention these days. This story would have attracted my attention anyway so the inclusion of Georgia Tech is just a bonus.

Continuous Descent Arrivals: Atlanta Flight Test Evaluates Technique For Saving Fuel And Reducing Noise In Airliners

”The changes were part of Georgia Tech’s flight-testing of “continuous descent arrivals,” a procedure designed to save fuel and time while producing environmental benefits by reducing both noise and emissions. Involving more than 600 flights, the Atlanta study was done in collaboration with the Federal Aviation Administration (FAA), FedEx and Atlanta’s two dominant air carriers: Delta Air Lines and AirTran Airways.“

Been there. Done that. I think I still have the T-shirt. I wish Professor Clarke the best of luck but I can’t offer him much hope. To understand why, take a look at this part.

”“Imagine a line of aircraft descending through a long tube that’s fixed laterally and limited vertically to be within a narrow band,” explained Clarke. “If each airplane were like a ball with a different coefficient of friction, then when you put the balls in the tube at equal intervals, they would begin to catch up with one another. The ball with the lower coefficient would tend to catch up with the ball with a higher coefficient. That’s something that we have to work very hard to avoid.” “

That is a good summation of the problem being addressed. The key is in understanding the airspace design that flows from however you intend to implement this program. “...a long tube that’s fixed laterally and limited vertically.” That “tube” will drive the design for the rest of your airspace. It will determine the actions the controller working that airspace will take and where aircraft operating around Atlanta -- aircraft not using the “tube” -- will be routed around that same tube. Form follows function.

That is all well and good. Until you need to move the “tube”. You can understand this by reading a linked article found (conveniently) on the same page.

Mathematician Calms The Skies With Turbulence Detection Algorithm

”A mathematician developed a system that creates a three-dimensional view of turbulence and transmits it to airliner cockpits. The new algorithm analyzes data gathered by Next Generation Doppler Radars and sends a real-time readout of turbulence every five minutes, covering an area up to one hundred miles out in front of a speeding plane. “

Cool stuff. Now imagine if this new radar showed an area of turbulence in that same “tube” of airspace from the previous article. It’s a no-brainer. Aircraft won’t be flying in the “tube”. But your airspace is designed around the “tube”. And in that we’re all shooting for NextGen, that means we’re running airplanes tighter -- closer together -- so we can improve the efficiency of the system. That means there is a whole different stream of airplanes (actually several different streams) really close to the “tube”. Do you see the problem ? And turbulence isn’t nearly as difficult to deal with as thunderstorms.

It might interest non-controllers to know that most of Atlanta Center’s arrival sectors are shaped like funnels (think both laterally and vertically.) They aren’t as precise (or efficient) as a “tube” but they are a lot more flexible. The only real choke point is where the “funnel” looks like a “tube” -- right at the spout.

If you’d like to read more on the basics of airspace design I’d recommend an article I wrote for AVweb, long ago: Say Again #19: ATC 302 — The Hub

If you’re in a rush, skip down to the section entitled “Airspace Modeling”.

”For those of you who are remembering to think in three dimensions you'll realize that the pieces of pie are actually more like funnels. I'd explain that there ain't no such thing as "funnel pie" but I think I lost the Yankees with the "Pi-R-squared" joke. Anyway, the departure sectors "funnel" the departures out from the airport and the arrival sectors "funnel" the arrivals in.“

Don Brown
February 4, 2009

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