Six Degrees For Separation: One Way To Solve The DFW Airspace Issue

The airspace over Addison (KADS) is slated to be changed soon if the FAA proceeds with its plan to reduce congestion into Dallas Love (KDAL) and Dallas/Fort Worth (KDFW). The airspace change includes a lowering of the Class D over Addison from 3000 MSL to 2500MSL. While that may not seem like much, it is in an area where operations are already in a very tight fit with DFW traffic to the west, DAL traffic inbound from the east-northeast and large amounts of corporate, fractional, cargo and training traffic underneath at ADS. In fact, the final approach fix (JERIT) for ADS rwy 15 is at 2000 MSL, which would leave only 500 feet separation between IFR arrivals into ADS and DAL traffic at 2500 if this airspace change goes through. As it stands, ADS is already the busiest GA airport in Texas and in the top 5 in the United States.

The area of concern: The 3000 MSL roof of Addison’s already highly modified class D is slated to be lowered to 2500 MSL, leaving very little space for aircraft as big as MD-80s and 737s to operate. The proximity to DFW and DAL is noteworthy.

There are numerous ways to avoid having to redesign the existing airspace. Although I’m sure some will suggest vectoring airliners further to the north and west before their southbound turn towards DAL, this is not efficient with respect to the jets. Anything that increases fuel consumption for the airlines is not only irresponsible environmentally, but financially. Likewise, the hundreds of businesses that rely on ADS should not be marginalized in the effort to reduce the impact to airliners. I am not writing this from the standpoint of “big airliners are against little piston planes”. Instead, I am writing this as the result of several years of observing, studying and testing new methods of utilizing existing airspace. After reading the NPRM on the changes to DFW’s airspace, I came to the conclusion that people may not be fully grasping the true capabilities of modern jet airliners.

The upside-down wedding cake design of Class B airspace is optimized for steep climbs and descents. Standard Class B has a floor gradient of 300 ft/nm out to the 10nm ring. This equates to only 1000fpm at 200ktas or 1250fpm at 250ktas. But again, this is for the floor and operations in excess of these values would be well contained within the airspace. With the advent of RNAV STARs and GPS approaches, creating 3D highways in the sky is no longer a fantasy but an easily employable system that works in VFR or IFR conditions. The only way to fit more aircraft into the volume of airspace already set aside is to increase the angle of descent at critical segments inside the Class B.

For separation and flow purposes, many congested terminal areas drop arrivals down 30 or 40nm out so that departures can climb unobstructed above them. This is because in areas like the DFW Class B, the proximity of DFW, DAL, ADS, AFW, NFW, FTW, GKY, GPM and RBD makes it very hard to get everyone where they need to be at the same time. When most of the non-RNAV STARs were designed, it was hard to conceptualize how to position aircraft three-dimensionally. Now that airliners and many corporate aircraft feature VNAV, FPA symbology and the ability to climb or descend in excess of 2000fpm, being able to follow a constant descent path is much easier to plan and execute.

As mentioned before, the standard floor gradient for Class B is 300ft/nm. Modern jet aircraft can climb at more than twice this rate under most conditions. Descending is actually more difficult to manage in some cases as an angle which is too steep will preclude deceleration to flap and gear speeds. Testing this theory in various sims, talking to pilots of different aircraft and flying the procedure in real aircraft has shown that an average glide angle of 6 degrees results in a power-off approach with no increase in airspeed (in reality the range was roughly 4.5 to 7.0). Depending on configuration, very low levels of power may be required to maintain airspeed. This power setting will invariably be less than that used during the current step-down method of approaching. This has tremendous advantages for noise abatement, fuel conservation, airspace utilization and wake turbulence avoidance.

Jets are not as responsive as light GA airplanes in the approach phase which is why a 6 degree glide path converts to a standard 3 degree glide path at some pre-dplanned distance from the runway, most likely 1500 to 1000 AGL (depending on the aircraft type and wind conditions). Further out in the Class B airspace, descent angles can be more conventional if satellite airport conflicts are not present, allowing jets to “pre-configure”; going to a minimal flap setting that would produce enough drag to keep speed from increasing in the descent. Some aircraft that are extremely clean with high inertia such as the A330 and B777 may require shallower angles or the use of speedbrakes.

Using this type of approach in the northeast sector of the DFW Class B would bring Love arrivals over Addison between 3000 and 4500 MSL (depending on where they are vectored from). This is a substantial safety margin for both Addison and Love arrivals. An additional benefit is that Love’s lateral spacing would not have to be modified from what exists today, reducing the potential for conflict with Dallas/Fort Worth traffic on the Cedar Creek Six arrival when a south flow is in use. Around the DFW Class B, departures leave the terminal area on north, east, south and west headings, while arrivals enter on northeast, southeast, southwest and northwest headings. This existing deconfliction works well with 6 degree descent angle as departures would not risk losing separation with arrivals.

The whole idea behind the 6 degree approach is to use what we already have without making any particular group of operators have to suffer. If the procedure works well in our airspace, it can easily be implemented nationwide for reasons as varied as traffic management, noise abatement and reduced emissions. Please try this procedure in whatever simulators you have access to. An example to test out is DAL runway 13L, crossing WADES at 7500 MSL, NITER at 1900MSL and conducting a normal visual or ILS once crossing the FAF. Since this is an angle-based and not a rate-based procedure, your VS will change as you descend and or change airspeed.

Runway 13L Dallas Love. Note the modified IF crossing altitude to produce a 6 degree glideslope to the FAF.

If you want to convert any IAP to a 6 degree variant, simply decide what your conversion altitude or intersection is (when or where you go from 6 to 3 degrees) and how far back from that point you want to commence the approach. Applying basic trig will net you the IF crossing altitude. For example using DFW’s runway ILS 17L:

FAF = GBUSH, 2300MSL

IF = RIVET, Unknown MSL, 12.6nm from FAF

sin descent angle x distance to FAF x nautical mile in feet + FAF altitude

((sin6 x 12.6) x 6076)) + 2300 = 10302 MSL at RIVET

Runway 17L Dallas/Fort Worth Intl. Notice the modified crossing altitude at the IF.

In the meantime, please send the FAA your comments and suggestions on the proposed airspace change. My solution is not the only one and the more minds that work on this issue, the better.

http://www.regulations.gov/#!docketBrowser;rpp=25;po=0;dct=PS;D=FAA-2012-1168