Duwamish Flight Track Study

by the FAA Northwest Mountain Region Air Traffic Division

dated December 19, 2000

This is the long-awaited study by the FAA to determine the feasibility of increased use of the Duwamish / Elliott Bay corridor for Sea-Tac jet departures during north flow conditions, as requested by the Port Commissioners in Resolution 3401 of June 27, 2000, and in a letter from Commission President Jack Block to the FAA on July 21, 2000. The report consists of a three-page Executive Summary; a main body of 17 pages plus six color figures and two tables; and 11 appendices containing data, correspondence, and the names of the people who contributed to the report.

The report analyzes four options for re-routing more traffic out the Duwamish, and it also examines the idea of extending the curfew hours on the east turn (currently 10 p.m. to 6 a.m.). The baseline data for the flight track study consists of the Sea-Tac operations on August 30, 2000 (see Table 2), when the airport was operating in north flow. The table below is a condensed version of Table 2, omitting arrivals and turboprop operations.

Departure	Destination	Jet Ops.
SUMMA / Lakeview	LA, San Diego	90
SUMMA / Baker City	Denver, Dallas, points S and SE	68
Mountain	Chicago, Boston, points East	168
Duwamish / SW	Honolulu, San Francisco	72
Duwamish / S	Portland	18
Duwamish / NW	Alaska and the Orient	63
North	Canada, polar flights to Europe	21
Other		3
Total		503

Figures 1-5 provide nice graphical illustrations of the current arrival and departure patterns and the changes that would result from the four alternatives.

Study Alternative #1

This would re-route all the SUMMA departures out the Duwamish / Elliott Bay corridor, along the same path as the 18 Duwamish /Portland flights in the above table. Then the planes would continue to the SUMMA point.

The report raises the following objections to this alternative. First, it says that Duwamish departures have to be radar vectored along the flight path by air traffic controllers. In other words, the controllers have to guide the planes along this route, which is "task intensive", and the controllers' attention should not be shifted to this task from their main mission of keeping aircraft separated. (It's not clear to me why planes have to be guided along the Duwamish route but not along other routes).

A second objection is that "The current arrival and departure routes were developed to facilitate efficiency as well as balance sector traffic loads. Moving an established departure route from an east turn to a west turn through Elliott Bay results in an unbalanced traffic load." It goes on to say that if departing aircraft are queued for take-off with alternating east turn and west turn departures, they can take off every 30 seconds, vs. three minutes for same-route departures. Therefore shifting aircraft to the west turn will create departure delays. (This makes absolutely no sense to me, since the number of east-turning departures is currently much greater than the number of west-turning departures. On August 30 (see table above) there were 326 planes making the east turn and 153 making the west turn. If they really wanted to balance the load and create more opportunities for alternating east-turn and west-turn departures, it seems to me they would want to shift planes from the east turn to the west turn).

Another objection is that "This alternative routing also creates conflicts with current sector boundaries". (I don't understand why, since the re-routed planes would fly along a path that is already used by some departures).

The Summary says that "A significant increase in departure delays during high traffic demand periods or during periods of low visibility would occur thus reducing efficiency well below current levels."

Study Alternative #2

This would re-route all the SUMMA departures out the Duwamish / Elliott Bay corridor, then make a sharp left turn and loop the planes back over the top of Sea-Tac Airport, then continue on to the SUMMA point.

Objections to this alternative are the same (almost word-for-word) as for #1. In addition, there are safety concerns with this route. Looking at Figure 3, it certainly does appear to be an odd and potentially dangerous flight path.

Study Alternative #3

This would re-route only the SUMMA / Lakeview departures out the Duwamish / Elliott Bay corridor. Otherwise it's the same as #1.

Objections to this alternative are the same (almost word-for-word) as for #1.

Study Alternative #4

This would re-route all the SUMMA departures out the Duwamish / Elliott Bay corridor, but the "cross over" would be handled slightly differently. The SUMMA / Baker City departures would proceed as in #1, crossing over the arrival path along the same route as the 18 Duwamish / Portland flights, and continuing to the SUMMA point. However, the SUMMA / Lakeview departures would proceed southwest and then south to Eugene, and then cross over to their original flight path.

The first objection to this alternative is that it would create traffic conflicts that were alleviated by the "four-post" plan of 1990. Also it would add significant mileage to the re-routed flights. And ditto the objections to alternative #1.

Analysis of Curfew Hours

Currently, northward-departing jets are not supposed to use the east turn between 10 p.m. and 6 a.m. Instead, they proceed along the Duwamish / Elliott Bay corridor, out to Puget Sound, then north until reaching an altitude of 10,000', when they may turn east.

The report says that "Late night traffic has been increasing at a steady pace" and that "the curfew should start later [my emphasis] as the traffic volumes increase". In other words, the Port Commissioners asked the FAA to consider extending the curfew hours, and the FAA is responding by saying that the curfew hours should be cut. In fact, the report says "it is highly probable that increasing traffic volumes during the first curfew hour (10 p.m. to 11 p.m.) will soon require that the curfew be shortened on weeknights to accommodate those increases". The report also says that "it could be feasible to increase the Saturday evening to Sunday morning curfew hours from 9 p.m. to 8 a.m. when traffic conditions permit [my emphasis]" and that air traffic control should "explore expanding and contracting the curfew accordingly". So much for the curfew.

Other Topics

Flight Management Systems. "Procedures for maximizing the use of the Duwamish / Elliott Bay Corridor is [sic] not compatible with a static FMS preprogrammed procedure. Due to the complexity of existing arrival and departure streams west and south of SEA, each Duwamish / Elliott Bay aircraft has to be vectored by an air traffic controller". (Again, I don't understand what's different about the Duwamish route. Figure 1, the existing arrival and departure diagram, shows a nearly symmetrical east/west pattern in arrivals and departures).

South Flow - Commencement Bay Nighttime Departure. Resolution 3401 indicated that increased use of this procedure had the potential to reduce noise for South King County residents. The report says, "This procedure is not an established FAA south flow nighttime procedure, contrary to the reference in Section 3, of the Port of Seattle Commission Resolution Number 3401". Thus it is summarily dismissed.

Jet / turboprop fleet mix. Besides the 503 jet departures on August 30 (Table 2) there were also 413 turboprop departures. The report says, "Airlines are in the process of replacing certain turboprop aircraft with smaller commuter jets (regional jets) and increasing overall jet operations at airports nationwide. As this transition from turboprops to regional jets occurs, more operations will be added to the jet departure and arrival flows. This will compound the air traffic complexity… "

North flow vs. south flow. Table 1 gives north flow and south flow by percent, for every month from 1987 through 1997. The mean for all 11 years is 34.5% north flow, 65.5% south flow. However, the mean for all 11 Julys is 52% north flow, and the mean for all 11 Augusts is 54% north flow.

Growth in traffic at Sea-Tac. Here are some figures on past and future annual operations (in thousands) at Sea-Tac, from various parts of the report:

Year	1970	1980	1986	1988	1998	1999	2004	2015
Operations	150	210	260	290	400	411	458	561

I wonder if we need another airport.

Harry Stern

January 8, 2001