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Geoffrey D. Gosling

Aviation System Planning Consultant
Berkeley, CA

February 15, 2001

U.S. Army Corps of Engineers
Regulatory Branch
P.O. Box 3755
Seattle, WA 98124
Attn:  Jonathan Freedman, Project Manager

Washington State Department of Ecology
Shorelands & Environmental Assistance Program
3190 – 160th Ave., S.E.
Bellevue, WA 98008-5452
Attn:  Ann Kenny, Environmental Specialist

RE:     Port of Seattle, Ref. No. 1996-4-02325
Comments relating to Seattle-Tacoma International Airport Project
Third Runway – Aviation Technology and Safety Issues

Dear Mr. Freedman and Ms. Kenny:

At the request of the Airport Communities Coalition (ACC), I have undertaken a further review of potential technology alternatives to the construction of a third runway at Seattle-Tacoma International Airport as described in the Final Environmental Impact Statement (FEIS) and Final Supplemental Environmental Impact Statement (FSEIS) for the Proposed Master Plan Update Development Actions for Seattle-Tacoma International Airport (Sea-Tac).  This letter supplements my initial findings submitted to the ACC in a letter report dated November 24, 1999.  It reviews recent developments in relevant communication, navigation and surveillance (CNS) and air traffic management (ATM) technologies, and discusses the potential for these technologies to enhance the capacity of Sea-Tac.

At the request of the ACC, I have also given some consideration to the operational factors that will arise at Sea-Tac as a consequence of operating three parallel runways simultaneously, and the implications of these factors for both airfield delays and runway safety.

As discussed in more detail below, the major conclusions from this review are that:

1)  The previous analysis of technology-based alternatives to the proposed third runway described in the FEIS and FSEIS failed to address recent developments in advanced air traffic management technologies, some of which have occurred since that analysis was undertaken, and thus fails to adequately reflect the potential of these technologies to enhance the capacity of the existing runways at Sea-Tac; and

2)  The increase in the number of runway crossings that will occur if the proposed third runway is constructed will increase the risk of runway incursions over present conditions, and may well limit the operational benefits that can be obtained from the new runway.

In my opinion, further study of both these issues is therefore necessary before the U.S. Army Corps of Engineers can properly assess the merits of the proposed third runway compared to other alternatives to enhance the capacity of the airport.  The information presented in the FEIS of the safety consequences of operating three runways in the manner proposed does not adequately address the issues involved, for the reasons discussed below, and a careful study of this issue would need to be undertaken before any firm conclusion can be drawn regarding the extent to which the third runway would affect safety levels at Sea-Tac.

For the record, I have attached a copy of a resume of my qualifications and experience in undertaking research and consulting activities relevant to the issues involved in this subject (Attachment A).

Technology Alternatives to the Proposed Third Runway

My previous report discussed the potential for recent advances in CNS and ATM technologies to open possibilities for entirely new ways to control air traffic that can overcome many of the limitations of the existing procedures based on radar and voice communications between pilots and controllers, and that could allow simultaneous instrument approaches to parallel runways separated by much smaller distances than currently allowed.  The key to these capabilities lies in the integration of Automated Dependent Surveillance –Broadcast (ADS-B) technology, in which each aircraft monitors its position using Global Positioning System (GPS) signals and broadcasts this information to surrounding aircraft, Differential GPS (DGPS) technology that greatly enhances the precision of the GPS information, and either the use of enhanced Cockpit Display of Traffic Information (CDTI) to allow the pilot to maintain safe separation for nearby aircraft under instrument conditions or the direct feed of the ADS-B information for other aircraft into the aircraft’s Flight Management System (FMS), the automated flight control system, so that the aircraft automatically maintains a safe separation from other traffic.

At the time of my previous report, three research and development efforts were underway to pursue the opportunities presented by these technologies.

A NASA research program termed the Terminal Area Productivity Program was exploring ways to increase the capacity of the existing airport and terminal area system.  One component of this program was a joint effort with Honeywell Technology Center and Honeywell Airport Systems to develop two related systems, the Airborne Information for Lateral Spacing (AILS) and the Closely Spaced Parallel Approaches (CASPER), that were designed to allow aircraft to perform independent instrument approaches to parallel runways spaced as close as 2,500 feet apart.  NASA and Honeywell performed in-flight demonstrations of the systems at Minneapolis-St Paul International Airport in November 1999.

The Cargo Airline Association (CAA), in partnership with the FAA Safe Flight 21 Program and a range of other industry organizations, had co-sponsored a large-scale Operational Evaluation test of ADS-B and CDTI technologies at Wilmington, Ohio in July 1999.  The test involved 12 cargo aircraft from three airlines, 12 other aircraft, including three FAA aircraft, a NASA Boeing 757, a U.S. Navy P-3 Orion and several light aircraft, and a ground vehicle.  ADS-B data were received by two ground stations, fused with radar data from an existing air traffic control surveillance radar and displayed on an air traffic control workstation.  During the test, pilots performed a variety of procedures using the traffic information displayed in the cockpit, including identification of other traffic and maneuvers to assist in the sequencing of arriving traffic and improve the efficiency of arrivals and departures.  The immediate goal of the CAA initiative was to allow the FAA to approve the use of ADS-B for traffic alert and conflict resolution functions in place of the current Traffic Alert and Collision Avoidance System (TCAS) equipment, which is based on radar transponder signals.  However, the larger interest of the CAA and the FAA Safe Flight 21 Program in the use of ADS-B is that it is not only potentially more accurate than existing TCAS equipment, but it can support a much wider range of future capabilities.

RTCA, Inc. had established Special Committee 186 to develop operational requirements and minimum performance standards for ADS-B.  A subgroup of Working Group 1, which was examining operations and implementation, had also been addressing closely-spaced parallel approaches, and had generated a number of concept papers that explored how ADS-B technology might be used to allow IFR operations on much closer spaced runways, including the existing runways at San Francisco International and Seattle-Tacoma International airports.  A paired approach concept is described in an October 1998 paper by Rocky Stone of United Airlines.

Subsequent developments in each of these areas are discussed in the following section.

In addition to more advanced CNS/ATM technologies, which will take some time to reach a stage where they can be implemented in a routine basis, there are existing radar and air traffic control display technologies that can support new operational procedures that could increase the capacity of the existing Sea-Tac runways in some weather conditions that currently restrict landings to one runway.  This display equipment, the Precision Runway Monitor (PRM), in conjunction with a high-update-rate radar and an offset Instrument Landing System (ILS) localizer and glideslope, allows a procedure known as Simultaneous Offset Instrument Approach (SOIA).  In this procedure, arriving aircraft approach one of two closely spaced runways on a course that is offset by a few degrees from the runway centerline.  This allows the aircraft on the two approach paths to be laterally separated at the missed approach point by 3,000 feet, the minimum required for simultaneous instrument approaches with a PRM.  After the missed approach point the aircraft need to have visual contact with other traffic and complete the approach under visual flight rules.  This procedure therefore requires a minimum ceiling and visibility to enable aircraft to operate under visual flight rules between the missed approach point and the runway.  Thus it cannot be used in all instrument meteorological conditions, but allows simultaneous instrument approaches under some conditions that would otherwise restrict operations to a single runway.  It is anticipated that this procedure could allow approaches to be performed with ceilings as low as 1,600 feet above ground level and visibility as low as 4 miles (FAA, 2000 Aviation Capacity Enhancement Plan, December 2000).  Efforts are currently underway to implement SOIA procedures at San Francisco International Airport.

Other ATM technologies that are currently being fielded have important benefits for the implementation of such procedures as SOIA and the use of ADS-B for paired approaches.  These include components of the Center-TRACON Automation System (CTAS), such as the Final Approach Spacing Tool.  These tools allow controllers to better position aircraft in the terminal airspace, in order to minimize the separation between successive aircraft and optimize the assignment of aircraft to different runways.  These tools are currently in the process of being fielded at selected ATC facilities as part of the FAA Free Flight Phase 1 Program.

Recent Developments in Advanced CNS/ATM Technologies

Since the end of 1999, several significant developments have occurred in the potential use of ADS-B technologies to enhance airport capacity.  The NASA Terminal Automation Program terminated at the end of fiscal year 2000, having met all its program goals and considered the AILS technology as sufficiently mature to hand off to industry for further development.  In May 2000, the AILS research team was awarded the NASA Turning Goals into Reality Award for exceptional progress toward affordable air travel.  The FAA Safe Flight 21 Program continued to work with the CAA and others on additional Operational Evaluations of ADS-B technology in the Ohio River Valley.  A second Operational Evaluation was conducted in late October 2000 at Louisville, Kentucky.  The goal of this Operational Evaluation was to evaluate air traffic controller use of ADS-B in the terminal area environment, concentrating on the following Safe Flight 21 Master Plan applications:  approach spacing, departure spacing/clearance, runway and final approach occupancy awareness, and airport surface situational awareness.  Further details are available on the Safe Flight 21 web site (www.faa.gov/safeflight21).

Most significantly for potential future applications at Sea-Tac, in December 2000 the Safe Flight 21 Program announced that it had entered into a cooperative venture with United Airlines and NASA to develop and evaluate the operational procedures for a Paired Approach landing concept that could be applied at San Francisco International and similar airports.  This concept exploits the features available in aircraft equipped with ADS-B and CDTI, and is based on concepts formulated by the Closely Spaced Parallel Approaches Sub-Group of Working Group 1 of the RTCA Special Committee 186 discussed in my initial report.  The program announcement is attached as Attachment B.  This concept is of particular relevance to the discussion of CNS/ATM technology applications at Sea-Tac because the runways at San Francisco International are even closer together than at Sea-Tac.  By keeping the trailing aircraft in each pair within 6,000 feet of the lead aircraft and on the upwind approach path, it is anticipated that the trailing aircraft will be close enough to the lead aircraft to remain clear of any wake vortices.  This solution to the capacity constraints caused by wake vortex considerations could negate the concern raised by the Port of Seattle in its responses to comments on the Master Plan Update Development Actions that wake vortex considerations would prevent simultaneous instrument approaches to parallel runways closer than 2,500 feet (e.g. see General Response 5 in Response to Comments on Permit Reference No. 1996-4-02325 – Master Plan Update Development Actions at Seattle-Tacoma International Airport, March 2000).

While it is certainly true that the paired approach concept will not allow independent simultaneous instrument approaches to both runways (the paired aircraft are obviously highly dependent), neither will the third runway under current air traffic control rules.  In any event, what matters is not whether approaches are independent or not, but the capacity provided by a given procedure.

The RTCA Special Committee 186 is continuing to meet and is currently working on developing Minimum Operational Performance Standards for cockpit display of traffic information, as well as Airborne Surveillance and Separation Assurance Processing.  Further details are on the RTCA web site (www.rtca.org).

Potential Implications for the Capacity of Sea-Tac Airport

If the paired approach concept being pursued by the Safe Flight 21 Program is found to be technically feasible, then in principle aircraft could continue to use the two existing runways in conditions of bad weather (low cloud and/or poor visibility) at similar arrival rates to those currently achieved in good weather.  However, this would require all aircraft using the airport to be suitably equipped and their crews approved to perform the relevant procedures.  To the extent that only a proportion of the aircraft fleet would be so equipped, at least initially, the capacity benefits would be reduced.

Whether the delay reduction benefits from the implementation of a paired approach concept using ADS-B technology would ever be as great as those offered by the proposed third runway, and how any difference in delay reduction benefits compares to the likely difference in implementation costs and environmental impacts, cannot be determined from the information presented in the FEIS and FSEIS.  This is not only because the paired approach concept was not considered in the discussion of technology alternatives in the FEIS and FSEIS, but also because the analyses of airfield capacity and delay upon which the FEIS and FSEIS were based were undertaken well before the details of the paired approach concept had been articulated.

Therefore in order to compare the potential capacity benefits of the paired approach concept to those of the proposed third runway, it would be necessary to perform a detailed simulation analysis of the paired approach concept at varying levels of aircraft equipage.  Until such an analysis is undertaken, it is simply not possible to say whether the proposed third runway would provide greater delay reduction benefits in the long term than implementation of the paired approach concept, or how the two approaches would compare in the nearer term.  Such an analysis would need to consider the effect of different implementation schedules for the paired approach concept, as well as different future traffic growth scenarios.  Clearly, fairly rapid implementation combined with slow traffic growth will give a very different outcome than a much slower implementation schedule combined with a rapid growth in traffic.

Whether or not the paired approach concept can be implemented at Sea-Tac, the combination of SOIA procedures and CTAS tools would appear to offer the prospect of significantly reducing delays during intermediate weather conditions (down to the ceiling and visibility limits of the SOIA procedure).  How effective this will be in reducing total delay will require a careful analysis that considers how often these intermediate weather conditions occur.  While this will not eliminate delays during very bad weather, it is critical to assessing this issue to know how often these conditions occur.  It is not sufficient to simply know what percent of the time these conditions occur.  What matters is how long they persist when they do occur.  There is a huge difference from a delay standpoint in a condition that occurs for one or two hours a day on many days a year and a condition that lasts all day on far fewer days per year.  In the former case, although delays build up while the conditions persist, once the weather improves and capacity returns to an unconstrained level, delays will gradually reduce as the backlog of flights is eliminated.  However, in the latter case delays will continue to build all day to such levels that it will become necessary to start canceling a large number of flights to preserve the integrity of the system.

While the capacity and delay analysis undertaken during the 1995 Capacity Enhancement Study Update for Sea-Tac, and used as the basis for the delay projections in the FEIS and FSEIS, considered the occurrence of different weather conditions over a ten-year period, the weather categories used in that study do not conform to the likely ceiling and visibility limits that would be allowable with a SOIA procedure.  Thus it would be necessary to re-run the analysis for both the third runway and the SOIA procedure in order to compare the delays under the revised weather categories.

Operational Considerations with Three Parallel Runways

Implementation of the proposed third runway will create a situation in which aircraft taxiing between the new runway and the passenger terminal complex will have to cross the two existing runways.  This has both delay and safety implications.

During periods of heavy arrival traffic and poor weather, arrivals will need to use the two outboard runways in order to be able to operate dependent instrument approaches to two runways, which require the full separation between the outboard runways under current air traffic control rules.  In this situation, departures will generally be assigned to the middle runway to avoid needing to increase the separation between successive arrivals to a given runway in order to be able to release a departure.  However, some departures may need to use the longest runway, Runway 16L/34R, which will generally require that a gap be created in the arrival flow to that runway, reducing the arrival capacity.  This also means that most departures will need to cross Runway 16L/34R to reach to middle runway.  This will significantly complicate the departure flow, since departing aircraft cannot be cleared across the arrival runway when an arriving aircraft is within 2 miles of the threshold, and there is limited space between the two existing runways to hold aircraft waiting to take off.  This is particularly problematical under north flow conditions since the thresholds of Runways 34L and 34R are not aligned, and departing traffic has to cross Runway 34R well down the runway.  Thus by the time an arrival on Runway 34R has passed the crossing taxiway, the next arrival may be too close to the threshold to allow a departure to cross.  Furthermore, the departure queue would obstruct the parallel taxiways in front of the South Satellite Terminal.

One solution to this problem would be to have departures cross both existing runways and form a departure queue on a parallel taxiway between Runway 16R/34L and the new runway.  The proposed taxiway system is well configured to support this during south flow conditions, but not at all well configured for north flow conditions.  Departures on Runway 34L would have to taxi all the way to the parallel taxiway adjacent to the new runway before looping back to the end of Runway 34L, resulting in longer taxi distances than necessary.  In both south and north flow, there is only one crossing taxiway that is well positioned to handle departing traffic.  Since each route involves two runway crossings, that generally will require a hold between the two runways, this is likely to result in traffic backing up onto the parallel taxiways in front of the terminals and may well require both arrival and departure rates to be temporarily reduced to allow enough departures across the runways to maintain the departure queue.  While this solution would prevent the arrivals on Runway 16L/34R from limiting the departure capacity, it will significantly increase the number of runway crossings.  This has potential safety implications, as discussed below.

Aircraft arriving on the new runway would need to cross the two existing runways to reach the terminal complex.  Apart from the delays inherent in any crossing of an active runway, the need for the local controller responsible for the existing runways to coordinate crossings in both directions will significantly increase the control task complexity and further delay crossing aircraft.  Each aircraft waiting to cross the runway will need to be given a separate clearance, and the time required to issue as many as four clearances (two crossings in each direction) will reduce the number of opportunities to clear aircraft across the runways without increasing the intervals between arriving and departing aircraft.  Since the space between the two existing runways generally only allows one aircraft to be held on each perpendicular crossing taxiway, the controller will need to clear an aircraft waiting between the runways to cross the runway in front of it before clearing the following aircraft into the hold position between the runways.

During peak departure periods and poor weather, there are two ways in which the airfield could be operated.  The first would have departures use both existing runways and assign most arrivals to the new runway.  The second would have departures use Runway 16L/34R and the new runway, while most arrivals use Runway 16R/34L.  The former has the advantage that an increase in arrival rate can be handled by dependent instrument approaches to the outboard runways, as in the peak arrival case.  This would also make the transition between arrival and departure peaks relatively easy.  However, the arrivals on the new runway would have to cross two departure runways, although this is easier than crossing an arrival and a departure runway, since departures can be held for crossing traffic, while arrivals cannot.

The large number of runway crossings that would have to be coordinated under these various operating conditions obviously increase the potential for runway incursions, either due to pilot or controller errors.  (A runway incursion is defined by the FAA as any occurrence at an airport involving an aircraft, vehicle, person, or object on the ground that creates a collision hazard or results in a loss of separation with an aircraft taking off, intending to take off, landing, or intending to land).  Although most runway incursions do not result in collisions, the few that do are often associated with great loss of life.  The worst accident in civil aviation history involved a runway collision at Tenerife in the Canary Islands between two Boeing 747 aircraft.  Several of the recent air carrier accidents in the U.S. have also involved runway collisions.  The FAA is rightfully concerned about the number of runway incursions that occur each year, and has a major program underway to reduce the frequency with which they occur.  However, to date this has not been particularly effective, and the number of reported runway incursions last year increased significantly over the previous year.

Among the factors that are believed to increase the risk of a runway incursion occurring are traffic intensity, airfield complexity, and poor visibility or darkness, for fairly self-evident reasons.  Thus it appears highly likely that the operational conditions that will occur at Sea-Tac if the third runway is constructed will tend to increase the risk of a runway incursion over the present conditions.  Whether this increase in risk is significant enough to be of concern, and whether steps can be taken to offset or reduce it, is not clear without a careful study of the issues involved.  Of particular concern is the possibility that operational procedures that may turn out to be necessary to reduce the risk of runway incursions could result in greater delays than predicted by the delay analysis undertaken thus far and upon which the FEIS and FSEIS are based.

Unfortunately, the analysis of air traffic safety presented in Chapter IV of the FEIS is based on some fairly simplistic, and erroneous, assumptions.  A separate analysis was performed for aircraft accidents and incidents and pilot deviations.  No consideration appears to have been given to analyzing controller operational errors.  The analysis of aircraft accidents and incidents assumed a constant ratio of accidents and incidents to aircraft operations, based on the average ratio over the period from 1984 to 1993.  This was used to project the number of accidents and incidents in the future.  The discussion of Future Conditions for aircraft accidents and incidents begins with the following, unsupported, statement:

No direct correlation exists between accidents and incidents and the number of aircraft operations at a particular airport.  (FEIS, p. IV.7-18)

The analysis then proceeded to assume the very correlation that it had just claimed did not exist.  Since the projected number of accidents and incidents in a given year depended only on the forecast traffic level, it follows that the same number was obtained irrespective of the development alternative being considered.  This clearly makes no sense at all.  The analysis made an implicit assumption that the different alternatives being considered had no effect on the accident or incident rates, and then concluded that there was no difference between alternatives.

The analysis of pilot deviations was based on 122 occurrences over an unspecified period since October 1987.  Most of the occurrences were airspace deviations and most were by general aviation pilots.  There were 9 runway incursions during this period, a rate of about 1.4 per year by 1994.  These were projected for future years for the do-nothing alternative based on a constant ratio to aircraft operations.  In the case of development alternatives, the number of runway incursions was projected for future years based on the projected increase in runway crossings due to the new runway.  However, the increase in the number of runway crossings assumed appears implausibly low, varying from about 44 percent for 2000 to 42 percent in 2020.  Since each operation on the third runway generates two runway crossings, while each operation on Runway 16R/34L only generates one under current conditions and of course operations on Runway 16L/34R do not generate any, even if the new runway only handles half as many operations as Runway 16R/34L the number of runway crossings would double.  If the departure queue for Runway 16R/34L is located to the west of the runway, as discussed above, or the new runway handles a larger share of the traffic, then the increase in the number of runway crossings would be even greater.

Another concern with the analysis is the assumption that the likelihood of a runway incursion occurring depends only on the number of runway crossings, and not where those crossings take place.  With the current operating pattern, most runway crossings in poor weather involve arriving traffic crossing a departure runway (Runway 16L/34R), since arrivals cannot use both runways under these conditions.  It is inherently easier for controllers to manage traffic crossing a departure runway, because departures can be held at the end of the runway to allow aircraft to cross, whereas inbound arrivals cannot be stopped, nor can aircraft be cleared to cross the runway if an arrival is within 2 miles of the threshold.

The discussion of runway crossings makes the following statement:

No direct correlation exists between the increase in runway crossings and safety, as the separation standards used by air traffic control will ensure adequate separation between aircraft, and aircraft and service vehicles.  (FEIS, p. IV.7-21)

This demonstrates a misplaced faith in the infallibility of both controllers and flight crew.  If this were true, obviously no runway incursions would ever occur.  It may well be the case that the risk of a runway incursion depends on more factors than just the number of runway crossings, but that is very different from saying that there is no correlation between runway crossings and incursions.

Because of these deficiencies in the analysis, the information presented in the FEIS of the safety consequences of operating three runways in the manner proposed does not adequately address the issues involved, and a more careful study of this issue would need to be undertaken before any firm conclusion can be drawn regarding the extent to which the third runway would affect safety levels at Sea-Tac.

Conclusion

The discussion of the potential contribution of advanced CNS/ATM technology to enhancing the capacity of Sea-Tac and the safety considerations involved in operating three parallel runways, that is contained in the FEIS, FSEIS, and subsequent responses by the Port of Seattle to public comments on these issues, has failed to consider several important recent developments and has not adequately analyzed the issues involved.  Therefore further studies of these issues should be undertaken before the U.S. Army Corps of Engineers can properly consider the potential role of new technology to enhance the capacity of Sea-Tac Airport, as well as assess any change in the risk of runway incursions that could result from the construction of the third runway.

Sincerely,

Geoffrey D. Gosling

Attachment A

Resume

Attachment B

Description of FAA Safe Flight 21 Project

Source: http://www.faa.gov/safeflight21 (Downloaded 2/14/01)