CDA Paper Hahn and Hoffman

8/3/2019 CDA Paper Hahn and Hoffman

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MP 070086

MITRE PRODUCT

Continuous Descent Arrival/Continuous DescentApproach Definitions and Variations

May 2007

Edward C. HahnJonathan Hoffman

Sponsor: Federal Aviation Administration Contract No.: DTFA01-01-C-00001

Dept. No.: F063 Project No.: 0207F901-IF

The views, opinions and/or findings contained in this report are those ofThe MITRE Corporation and should not be construed as an off icialGovernment position, policy, or decision, unless designated by otherdocumentation.

This document was prepared for authorized distributiononly. It has not been approved for public release.

2007 The MITRE Corporation. All Rights Reserved.

Center for Advanced Aviation System Development

McLean, Virginia


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Abstract

This product documents the state of the art of the concept known as Continuous Descent Arrival

(CDA), sometimes also called Continuous Descent Approach. At the present time, there is nosingle agreed-upon detailed description of this concept, but there is a shared high-level idea: an

aircraft that is allowed to descend continuously, without interim level segments, will produce less

noise and emissions, and burn less fuel, than conventional arrivals in widespread use today. Thisdocument provides a basic definition of CDA; describes several variations to the procedure that

provide many of the noise and fuel benefits of a CDA, but require less airspace; describes CDA

trials; and summarizes the general constraints that will affect widespread implementation of CDAprocedures.

Keywords: Arrival, CDA, Continuous Descent Approach, Continuous Descent Arrival,

Emissions, Environment, Noise, Profile Descent, RNAV, Vertical Profile


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Acknowledgements

The authors wish to thank Dr. John-Paul Clark of the Georgia Institute of Technology for

organizing industry workshops on CDA, and publishing the briefings on the Internet. Theseactivities have significantly simplified the task of cataloging the various industry activities related

to CDA. The authors would also like to recognize the efforts of Angela Signore in the production

of this document.


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Table of Contents

Introduction 1

Baseline Definition 1

Variations on the Baseline Definition 2

Summary of Industry Efforts in CDA 2

Constraints on Widespread Implementation of CDAs 16

Conclusions and Summary 20

List of References 22

Glossary 23


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List of Figures

Figure 1. UPS Benefits from CDA Trials 4

Figure 2. UPS Arrival Routes into SDF for 2007 CDA Trials 5

Figure 3. The CIVET Five Arrival to LAX 8

Figure 4. RNAV Runway 30 Approach to LGB 10

Figure 5. Vertical Profile for IAH CDA 11

Figure 6. RNAV STAR at Stockholm Arlanda 13

Figure 7. CDAIA for UK Nottingham-East Midlands Airport 15

Figure 8. Crossing Points of PHL Arrivals and Departures 17

Figure 9. Use of Airspace above Altitude Restrictions 18

List of Tables

Table 1. Summary of CDA Trials 21


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Introduction

The purpose of this paper is to document the state-of-the-art concept known as Continuous

Descent Arrival (CDA), sometimes also called Continuous Descent Approach. At the present

time, there is no single agreed upon detailed description of this concept, but there is a shared high-level idea: an aircraft that is allowed to descend continuously, without interim level segments,

will produce less noise and emissions, and burn less fuel than conventional arrivals in widespread

use today.

Baseline Definition

As there is no single concept for CDA, a baseline definition is proposed for the purposes of thisdocument:

Continuous Descent Arrival: an arrival where an aircraft is cleared to

descend from cruise altitude to final approach using a best-economy

power setting (usually identified as flight idle thrust) at all times. Such

an arrival is continuously descending, except for the provision of

momentary level segments used to slow aircraft without need to change

thrust settings (e.g., to meet the 250 knot restriction at 10,000 feet

altitude). At final approach, thrust may be added to permit a safe,

stabilized approach speed and flap configuration down a glideslope to

the runway.

This definition of CDA provides the most noise, fuel, and emissions benefits from the aircraft.

However, it does not specify how the descent trajectory is calculated or coordinated, or how

streams of aircraft that perform CDAs are managed. These details are where many of thedifferences among concepts are manifest.

In particular, it does not assume the presence of an airline or Air Navigation Service Provider

(ANSP) metering, sequencing, or spacing capability to manage multiple arrivals. (An example ofsuch an airline capability includes United Parcel Service of Americas (UPS) Airline-Based En

Route Sequencing and Spacing [ABESS] tool; an example of an ANSP capability is the Federal

Aviation Administrations [FAA] Traffic Management Advisor [TMA].)

Another concept that is not included in the baseline definition of CDA is the presence of any flight

deck capability used to ensure efficient spacing at the runway threshold, such as a Flight

Management System (FMS) Required Time of Arrival (RTA) mode, or Airborne Separation

Assurance System capability as proposed by UPS.


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Variations on the Baseline Definition

Several industry activities being performed under the umbrella of CDA do not technically meet

the above definition of CDA if applied strictly; rather, they are slight variations on this definition.

As these concepts are intended to provide the same benefit as CDA, these variations are discussedhere.

Descent Point Variation

These are CDAs that do not start at cruise altitude, but rather start at some intermediate altitude

(typically around 10,000 feet). These applications do not provide the maximum fuel benefit, but

do provide partial fuel benefit and most of the noise benefit of CDA, especially during lowaltitude flight.

Note that some of these applications are called Continuous DescentApproaches, using the same

acronym CDA. To avoid confusion, the term Continuous Descent Arrival from an IntermediateAltitude (CDAIA) will be used throughout this document in place ofContinuous Descent

Approach.

Non-Idle Descent Variation

This variation of CDA does not use the specific optimum descent profile for each aircraft, but

rather defines a flight profile based on a generic model of the aircraft (or a generic model ofmultiple aircraft). Thus, while some or most aircraft may fly an actual idle descent at the optimum

FMS-calculated economy speed, other aircraft will fly at a non-optimum speed. In some cases,

non-idle thrust or speedbrakes may need to be used to adhere to the designed procedure.

The intent of non-idle descent variants of CDA is to trade off achieving maximum fuel or noisebenefit against the need to reserve large blocks of airspace for aircraft on descent. In many cases,

the majority of the noise and fuel benefit is obtained across the fleet, while reducing the impact of

CDA traffic on crossing traffic streams or departures, and preserving capacity.

Interrupted CDA Variation

This CDA variation is a baseline CDA that has been split into more than one descent segment,

perhaps including a level segment as well. The purpose of this split is to compensate for

imperfect trajectory prediction during the early descent segments, and to provide more flexibilityin the latter descent segments.

Summary of Industry Efforts in CDAThis section of the document will describe the industry activities on CDA known to the authors at

the time of writing, and to compare and contrast the different approaches taken by different

industry groups. These efforts are summarized in Table 1 at the end of the document.


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Louisville Standiford International Airport (SDF) CDAs

This activity is being led by UPS Airlines. It is aNon-Idle Descent Variantof CDA, which uses

an airline-based tool to perform initial sequencing and spacing of aircraft to the Top of Descent(TOD) point, and then relies on flight-deck spacing tools to maintain spacing during descent.

The concept thus far has been applied to UPSs B767 aircraft arriving from west coast airports, at

the end of the arrival bank during the nightly sort at SDF Airport. At this time of night, the trafficenvironment is predominantly UPS, and at the time of arrival most of the arriving UPS aircraft

from other parts of the country have already landed.

During the cruise phase of flight, the ABESS tool, located in the UPS Airline Operations Center,

uses Enhanced Traffic Management System (ETMS) and Automatic Dependent Surveillance -Broadcast Mode (ADS-B) surveillance data to estimate when aircraft will reach the merge fix for

SDF, and the arriving sequence of aircraft.

Based on arrival priority, UPS may issue a speed advisory to the flight crew (via Aircraft

Communications Addressing and Reporting System [ACARS]) to adjust the cruise speed, such

that the spacing between aircraft at the merge fix for SDF is adequate for compression and winds

(e.g., 15 nautical miles [NM]) and in the sequence desired by UPS. Also at this time, the flightcrew is told of the traffic sequence (i.e., which aircraft they will be following on descent). This

spacing activity is accomplished between 750 and 60 miles prior to the merge fix, which is

approximately the location of the TOD Point. Due to the relatively small changes in speed, andthe geographic dispersion of aircraft, these speed advisories are relatively transparent to air traffic

control (ATC).

Once the aircraft reach the merge point, the flight crew is cleared to descend via a pre-defined

Standard Terminal Arrival Route (STAR) with crossing restrictions, and to use a flight deck toolbased on ADS-B to manage their spacing from the aircraft in front of them. This tool uses a

generic model for the B767s descent profile rather than the optimum calculated by the FMS for

each flight, in order to place the aircraft at an efficient spacing at the runway threshold. Thus,while the aircraft fly a descent profile that is close to optimum, some adjustment in speed (and

thus engine power setting) may occur.

Benefits to date are estimated to provide up to a 30% reduction in noise (6 dB at 7.5 and 15 milesfrom the runway), a 34% reduction in NOx emissions, and 250-465 lbs of fuel per flight. Note

that some of the noise, NOx and fuel savings are a result of shorter routes when flying the CDA

procedure compared against baseline routing. (The CDA path was more direct to the runway, seeFigure 1.)


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Figure 1. UPS Benefits from CDA Trials [1]

Activities in 2007 will include redesigned arrival procedures to multiple runways (shown inFigure 2), and to begin the necessary activities to allow for multiple aircraft types to fly the

arrivals. Note that UPS foresees that the computations performed by the ABESS tool may be

transferred to the FAA (e.g., in TMA) as the procedure matures, especially if additional carriersparticipate at a single airport.


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Figure 2. UPS Arrival Routes into SDF for 2007 CDA Trials [2]

Louisville/UPS Airlines

Type: Non-Idle Descent Variation CDA

Route: Predefined Area Navigation (RNAV) STAR

Spacing Point: TOD (cruise), Runway Threshold (descent)

Spacing Manager: Airline Operation Center (AOC) tool (cruise), Flight Deck tool (descent)

Notes: Requires Airborne Separation Assurance System Avionics, closed-loop flight-deck

control of spacing (descent)

San Francisco (SFO) Tailored Arrivals

Tailored Arrivals is a concept proposed by Boeing, and is described as a CDA-modified for local

conditions such as traffic or Traffic Flow Management (TFM). As such, any particular TailoredArrival may not strictly conform to the baseline definition (i.e., may be a non-idle descent variant

or an interrupted CDA.) There are two main timeframes for which Tailored Arrivals are being

developed, Near-Term and Long-Term.


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The Near-Term concept is intended to use existing aircraft capabilities such as Future Air

Navigation System 1/A (FANS 1/A). It uses pre-defined arrival routes similar to STARs,including crossing restrictions. This concept is intended to be used in low-density environments

where other traffic is unlikely to interfere with the ability to perform the Tailored Arrival. Trialsto date in the U.S. have used a single arrival (UAL 76) to SFO, scheduled early in the morninglocal time.

The Long-Term concept is intended to use routes that are generated on-the-fly by the groundsystem, using random latitudes and longitudes as waypoints. These waypoints are transmitted via

data link to aircraft. Since the application is targeted for medium-to-high density environments,each route is tailored to the specific aircraft traffic situation. However, as the current ground

system and avionics are unable to handle generation and communication of random routes, this

capability will be a longer-term application requiring significant avionics and ground system

development [3].

As currently performed in trials at SFO, the spacing point to which the traffic stream is managedis the TRACON entry point (BRINY). The ground system predicts when the aircraft will arrive atthe TRACON entry point, given the predefined procedure being used, prior to the aircraft

reaching the TOD. Once a time of arrival is estimated, the ground system uplinks the pre-defined

route of flight to the runway, and a cruise and descent speed. Finally, the latest winds areuplinked to the aircraft just prior to the TOD. The aircraft will then fly the route using the

estimated speeds.

Trials to date have shown that the ground system is able to predict the time of arrival at theTRACON entry point within a mean of less than ten seconds, with a standard deviation of

approximately 20 seconds [4]. It should be noted that this produces a 95% window of

approximately 80 seconds around which the aircraft may arrive at the TRACON entry point,assuming a normal distribution in error. Air Traffic Controllers in todays system are typically

able to deliver aircraft with much better precision (e.g., a window of 5 seconds).

Benefits from the trials at SFO indicate potential fuel benefits of 120 kg 210 kg of fuel per flight

[5], as well as comparable CO2 and NOx benefits, depending on the size of the aircraft. Theseyield annualized benefits that are quite large if the majority of arrivals can be performed using

Tailored Arrivals.

Future plans at SFO include the inclusion of additional carriers and fleet types, and thedevelopment of a ground automation capability that can perform random route generation and

conflict detection/resolution, in addition to trajectory prediction. (Note that Boeing is also

performing trials outside the U.S. with other carriers and Air Navigation Service Providers; resultsand future plans are similar to those for the U.S.)


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SFO Tailored Arrivals/Near-Term:

Type: Baseline CDA (may be non-idle or interrupted if there are traffic issues)

Route: Predefined STARSpacingPoint: TRACON entry point

SpacingManager: NASA En Route Descent Advisor (used by FAA)

Notes: Requires Data Link

SFO Tailored Arrivals/Far-Term:

Type: Baseline CDA (may be non-idle or interrupted if there are traffic issues)

Route: Tailored for each aircraft (random waypoints)

SpacingPoint: TRACON entry point

SpacingManager: NASA Enhanced En Route Descent Advisor (to be used by FAA)Notes: Requires Data Link

Los Angeles International Airport (LAX)/RNAV Profile Descents

Phoenix Sky-Harbor International Airport (PHX)/RNAV Profile Descents

These two airports have developed several procedures that have been published as RNAV

STARs. These are Non-Idle Descent Variations of CDA, in that they have been designed such

that most (~75%) of the aircraft capable of performing CDAs are able to meet the crossingrestrictions using idle thrust. The remaining aircraft must fly off-idle or with speedbrakes

deployed to meet the crossing restrictions.

The LAX procedure has an anchor point designed into the procedure, which provides aconstrained altitude window about 2000 feet high, at 280 knots. This serves to force differentaircraft types into a smaller overall altitude window for the entire procedure, preserving space for

streams of crossing traffic. Constraining the overall altitude window along the approach makes

the procedure viable in the high-volume LA Basin environment (see Figure 3). Note that sincethis procedure is defined as a regular STAR rather than as a special procedure, controllers will still

intervene as necessary to ensure separation and spacing.

Controllers in the LA Air Route Traffic Control Center (ARTCC) have been given a look-up table

with recommended spacing at the handoff point between ZLA and Southern California TRACON

(SCT), based on the worse-case spacing required between aircraft of different wake vortex

categories, and for various wind conditions at altitude. These tables have been computed to assure

that required separation will be maintained between successive aircraft throughout the arrival. Inthe future, these tables may serve as a basis for making modifications to the TMA.


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The CIVET Five Arrival to LAX contains vertical constraints that allow for a variety of aircraft to descend via a CDA [6

Figure 3. The CIVET Five Arrival to LAX


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Findings by airspace specialists at SCT suggest that CDAs can have a positive impact on overall

procedure design by making aircraft profiles more consistent, with the understanding that certaindesign pitfalls need to be avoided. These include consideration of the following:

Aircraft at flight idle have limited ability to show further

Aircraft at the upper end of the CDA altitude window have limited ability to increasespeed

Handling multiple CDA paths requires additional flow planning

All aircraft do not have the same performance

Some aircraft have more predictable performance than others

If care is taken with defining anchor points, then a procedure can be designed that provides an

overall net neutral or positive effect on airport capacity, with environmental benefits only slightly

less than a baseline CDA [6].

LAXs procedure was initially published in February 2006 (CIVET Five), and based on field

feedback is to be republished as the RIIVR arrival with minor revisions. PHX has two procedures

designed (EAGUL and MAIER) that accomplish similar profile descents, and were publishedearly 2007, and other locations are expected to use similar design principles in the future.

LAX/PHX RNAV Profile Descents

Type: Non-Idle Descent CDA

Route: Published STAR

SpacingPoint: TRACON entry pointSpacingManager: Pre-Computed Look-up Table (LAX), Procedural

Southern California TRACON/RNAV Approaches

In addition to the arrival procedure at LAX, SCT is also investigating the use of CDAIA for

several airports in the LA Basin. Noise issues are a significant driver for the local community,

and influence operations at many airports in the basin.

Thus far, the RNAV Required Navigation Performance (RNP) Y approach to Runway 30 at Long

Beach Daugherty Field (LGB) has been evaluated for suitability for a CDAIA, starting at the

KAYOH fix at or below 9000 feet altitude (see Figure 4). This approach is being tested with Jet

Blue A320 and UPS B767 aircraft; preliminary results show that the route is successful insegregating the LGB traffic from other traffic flows. One issue that may prevent extension of the

CDAIA to higher altitudes for LGB is interference with departure traffic at BANDS intersection[7].


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RNAV Runway 30 Approach to LGB is being tested for suitability as a CDAIA [6].

Figure 4. RNAV Runway 30 Approach to LGB

Another procedure at John Wayne Airport (SNA) has been defined, but test results using UPS

B757 aircraft were not available at this writing. As these procedures have been designed asdemonstration projects, there has been limited discussion of spacing management for multiple

aircraft.

At both locations, it was noted that, a complete picture of all traffic patterns must be consideredfor a successful CDA procedure design (including departure and arrival streams from nearby

airports, Class B airspace restriction, etc.) [8].

LGB/SNA RNAV Approaches

Type: CDAIA

Route: Published RNAV (RNP) Approach

SpacingPoint: Not specified

SpacingManager: Not specified/procedural

Houston George Bush Intercontinental Airport (IAH) Continuous Descent Arrivals

The goal of this concept is to design CDAs that can feed dual independent parallel runways at

Houston George Bush Intercontinental Airport (IAH). Note that as designed, this Boeing concept

is not a Tailored Arrival.


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The design approach is to split the descent into two segments: one from cruise to TRACON entry

(approximately 10,000 feet), and one from TRACON entry to final approach (see Figure 5).Thus, this arrival can be called an Interrupted CDA. Both segments are provided with multiple

pre-defined paths, to allow for absorption of different amounts of delay while on descent, forcompatibility of the procedure with a medium to heavy traffic environment.

Note level segment at 10,000 feet.

Figure 5. Vertical Profile for IAH CDA

The first CDA segment, from cruise to TRACON entry, is designed to be performed at idle thrust.

This segment is followed by a short level segment, to compensate for differences in the idledescent trajectory caused by inaccurate wind data in the FMS and Flight Technical Error, and to

allow for the option of a closer-in reselection of TRACON flight paths for flexibility [9].

The second descent segment is along a 2 degree flight path angle, and thus is a Non-Idle DescentVariant of CDA. This provides for glideslope intercept at final approach that is consistent with

standard operational practices (e.g., altitude separation at turn to final, flap schedule, landing gear

configuration, etc.).

Ground automation is proposed as a means to choose paths (especially through the TRACON) toprovide sequencing and separation under medium to heavy traffic demand.

Preliminary simulation results provide estimates of benefits for a B737-700W aircraft of 17 lbs of

fuel (2.1%), and 24 seconds shorter flight time over standard operations.


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At this point in time, activities at IAH are still in the conceptual planning stages.

IAH CDA

Type: Interrupted CDA, using baseline CDA and Non-Idle Descent CDA segments

Route: Multiple pre-defined RNAV procedures (path options)

Spacing Point: TRACON entry point, Runway Threshold

SpacingManager: Ground automation

Notes: Data Link not required

Stockholm Arlanda International Airport Green Approaches

Activities since October 2005 at Arlanda International Airport in Stockholm are part of an effortto develop Green Approaches that use CDA concepts from TOD. Unlike other concepts

discussed to this point, this concept uses capabilities of the aircrafts FMS much more extensively[10].

The FMSs calculated trajectory along a pre-defined arrival route (see Figure 6) including

estimated time at the runway is sent via data link to a ground automation system. This ground

system uses the trajectory prediction as a baseline; the system will propose a RTA via datalinkback to the aircraft FMS, including small changes to the arrival time if necessary for spacing.

The flight crew will then assess whether the proposed RTA is achievable using the FMS, and if

so, will downlink the revised trajectory prediction for meeting the RTA. At this point, the aircraft

will actively manage its trajectory to meet the RTA time. As such, the CDA may require use ofnon-idle thrust at points during the descent.

As of September 2006, 250 approaches have been flown. Without using the FMS RTAcapability, the aircrafts prediction of arrival at the runway threshold was 3 minutes. With RTAcapability, the aircraft were able to meet a window of 5 seconds [11].

Other operational issues related to the design of the arrival procedure have been identified (e.g.,excessively slow speeds at high altitudes, long path lengths), but these are expected to be

addressed through revised procedures.

Stockholm Green Approaches

Type: Non-Idle Descent CDA

Route: Predefined arrival procedure

SpacingPoint: Runway Threshold

SpacingManager: Combination of Ground Automation and Aircraft FMS

Notes: Data Link needed for trajectory negotiation, RTA capability required on FMS, closed-

loop flight-deck control of arrival time


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RNAV STAR at Stockholm Arlanda is being evaluated for Green Approaches [11].

Figure 6. RNAV STAR at Stockholm Arlanda


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UK Nottingham East Midlands CDA

This trial is an idle thrust CDA from an Intermediate Altitude, involving several aircraft types(B757-200F, MD11F, B767-300F, and A319) at Nottingham-East Midlands Airport (NEMA).

Trials have been conducted between May and November 2006 to show proof-of-concept and to

estimate noise and fuel consumption benefits.

Two RNAV approaches have been defined, starting at approximately 9000 feet (entry to NEMA

terminal airspace; Figure 7 shows one of the approaches). A preliminary comparison of fuel burn

for the procedure shows a reduction of 35 kg (9%) for B757 aircraft, and 150 kg (20%) for MD11aircraft, vs. baseline runs of the same aircraft on conventional approaches. In addition, an

estimated 6dB reduction in peak noise (i.e., from ~67dB to 61dB) has been measured 11 NMfrom the runway threshold for A319 aircraft [12].

Spacing results have not been reported as part of this trial.

UK Nottingham-East Midlands Airport CDA

Type: CDAIA

Route: Predefined RNAV approaches

Spacing Point: None indicated

Spacing Manager: None indicated

Other Proof-Of-Concept Trials

There are several proof-of-concept flight trials involving CDAs that will be performed in 2007.

Delta Airlines will be conducting CDAs for small number of flights into Atlanta Hartsfield-Jackson International Airport (ATL), using a procedure based on the ERLIN arrival, on a trialbasis. These trials will occur during night operations starting in April 2007. The intent of this trial

is to gather information on fuel burn and aircraft descent profiles to assist with the refinement of a

CDA arrival procedure. This effort is a prelude to a larger trial anticipated for the 2008/2009

timeframe, which will use an airline-based spacing tool to manage spacing at the TRACON entrypoint [13].

The Atlanta effort is also expected to contribute to a Joint Program Development Office (JPDO)project to define 2015 and 2025 operations concepts for the Atlanta Metropolitan Area. These are

intended to be holistic concepts, and thus will include concepts such as Equivalent Visual

Operations or Super Density Operations in addition to CDA.

Northwest Airlines will be performing single-aircraft CDAs into Fargo/Hector InternationalAirport during night hours. The intent of these flight trials will be to estimate potential fuel

savings.


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Figure 7. CDAIA for UK Nottingham-East Midlands Airport

Other Proof-Of-Concept Trials

Type: Baseline CDA (Proof-Of-Concept only)

Route: Predefined arrival procedures

Spacing Point: none (future ATL TRACON entry point)

SpacingManager: none (future ATL airline based ground automation)


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Constraints on Widespread Implementation of CDAs

(adapted from [14])

The practical application of CDA during medium to heavy traffic conditions will be limited byseveral factors.

First, depending on the exact variation of CDA employed, there may be a larger or smaller

population of aircraft that are equipped to execute the proposed application that is, are equippedwith Data Link, Cockpit Display of Traffic Information (CDTI), RTA, or other capabilities.

However, even in the most basic form, many existing aircraft that are flying today do not have

FMSs that are capable of calculating continuous descent profiles. Instead, these aircraft computeearly descents with level segments to ensure adherence to crossing restrictions (these aircraft are

sometimes referred to as dive and drive aircraft.) With careful procedure design, such as those

implemented at LAX, this factor can be mitigated to some extent.

Second, for aircraft that are arriving to a single airport, there is an uncertainty associated with thearrival of aircraft performing CDAs that must be incorporated into the overall airport operation.

For example, in the SFO Tailored Arrival trials, a standard deviation of 20 seconds has been

demonstrated in the prediction of arrival time at the TRACON boundary from the top of descent.

This translates to an 80 second window of uncertainty (at 95% confidence) that must be protectedfor the traffic, assuming a normal arrival error distribution. This in turn implies that the TRACON

must have the flexibility to move other arriving aircraft around the CDA arrival if landing

throughput at the runway is not to be lost.

In addition to these factors, there are two others: the presence of crossing traffic, and the presence

of co-linear traffic that is subject to different speed requirements.

Crossing Traffic

In the current system, it is most expeditious to separate crossing flows of traffic by altitude.

Where flows must be crossed, the altitude flexibility that makes CDA effective may need to berestricted, so CDA applications may be limited. Crossing flows of concern are different in the

different altitude strata. Below 14,000 feet, departure flows from the destination airport and

arrival and departure flows from satellite airports are most likely to cross the approach. Above14,000 feet, the primary concern is overflight traffic to other, unrelated airports.

Low Altitude Example: Crossing Traffic at Philadelphia International Airport (PHL)

Many conventional approaches to Philadelphia involve two or more extra turns before thedownwind and base legs. Philadelphia approach control airspace is wedged between New YorkTRACON to the northeast and Potomac TRACON to the southwest, so arrival and departure

operations must share a limited space. Overflight traffic on Victor airways creates additional

complexity. Aircraft descend rapidly, with level segments at 4,000, 6,000, or 7,000 feet.


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The reasons for the level segments are visible in Figure 8, which shows radar tracks1 for PHL

arrival and departure traffic. Arrival tracks are shown with grey lines, departure radar returns areshown with red crosses. The airport is in west configuration, with arrivals to Runways 27R, 26,

and 35. Departures are using Runways 27L and 35. The three most important crossing situationsfor Runway 27 arrivals are marked with black circles.

Figure 8. Crossing Points of PHL Arrivals and Departures

High Altitude Example: Arrivals to the New York/Philadelphia Airports

Arrivals to New York and Philadelphia begin to step down from their cruise altitude as far as 200NM from their destinations. In situations where the low altitude airspace permits CDAIAs, the enroute traffic flows determine whether the approach can be extended upward and outward to create

a continuous-descent arrival.

Figure 9 shows that the airspace in the New York/Philadelphia area is tightly constrained. Each

point where an arrival altitude restriction exists in the proposed future New York airspace design

1PHL TRACON Automated Radar Terminal System (ARTS) data for August 18, 2006.


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is marked with a black triangle. Traffic in the vicinity (except arrivals to modeled airports and

low-altitude flights beneath them) is compared to the altitude restriction.

Crossing Traffic above NY/PHLaltitude restrictions by

0-2,000 ft2-4,000 ft

Crossing Traffic above NY/PHLaltitude restrictions by

0-2,000 ft2-4,000 ft

Figure 9. Use of Airspace above Altitude Restrictions

Where the 25th

percentile of the crossing traffic altitudes is less than 2000 feet above the altituderestriction, the point is marked red. If there is a gap of 2000 to 4000 feet between the restriction

and the 25th percentile of crossing altitudes, the point is colored orange. If the crossing traffic is

more than 4000 feet above the arrivals, the point is not marked.

Opportunities for raising restriction altitudes would be seen as black triangles without nearby redpoints. These locations are few. Where altitudes are not tightly constrained from above such as

southwest of DuPont VOR (DQO) or west of Wilkes-Barre VOR (LVZ) there are other placescloser to the destination airport where crossing traffic requires the altitude restriction in the

preferred alternative. To lift one altitude restriction while leaving restrictions before and after thatpoint on the flights path would not facilitate creation of a CDA.


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Collinear Traffic

Most airways are used by flights to many different destinations. An en route controller separates

aircraft in trail along the airways, matching speeds between flights at the same altitude to createorderly flows of traffic. Once a flow has been set up, the aircraft will maintain proper separationbecause their speeds are the same. En route controllers are frequently called upon to put extra

spacing between two arrivals to the same airport for flow management purposes; they accomplish

this by putting the appropriate number of aircraft bound for other airports (if such flights are in the

sector) between two flow-managed aircraft. Since the operating conditions at the differentdestinations are unrelated to each other, it is common for several different spacing requirements tobe in force at any given moment. For example, the controller may need to space PHL arrivals 20

miles apart while EWR arrivals must be 10 miles apart and Teterboro Airport (TEB) arrivals can

flow freely. This is a highly complex situation. More than one controller shares the responsibility

for spacing in situations like this.

In the current system, where arrivals step down early, the different destinations can be stratified byaltitude. That is, flights bound for the closest airport are put at the lowest altitude stratum; flightsbound for the furthest airport are kept highest, and so forth. When the traffic is arranged this way,

the airspace can be split into air traffic control sectors by altitude. The controller of the low-

altitude sector has responsibility for spacing aircraft to the closest destination; the traffic to otherairports is handled by other controllers in higher sectors.

When aircraft are cleared on CDA at cruise altitudes, the controllers job becomes more complex

in three ways. First, the necessary spacings between aircraft are more complicated. Waketurbulence separation on final approach is not typically a problem for en route controllers in

todays system because the spacing can be increased in approach control airspace if necessary.

Aircraft on CDA are not to be maneuvered by approach controllers, so spacings that protect waketurbulence separation must be applied in the en route airspace. Second, altitude stratification is no

longer possible after the CDA begins. Third, aircraft speeds are no longer at the controllersdiscretion. Instead of matching the speeds of the aircraft in front and behind, the aircraft speed is

set for efficient descent (with the notable exception of the UPS CDA application at Louisville

which uses CDTI for spacing along the descent path).

A single controller is now responsible for making a single, well-separated flow out of aircraft

which may have high speeds to compress spacing to one airport, mixed with others that need low

speeds for delay absorption at another airport. Before CDA can be used to more than one airport,it must be proven that this situation is never severe enough to present the en route controller with

an insolvable problem.


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Conclusions and Summary

While the industry has not yet coalesced around a common definition or procedure for Continuous

Descent Arrivals, the basic concept of using an idle or near-idle descent shows promise for

reducing fuel burn, noise, and emissions. Trials to date have shown benefit during non-peakhours, and work is underway to demonstrate the concept during more heavy traffic conditions,

with a wider variety of aircraft types and with a larger percentage of aircraft that are performing

CDAs.

As described above there are many issues surrounding the widespread implementation of CDAs

and its affect on airspace and airport throughput. The FAA has requested that The MITRE

Corporations Center for Advanced Aviation System Development (CAASD) use its modelingand simulation capabilities to investigate the impact of broader use of CDAs. CAASD is

modeling the effect of CDAs during times with higher traffic levels, with a mix of aircraft types

and capabilities, with multiple arrival fixes and routes with CDA traffic, and when there aredependencies between arrival flows or between arrivals and departures. The results of this

modeling will provide additional insights on when CDAs or CDAs with variations should be

used.


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Table 1. Summary of CDA Trials

Louisville/

UPS Airlines

SFO

Tailored

Arrivals(Near)

SFO

Tailored

Arrivals(Far)

LAX/PHX

RNAV Profile

Descents

LGB/SNA

RNAV

Approaches

IAH CDAs

Stockhol

Green

Approach

Type

Non-Idle Descent

CDA

Baseline

CDA (subject

to traffic)

Baseline

CDA (subject

to traffic)

Non-Idle

Descent CDA

CDAIA Interrupted CDA, with

Baseline CDA and

Non-Idle Descent

Segments

Non-Idle

Descent CD

Route

Predefined RNAV

STAR

Predefined

STAR

Tailored for

each aircraft

Published

STAR

Published

RNAV (RNP)

Approach

Multiple pre-defined

RNAV procedures

(path options)

Pre-defined

Arrival

Procedure

Spacing

Point

Top of Descent

(Cruise), Runway

Threshold

(Descent)

TRACON

entry point

TRACON

entry point

TRACON entry

point

Not specified TRACON entry point,

Runway Threshold

Runway

Threshold

Spacing

Manager

AOC Tool

(Cruise), Flight

Deck Tool

(Descent)

NASA En

Route

Descent

Advisor

NASA

Enhanced En

Route

Descent

Advisor

Pre-Computed

Look-up Table

(LAX),

Procedural

Not specified /

procedural

Ground automation Ground

Automation

Aircraft FM

(negotiated)

Notes

Requires Airborne

Separation

Assurance System

Avionics, closed-

loop flight deck

control of spacing

(descent)

Requires Data

Link

Requires Data

Link

Not a Tailored Arrival Needs Data

for trajector

negotiation,

RTA capabi

closed-loop

flight deck

control of ar

time


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List of References

1. Walton, J., Dramatically Improving Gate-to-Gate Operations, October 2006,http://www.arinc.com/aeec/general_session/gs_reports/2006/presentations/11_Gate_to_GateMontreal.pdf.

2. Walton, J., RNAV/CDA Arrival Design: 2004 Flight Test Trials, Louisville InternationalAirport, January 2006,

http://www.ae.gatech.edu/people/jpclarke/cda/workshop1/Presentations/Day1-Thu19Jan2006/3a.Walton.pdf.

3. Mead, R., Tailored Arrivals Overview, Boeing 2007 Tailored Arrivals Symposium,March 2007, Seattle, Washington.

4. Coppenbarger, R., Oceanic Tailored Arrivals: Project Overview, Boeing 2007 TailoredArrivals Symposium, March 2007, Seattle, Washington.

5. Peake, R. and G. McDonald., Airservices Australia: Boeing Tailored ArrivalSymposium, Boeing 2007 Tailored Arrivals Symposium, March 2007, Seattle, Washington.

6. White, W., Optimized Profile Descent Design, November 2006,http://www.ae.gatech.edu/people/jpclarke/cda/workshop4/white.pdf.

7. Wat, J., SNA & LGB Continuous Descent Arrival Demonstration Project, November2006, http://www.ae.gatech.edu/people/jpclarke/cda/workshop4/wat.pdf.

8. Ibid.

9. Tong, K., Continuous Descent Approach Design for Independent Dual RunwayOperation at IAH, April 2006,http://www.ae.gatech.edu/people/jpclarke/cda/workshop2/presentations/Tong.pdf.

10.Klooster, J., FMS Considerations in CDA Design, November 2006,http://www.ae.gatech.edu/people/jpclarke/cda/workshop4/klooster.pdf.

11.Manzi, P., Green Approaches in NUP 2+, September 2006,http://www.ae.gatech.edu/people/jpclarke/cda/workshop3/presentations/Manzi.pdf .

12.Reynolds, T, Advanced Continuous Descent Approach Activities at Nottingham EastMidlands Airport, UK, September 2006,

http://www.ae.gatech.edu/people/jpclarke/cda/workshop3/presentations/Reynolds.pdf .

13. Nagle, G., and T. Staigle, Project ATL, September 2006,http://www.ae.gatech.edu/people/jpclarke/cda/workshop3/presentations/NagleStaigle.pdf .

14.Boan, L., A. Cooper, et al., Operational Analysis of Mitigation of the NY/NJ/PHLAirspace Redesign, April 2007, MP 070049, McLean, VA.


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Glossary

ABESS Airline-Based En Route Sequencing and SpacingACARS Aircraft Communications Addressing and Reporting System

ADS-B Automatic Dependent Surveillance - Broadcast Mode

ANSP Air Navigation Service Provider

AOC Airline Operations Center

ARTCC Air Route Traffic Control Center

ARTS Automated Radar Terminal System

ATC Air Traffic Control

ATL Atlanta Hartsfield-Jackson International Airport

CAASD Center for Advanced Aviation System Development

CDA Continuous Descent Approach

CDAIA Continuous Descent Arrival from an Intermediate Altitude

CDTI Cockpit Display of Traffic Information

ETMS Enhanced Traffic Management System

FAA Federal Aviation Administration

FANS Future Air Navigation System

FMS Flight Management System

IAH Houston George Bush Intercontinental AirportJPDO Joint Program Development Office

LAX Los Angeles International Airport

LGB Long Beach Daugherty Field

NEMA Nottingham-East Midlands Airport

NM Nautical Miles

PHL Philadelphia International Airport

PHX Phoenix Sky-Harbor International Airport

RNAV Area Navigation

RNP Required Navigation Performance

RTA Required Time of Arrival

SCT Southern California TRACON

SDF Louisville Standiford International Airport


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SNA John Wayne Airport

STAR Standard Terminal Arrival Route

TEB Teterboro Airport

TFM Traffic Flow Management

TMA Traffic Management Advisor

TOD Top of Descent

TRACON Terminal Radar Approach Control

UPS United Parcel Service of America

Documents

CDA Paper Hahn and Hoffman