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PAIRED APPROACH FLIGHT DEMONSTRATION RESULTS Kenneth Leiden, Mary Ellen Miller, Curt Kaler, Jon Cunningham, Mosaic ATM, Inc., Leesburg, VA Rick Berckefeldt, Bill True, Jason McMahon, Honeywell, Olathe, KS; Redmond, WA; Phoenix, AZ

Paul Harrison, Alaska Airlines, Huntington Beach, CA Rocky Stone, United Airlines, Panama City, FL

Mark Palmer, Federal Aviation Administration, Washington, DC Chris Lawler, Cavan Solutions, Washington, DC

Abstract

To increase capacity to closely spaced parallel runways (CSPR) during instrument meteorological conditions (IMC), the Paired Approach procedure has been developed. The Paired Approach procedure is a part of Interval Management (IM), which leverages automatic dependent surveillance-broadcast (ADS-B) Out for the lead aircraft (referred to as the Target aircraft) and ADS-B In for the trail aircraft (referred to as the IM aircraft). During the Paired Approach procedure, the IM aircraft uses speed commands from the flight-deck IM (FIM) equipment to maintain precise spacing between itself and the Target aircraft. The IM flight crew is given a time-based spacing parameter referred to as the assigned spacing goal, which is the desired spacing behind the Target aircraft. The desired spacing between the IM and Target aircraft in a Paired Approach operation is the spacing that will keep the IM aircraft far enough behind the Target aircraft to avoid the risk of a collision while still being close enough to the Target aircraft to avoid an encounter with its wake.

The FAA successfully conducted a Paired Approach Flight Demonstration at San Francisco International Airport (SFO) and Tucson International Airport (TUS) in February, 2019. Honeywell developed the prototype avionics and Alaska Airlines, United Airlines, and Honeywell provided aircraft during the demonstration. Fourteen PA procedures (seven at SFO and seven at TUS) were performed. Overall, the FIM spacing performance was excellent: the mean spacing error at the planned termination point (i.e. the final approach fix, where closed loop spacing control ended) was -2.26 seconds, with a

1 The airport acceptance rates shown in this figure for visual approaches, Simultaneous Offset Instrument Approach (SOIA), single runway, and .JO 7110.308C operations are the current rates identified at http://www.fly.faa.gov/ois/. The Paired Approach

standard deviation of 1.15 seconds. This compares favorably with a requirement of ± 7 seconds with a 95% confidence interval. The IM pilot operational acceptability of the FIM speed guidance was rated favorably.

Background The Paired Approach (PA) procedure has the

potential to more closely emulate the throughput that is achievable using visual approaches to CSPR (see Figure 1). The PA procedure requires the lead aircraft (referred to as the Target aircraft) to be ADS-B Out equipped, while the trail aircraft (referred to as the IM aircraft) must be equipped with ADS-B In and FIM avionics[1].

Figure 1. Anticipated Arrival Capacity (per hour)

for Paired Approach Compared to Other Procedures Utilized at SFO [2].1

The FIM avionics support the flight crew’s task of precisely spacing the IM aircraft between the collision hazard area and the wake hazard area as

value of 46 is referenced in [2], but is based on different assumptions than the current concept so the value is subject to change.

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shown in Figure 2. Staying in this region (denoted the normal operating zone (NOZ) in Figure 2), mitigates the collision and wake risks associated with the PA procedure. The desired distance behind the Target aircraft that will ensure the IM aircraft is within the NOZ under nominal conditions is called the assigned spacing goal (ASG), and will be determined for each aircraft pair using Air Traffic Control (ATC) ground automation dependent on how the full concept is implemented.

Figure 2. Overview of Paired Approach Procedure.

A significant amount of work has been done to develop prototype FIM avionics using the DO-361 Minimum Operational Performance Standards (MOPS) that was developed by RTCA Special Committee 186 (SC-186) Working Group 4 (WG-4) [3]. The Air Traffic Management (ATM) Technology Demonstration 1 (ATD-1) activity conducted by NASA in 2017 developed prototype IM avionics for aircraft merging into a single stream [4]. The results of the successful ATD-1 flight tests indicated the prototype avionics may be updated to included PA-specific functionality.

The PA Flight Demonstration project has been conducted over a multi-year period and broken up into three phases; planning, development, and execution. This paper presents outcomes of the third phase, which includes the conduct of the Flight Demonstration, data gathering activities, and analysis.

Paired Approach Operational Concept The PA procedure enables dependent approaches

to CSPRs by utilizing FIM avionics to achieve precise inter-aircraft spacing. In the PA procedure, one aircraft flies an ILS approach, while the other flies an RNAV approach. Some aspects of the approaches used for PA are site-specific. For instance, at some sites the approaches are both straight-in, while at other sites, such as SFO, one approach is a 3-degree offset approach. PA operations can be used down to ILS Category I minima, but the actual minima are dependent on the type of approaches used in the PA procedure. Although previous work examined delegated separation [1], the responsibility for separation remains with the controller.

The IM aircraft’s FIM avionics compute speed guidance for the IM flight crew to implement in order to capture and maintain the ASG. The ASG represents the desired time-based spacing between the Target aircraft and IM aircraft. The ASG is calculated to ensure that in nominal conditions, the IM aircraft will remain in the safe zone shown in Figure 3. The ASG is determined by ground automation and communicated to the flight crew who manually enter it into the FIM avionics. The IM flight crew is responsible for implementing the corresponding speed guidance from the FIM avionics until the IM aircraft reaches the planned termination point (PTP) (see Figure 3) or otherwise instructed by the controller.

The PA separation standard has a collision safety limit (CSL) and a wake safety limit (WSL) as shown in Figure 3. Both limits represent along-path projections of the required separation measured relative to the Target aircraft. The IM aircraft is safely separated (i.e., protected from collision and wake encounters from the Target aircraft) if it remains in the safe zone between these two limits. The safe zone is composed of a NOZ and two non-transgression zones (NTZs). Although the NOZ and NTZ are useful concepts for determining avionics requirements and performance assessment of PA, they may not play a role in the final PA procedure. For instance, there may not be a requirement for the NOZ and NTZ to be displayed to the controller. Instead, ground automation may display the separation standard and use an alert when it is predicted the IM aircraft may violate the separation standard.

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The ASG is determined by ground automation primarily as a function of Target and IM aircraft planned final approach speeds (PFASs). When an IM aircraft captures the ASG, it is within the NOZ by design. Throughout the operation, the controller must monitor the separation between the Target and IM aircraft closely, and intervene if required to avoid a loss of separation. The FIM avionics will not display CSL, WSL, or either NTZ on the flight crew displays, however, pending further research, this information may be displayed to the controller.

At the PTP (expected to be the final approach fix [FAF] for most situations), the PA procedure and associated FIM speed guidance are terminated. The IM aircraft then slows to final approach speed and follows the instrument approach procedure (IAP) for the remainder of the approach. This segment is referred to as the open-loop segment of the PA concept (see Figure 3). Since speed guidance ends at the PTP, the spacing at the PTP must account for the compression or decompression in spacing that will occur as a result of the difference in final approach speeds between the IM and Target aircraft. For this reason, the IM aircraft’s ASG is dependent on the difference in final approach speeds of the paired aircraft. For example, prior to the PTP, the ASG may place the IM aircraft in the rear of the NOZ for IM aircraft faster than the Target aircraft and in the front of the NOZ for IM aircraft slower than Target aircraft. To keep the IM aircraft in the NOZ during the open-loop segment, the final approach speeds of the Target and IM aircraft must be compatible as determined by ground automation.

Flight Demonstration Topics

Differences between PA Operational Concept and Flight Demonstration

There are several differences between the PA operational concept and the PA Flight Demonstration that were identified in last year’s ICNS paper [5]. These differences are not reiterated here except for the following difference pertaining to flight rules and meteorological conditions:

Operational Concept: The PA procedure is used under instrument flight rules enabling dependent approaches to CSPRs down to Category I minima.

Flight Demonstration: The PA procedure was conducted in visual approach conditions and both the Target and IM aircraft were given visual approach clearances to ensure the required levels of safety are satisfied. There are three safety issues alleviated by requiring a visual approach clearance for the Target and IM aircraft:

1) The reduced separation standard used in PA is currently only achievable in today's operations via a visual approach clearance, allowing the IM flight crew to assume separation responsibility.

2) The FIM avionics for the IM aircraft are uncertified.

3) Experimental IAPs are flown by both the Target and IM aircraft.

Flight Demonstration Overview The SFO flight demonstration was planned for

four trials, one trial per day, with six PA runs being conducted during each trial. Weather and traffic permitting, the trials were planned for consecutive

Figure 3. Key Elements of the Paired Approach Concept.

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days starting on a Saturday (lightest traffic levels). Each trial consists of approximately 2 hours of flying time at approximately 20-25 minutes per run). The PA runs are followed with planned go-arounds except for the last PA run, which concludes with both aircraft landing on their respective runways. The aircraft operator, type, and wake category for both the Target and IM aircraft along with the rationale for including the aircraft pair in the flight demonstration are shown in Table 1.

The demonstration was postponed until late February due to scheduling constraints with ATC facility restrictions. The most significant impact of the postponement was that Honeywell’s Falcon 900 aircraft was not available during the rescheduled time period. Moving the flight demonstration into the March timeframe was not feasible because traffic levels increase with spring break, making it very difficult to not impact commercial air transport arrivals into SFO. Collectively, this forced the

elimination of the original trial #1. A revised flight demo matrix is shown in Table 1.

Experimental IAPs Table 2 summarizes the key characteristics of the

experimental IAPs for SFO. For wake category D/D pair (A320/ B757), the Target aircraft is assigned to the ILS 28L approach regardless of crosswinds. However, for the wake category B/D pair (B777/B757), the Target and IM aircraft may be swapped depending on wind direction and magnitude to ensure that crosswinds are not carrying the Target’s wake toward the IM aircraft’s approach path.

To ensure FIM performance is adequate, speed constraints are required in the Target aircraft IAP. Speed constraints of 190 and 170 kts are inserted along the flight profile, with the 170 kt speed constraint ending at the PTP (see Figure 4). Upon crossing the PTP, the Target aircraft slows to final approach speed so no further speed constraints are needed.

Table 1. Original and Revised Flight Demonstration Matrix with Rationale for Selected Aircraft Pairs.

Flight Demo Trial #

Target aircraft

Target aircraft wake

category IM

aircraft

IM aircraft wake

category Rationale for including in

flight demonstration

1 Honeywell B757 D Honeywell

F900 E

B757 not allowed as lead aircraft in .308 operations at SFO. In addition, Honeywell aircraft pair provide low project risk since these aircraft are dedicated test aircraft -- they do not need to be taken out of revenue service like United or Alaska Airlines aircraft.

2 United B777-200 B Honeywell

B757 D Wake category B aircraft not allowed as lead aircraft in .308 operations.

3 Alaska A319,

320, 321 D Honeywell

B757 D Demonstrate wake pair D/D (most common pairing)

4

Alaska A319,

320, or 321

D Honeywell F900 E Repeat of wake pair from #1, but with Alaska Airlines providing Target

aircraft.

Revised flight demonstration matrix

1

Alaska A319,

320, or 321

D Honeywell B757 D Demonstrate wake pair D/D (most common pairing)

2 United B777-200 B Honeywell

B757 D Wake category B aircraft not allowed as lead aircraft in .308 operations.

3

Alaska A319,

320, or 321

D Honeywell B757 D

This is a repeat of revised trial #1, but with a different range of PFAS values. Only conducted as a back-up if trial #1 or #2 were not deemed fully successful.

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Table 2. Summary of SFO Experimental IAPs Needed to Support PA Flight Demonstration.

Runway 28L Runway 28R ILS PA RWY 28L (nominally used by Target, but can be used IM aircraft for wake category B/D pair depending on crosswinds) ILS with 2.85° glideslope. Includes speed constraints (Target only) and PTP.

IM aircraft IAP: RNAV with 3° lateral offset and 3° glidepath. Includes PTP. Target aircraft IAP: (for B/D wake pair only) RNAV with 3° lateral offset and 2.7° glidepath Includes speed constraints and PTP.

Figure 4. Approach Plate for Runway 28L for Target Aircraft Depicting Speed Constraints.

2 Note that this was also a problem for the old Class B airspace, but the IAP was not modified in that timeframe as there was an

Class B Considerations The SFO Class B airspace redesign was

completed in August, 2018. The new airspace posed a problem only for the Target aircraft IAP for RNAV RWY 28R with the shallower 2.7° glidepath (needed for wake category B/D pair only), which was not fully contained in the new Class B airspace.2 The altitude constraint at BILLT was increased by 200 ft to keep the glidepath within Class B airspace. As a result, the glidepath for the PALLH/BILLT segment is shallower (~2.3°) and the glidepath for the BILLT/LEIDN segment is steeper (~3°). The SFO Class B with respect to the lateral profile of the IAPs is depicted in Figure 5. Figure 6 depicts the vertical profiles of the IAPs, the SFO Class B boundary, and the San Jose (KSJC) Class C boundary.

Figure 5. Lateral Profile of Experimental IAPs

with Class B Airspace Boundaries (ILS RWY 28L in Blue and RNAV RWY 28R in Red).

The IAPs specify that the glideslope/glidepath for the Target aircraft is 0.15° lower than the IM aircraft glideslope/glidepath to mitigate potential wake encounter issues for the approach segment between the PTP and the runway threshold. The difference between the Target and IM aircraft vertical profiles is not linear (see Figure 7). This is due to the RNAV glidepath angle being constant as referenced to local horizontal along the approach based on the earth’s curvature. In contrast, the ILS glideslope is referenced to a fixed horizontal plane at the glideslope equipment’s location and thus does not account for the earth’s curvature. Although it is not intuitive, the ILS glideslope of 2.85° is higher than the RNAV glidepath of 3° for distances significantly far from the airport (roughly 20 nautical miles [NM]).

expectation that the new Class B boundaries might resolve this issue, but it did not.

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Figure 6. Vertical Profile of the Experimental IAPs with Class B and C Airspace Identified.

Figure 7. Vertical Profiles and Differences Between Vertical Profiles for the Experimental

IAPs.

Traffic Management Impact Study to Support Flight Demonstration Procedures.

During the early stages of the project it was important to reach out and establish a good working relationship with the FAA facilities that were going to host the PA research team, and allow us to conduct the PA Flight Demonstration in their airspace.

Workgroups were formed and numerous meetings were held to discuss procedures and identify each facility’s participants’ roles and responsibilities. A good example of this would be the collaboration that took place to develop the procedures for the South Side Pattern (see Figure 8).

Figure 8. South Side Pattern Used for Multiple Go-Arounds During the Flight Demonstration.

Since one of the primary project objectives was to conduct the PA Demo during a time period that would have the least amount of impact to other SFO arrivals and departures, a Traffic Management Impact study was conducted at the Oakland Air Route Traffic Control Center Traffic Management Unit (ZOA TMU)

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to determine the best course of action to accomplish this.

The resulting procedures included analyzing historical traffic data from both Official Airline Guide (OAG) and Traffic Flow Management System (TFMS), and to also study Flight Schedule Monitor (FSM) traffic data right up until an hour before the PA flights depart (see Figure 9). Slots for each PA aircraft pair were reserved in the arrival sequence by scheduling “Super Heavy Blocked Slot Intervals” at the appropriate times in the Time-Based Flow Management (TBFM) timelines, used for metering. This preserved the integrity of the arrival flows so that other arrivals didn’t compress into the gaps reserved for the PA flights, which also prevented any undue delay to any flights on approach.

Readiness Reviews The Flight Demonstration Readiness Reviews

(FDRRs) are a series of web conference meetings to address a checklist of challenge-response status items to determine if the countdown to fly can proceed. The FDRRs occur at prescribed time instances relative to a scheduled flight demonstration trial date. These time instances included 14 days prior, 3 days prior, day prior, 10 am morning of trial, and preflight. The 3 days prior FDRR was an important decision point as it determined whether the PA research team should travel to SFO based on the weather forecast.

Because weather forecasts were to play such a critical role in the FDRRs, a series of meetings was schedules to dry-run or “walk through” the forecast activities. Meteorologists from the Center Weather Service Units (CWSU) from both ZOA and Albuquerque Center (ZAB) participated in these meetings in early December 2018. The 3 days prior, day prior, and 10 am morning of trial were scheduled as if the trial were to be held on Friday Dec 7. The walk-throughs demonstrated the effectiveness of the series of forecasts.

The FDRRs include the PA research team members, pilots, and FAA personnel from SFO and TUS Air Traffic Control Towers (ATCTs), Northern California Terminal Radar Approach Control (TRACON) [i.e., NCT], Tucson TRACON (i.e., U90) and ZOA TMU, ZOA CWSU, and ZAB CWSU. The status items include weather forecast (to support visual approaches and landing on runways 28L/R), facility staffing, aircraft and equipment readiness, and NOTAMs. A list of FDRR checklist items is shown in Figure 10. This particular checklist is the completed preflight FDRR checklist for Feb. 23, resulting in a Go decision to fly the first trial of the flight demonstration at SFO.

Decisions and Time Sequence Early in the planning process, the PA team had

decided that Saturdays at SFO were the best day of the

Figure 9. FSM Depiction of SFO Arrival Demand and Preferred Time Window for PA Flight Demonstration.

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week for the first trial in the flight demonstration. This is due to Saturdays have the lightest traffic, giving controllers more spacing buffer for merging the PA flights with the normal arrival traffic. The dress rehearsal was successfully conducted at TUS ATCT and TRACON (U90) on Wednesday, Feb 13. The first flight demonstration trial was rescheduled for Saturday, Feb 23 (due to the scheduling conflict previously mentioned).

On Wednesday Feb 20, the 3 day prior FDRR was held and all checklist status items resulted in a Go decision. The one concern was that although the weather forecast was favorable, the forecast was only of medium confidence. The research team decided to have a 2 day prior FDRR to see if the confidence in the forecast would improve. In the meantime, research team members that needed to travel early on Thursday from the east coast proceeded with their travel plans.

Figure 10. Preflight FDRR Checklist for February 23, 2019.

On the afternoon of Thursday Feb 21, the 2-day FDRR indicated the weather forecast was again favorable and also the confidence of the forecast increased to high. The rest of the research team with flights later on Thu or early Fri proceeded with their travel plans to SFO. In addition, Honeywell relocated their B757 from their hangar at PHX to SFO on Friday.

On Friday Feb 22, the day prior FDRR was held with the full PA research team in-person at a FAA conference room at the base of SFO ATCT. This FDRR confirmed the Go decision to fly the first trial.

On the morning of Saturday Feb 23, the PA research team members were dispatched and in place at SFO ATC, NCT, and ZOA TMU, and the FAA Hotline was activated. During the morning FDRR, FSM still indicated that the best time to conduct PA trail #1, with the least amount of impact to other SFO arrivals and departures, was 2330Z (3:30 pm local). As time progressed into the afternoon, FSM continued to update as flights across the NAS departed for SFO. ZOA TMU selected and scheduled the first “Super Heavy Slots” in the TBFM timeline, and the first PA Demo ETA of 2330Z (3:30 pm) was announced during the preflight FDRR. Additional calculations identified the ETD (3:05 pm), staging area arrival times (2:55 pm), and aircraft push times (2:35 pm for Alaska Airlines; 2:45 pm for Honeywell), which were coordinated with the pilots and SFO ATCT personnel.

The communication modes identified in Table 3 were used for research team members and FAA operational personnel to communicate prior to the departures and during the airborne portion of the flight trial. After the PA flights departed, they were vectored into the South Side Pattern and eventually vectored onto their respective approach courses, utilizing the available slots in the approach sequence (see Figure 11). With the most recent sensed winds from the IM aircraft, ASGs were calculated by the Airborne Flight Demonstration Direction (A-FDD) and confirmed by the FDD Assistant on the ground. Traffic ended up being light, which allowed the PA team to coordinate, execute and complete an extra 7th run.

On Sunday Feb 24, the 10 am and preflight FDRRs indicated borderline condition for ceilings with medium confidence (scattered clouds at 3000 ft, and broken clouds at 5000 ft.) The ZOA CWSU meteorologist stated that high pressure from S CA might prevent the clouds over the ocean from pushing into San Francisco Bay in the afternoon. Based on that,

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a Go decision was made to continue with the flight trial preparations. A United Airlines B777 had arrived from Asia via a commercial flight at 1 pm local and was quickly made available for the flight demonstration.

The first arrival slots were scheduled into TBFM, with 2335Z (3:35 pm) coordinated as the first ETA. Unfortunately, prior to the PA flights ever departing, winds developed and pushed an offshore cloud deck into the bay in the vicinity of the RWY 28L/R approaches (although the bay to the east stayed clear). As a result, SFO went from VMC to IMC, resulting in a ground stop and eventually a ground delay program. At that point, trial #2 for Sunday was terminated.

Since the extended forecasts at SFO for the following days were unfavorable, the PA Demo was moved to Tucson, the coordinated back-up airport, where the weather forecast was favorable. On the morning of Tuesday Feb 26, the Research Team members were dispatched to both TUS ATCT and U90 (TUS TRACON), where a PA Demo start time of 1330Z (7:30 am local) was coordinated with the flight crews. The United Airlines B777 aircraft departed from SFO at 6:36 am (delayed due to weather) and the Honeywell B757 departed from PHX at 7:32 am. Both aircraft arrived in TUS airspace at 8:00 am, at which point the PA flight trial #2 commenced.

Table 3. Modes of Communication to Support PA Flight Demonstration. Communication Mode Audience/access/restrictions

Hot Line Anyone with hot line number and access to a phone. The hot line will be opened on days where flight trials are planned beginning with the 10 am morning of trial FDRR through the post-trial recap meeting.

VHF Air-Air Target and IM flight crew, Airborne Flight Demonstration Director (A-FDD), other airborne-based research team members.

Frequency: Primary - 128.65 (TWR backup) Plan B - 131.4 (UAL Ramp backup) Plan C - 123.45 (Common freq)

GoToMeeting Primary mode for all FDRRs.

FAA Voice Frequency Communication between controller and pilot.

Landlines (VSCS) for ATC communication

Used primarily for TBFM slot from ZOA TMU to NCT TMU to ATC Coordinator and out to Research Team.

Airborne Internet Honeywell 757 and United 777 fleet have satellite Internet. A-FDD and airborne-based research team members can communicate with ground-based research team via email. Email will be the primary mode of communication between the FDD and A-FDD.

Mobile (voice/SMS) Use of mobile phones are not allowed in operational areas of FAA facilities. Mobile phones permitted for airborne use if there is an operational need. Thus, this is a backup means of communication between airborne-based and ground-based research team members.

ACARS uplink/downlink Used for FDD/pilot comm. Backup if there are Internet problems preventing FDD/A-FDD comm.

Figure 11. Alaska Airlines A320 and Honeywell B757 on Final Approach During PA Trial #1 at SFO on Feb 23, 2019.

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Since arrival/departure volume at TUS is quite low from 7-9 am, and since TBFM is not used at U90, the controllers vectored the PA flights onto the approach course utilizing the numerous slots available in the arrival flows. The weather and traffic cooperated at TUS enabling the pilots to get 7 runs in on Tuesday, completing a second successful PA Demonstration.

FIM Performance Summary As described above, the first demonstration day

was conducted at SFO with the Honeywell 757 executing paired approach spacing behind the Alaska Airlines A320 and the second demonstration day was conducted at TUS with the Honeywell 757 spacing behind the United Airlines 777. A total of 7 approaches were performed each day, with the SFO results summarized in the left half of Table 4 and the TUS results summarized in the right half.

Table 4 shows the key parameters for each run. The ASG was calculated for each run based on the planned final approach speeds for the Target and IM aircraft and the current wind conditions. The spacing at initiation term refers to the longitudinal spacing that was achieved between the two aircraft at the first point when the system could be engaged displaying AVAILABLE on the human-machine interface (HMI). A range of 1.5 – 2.5 NM has been specified for the demonstration flights. This is also referred to as the controller initiated spacing objective (CISO). The Measured Spacing Interval (MSI) at initiation shows the initial spacing between the two aircraft. The difference between this MSI and the ASG determines the amount of 'Capture' required and is shown as the spacing error at initiation (negative values indicate that ownship is too close to the traffic, positive values too

far away from the traffic). The distance available for the Control Law to capture spacing is reflected by the Closed Loop Length; note that this distance is measured to the PTP, not the runway threshold. The Final Spacing Error records the difference between the MSI and the ASG, taken when ownship crosses the PTP to terminate the clearance. The 7 second spacing performance required for FIM-PA operations is to be evaluated against this spacing error value.

The continuous spacing performance for all 14 runs is shown graphically in Figure 12. This chart shows the initial spacing error (MSI – ASG) for each run and the evolution of the error against distance to go (DTG) to the planned termination point. The graph should be read right to left, with the initial spacing shown near the right hand side of the plot and the final spacing shown at DTG = 0. This point corresponds to the planned termination point and the start of the open loop portion of the procedure. For both SFO and TUS, the PTP was 6.25 NM from the runway threshold.

For San Francisco, the controllers were asked to achieve 2.0 ± 0.5 NM spacing at the beginning of each procedure; for Tucson, a combination of controller vectors and pilot speed discretion was used to achieve the same spacing. The perfect spacing at initiation (MSI = ASG) varies from run to run and depends on ASG, wind speed and direction and the history of traffic airspeed over the initial segment of the approach course. Prior to the demo, an analyses was performed on the sensitivity of the initial spacing to these variables and 2 NM was selected as the best compromise for the range of expected conditions for the demo.

Table 4. Summary Data for All Runs.

Day 1 KSFO - A320/757 Day 2 KTUS - 777/757 run1 run2 run3 run4 run5 run6 run7 run1 run2 run3 run4 run5 run6 run7 Assigned Spacing Goal (s) 27 28 32 32 39 33 30 23 24 27 27 31 30 30

Spacing at Initiation (NM) 2.27 1.86 1.64 1.19 2.25 2.13 1.68 2.39 0.06 0.72 1.66 2.39 0.95 2.85

Measured Spacing Interval at Initiation (s) 40.33 37.64 30.20 21.29 41.25 39.67 31.40 42.44 0.87 12.27 26.76 38.75 15.02 45.51

Spacing Error at Initiation (s) 13.33 9.64 -1.80 -

10.71 2.25 6.67 1.40 19.44 -23.13

-14.73 -0.24 7.75 -

14.98 15.51

Closed Loop Length (NM) 18.15 18.55 20.43 16.62 16.75 18.00 18.57 24.89 16.83 24.70 24.76 25.27 24.00 24.61

Final Spacing Error (s) -3.32 -1.09 -3.09 -3.34 -3.65 -0.53 -3.04 -1.68 -1.34 -2.41 -3.92 -1.99 -0.51 -1.77

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Figure 12. Spacing Performance for All Demonstration Runs.

Figure 12 and Table 4 show that in general the San Francisco runs were shorter but the initial spacing was slightly better controlled. However in all cases at both airports, capture rates and closed loop lengths were sufficient for the initial errors to be corrected.

Overall, the spacing performance was excellent: the mean spacing error at the planned termination point (i.e. the final approach fix, where closed loop spacing control ended) was -2.26 seconds, with a standard deviation of 1.15 seconds. This compares favorably with an assumed requirement of ± 7 seconds with 95% confidence interval [6].

In addition to the summary statistics shown above, each run was analyzed using a standard set of graphical tools. Only one run is analyzed here, however the complete set of data for all 14 runs will be published in PA final report in May, 2019.

Figure 13 shows a representative run from SFO. The upper graph shows the IM aircraft measured calibrated airspeed (CAS) and the commanded speed from the PA avionics. The lower graph shows the evolution of the spacing as a function of the distance to go to the PTP.

While initial spacing was nearly perfect (CISO = 1.64 NM and capture amount only 1.8 seconds), spacing deteriorated after the start of the clearance because ownship came into the procedure vectored at a higher speed than the traffic (the traffic was following the profile speed of 190 knots while the IM aircraft entered the procedure at approximately 210

knots). Since there was so little spacing error at the beginning of the procedure, the first speed command to the IM aircraft was just to fly the same speed as the traffic.

However, since the IM aircraft had started at a higher speed and couldn’t instantaneously slow down, after about 2.5 NM the error had built up enough that a speed command to reduce speed by an additional 10 kts was issued to restore spacing. After spacing was fully closed, ownship was commanded to rejoin the nominal speed. The final sequence of deceleration commands was issued to follow the profile. Final spacing was -2.41 seconds, and a total of 4 speed commands were used.

Although this example did not have a significant initial spacing error, there was a speed difference between the Target and the IM aircraft to correct. A similar analysis of runs with larger initial spacing errors shows that the general form of the speed commands and spacing in this example is typical for both speed difference and spacing errors: the control law issues a speed command different from the base profile to null the error, the IM aircraft implements the speed command and reduces the error, then the control law returns the IM aircraft to the nominal profile. Finally, when the Target aircraft starts the final deceleration, a small series of speed commands are generated to slow the IM aircraft down to the new target speed.

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This example also shows that the error at the end of the procedure is negative, i.e. the IM aircraft is slightly closer to the Target aircraft than the ASG. Figure 12 and Table 4 show that this is a general result: there was a bias towards errors in this direction for all the runs in this demo. The research team attributes this to the deceleration profile (the traffic is always slowing down near the end of the procedure to meet a 170 knot speed constraint at 2.25 NM from the PTP) and a reactive control law that only responds to changes in the spacing rather than predicting the Target’s trajectory.

Pilot Perspective – IM Aircraft Data Entry

The FIM avionics consists of an application hosted on an installed Astronautics NEXIS electronic flight bag (EFB) system that served as the primary pilot interface and display, along with two auxiliary guidance displays (AGDs) positioned in each pilot’s field of view. The AGDs are Apple iPhones connected to the EFB via an onboard wireless network, hosting an application that repeats a subset of the EFB display

elements, including speed commands. The FIM avionics features its own internal navigation database, and it extracts ADS-B data and ownship state data from the IM aircraft’s data busses and surveillance equipment to compute a pairing solution. The database contains the experimental Paired Approach procedures that align with an experimental aircraft FMS navigation database hosting the same procedures. For the Demonstration, a single EFB was installed to the right of the Pilot Monitoring (PM) who handled all EFB data entry and functions.

No EFB was installed for the Pilot Flying (PF) in this configuration. Each AGD was installed over the forward edge of the glareshield in front of each pilot.

To enable pairing, the FIM avionics requires the pilot to manually enter three groups of data: 1) wind information, 2) intended approach procedure, and 3) Target ID, ASG, and Paired Approach clearance type. It is envisioned that data entry can be automated in the future by extracting information from the aircraft flight management computer (FMC) and via ATC datalink.

Figure 13. Speed Command and Performance Details for an Example Run.

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Forecast Wind Data: To optimize FIM avionics performance, descent forecast wind speed and direction is entered for various altitudes (including the surface) along the Paired Approach flight path. Wind data is also required to determine if an adverse crosswind condition exists for a category B/D pairing, which would prompt a runway swap. During the Demonstration, forecast winds were sourced for the surface, 3000, 6000, and 9000 feet AGL prior to flight. These winds were also entered into the ASG calculator to generate preliminary ASG figures for each approach. This wind information was entered into the FIM avionics prior to departure. It was not necessary to reenter the winds for each approach.

Ownship Data: Entry of the specific approach to be flown by the IM aircraft (ownship) is required. For the Demonstration, this was accomplished prior to flight for the first approach, and for each subsequent approach during downwind vectors. The pilot is prompted to enter the destination airport ID, intended runway, and the Paired Approach procedure to be flown. During downwind vectors, the IM crew also loaded and cross-checked the experimental procedure in the FMS, and configured the ship’s avionics to fly the approach.

IM Clearance Data: After entry of Ownship Data, selection of the Paired Approach clearance type (five FIM clearance types are available) is required prior to every approach. Avionics logic ascertains that the Target will fly the adjacent experimental approach

to the corresponding parallel runway. The pilot is then prompted to enter the ASG and Target ID: the Target ID field provides a drop-down list of nearby Target ADS-B IDs (closest Target at the top of the list), but also allows for manual Target ID entry. During the Demonstration, the Target was always shown at the top of the list and was selected.

Upon completion of the above data entry steps, the FIM avionics system is ready to ARM. In the ARM state, the FIM avionics evaluate the track history of both aircraft and wait for pairing criteria to be met. When both aircraft are established on their respective final approach courses and all pairing criteria is satisfied, EXECUTE mode becomes available. Upon selection of EXECUTE, PAIRED mode is active and speed commands are provided on the EFB and AGDs. During the Demonstration, the IM crew selected ARM mode during the final ‘base’ vector and was able to select EXECUTE shortly after joining the final approach course.

Flying the Approach For pilots, the Paired Approach procedure is

novel in that speed adjustments on approach are neither dictated by ATC, nor are they subject to the pilot’s discretion (except in the interest of safety); they are purely reactionary to the state of another aircraft (see Figures 14-15). This entails new considerations and challenges.

Figure 14. United Airlines B777 and Honeywell B757 on Final Approach During PA trial #2 at TUS on Feb 26, 2019.

Honeywell Internal

During the Demonstration, the approach procedure was flown in LNAV/VNAV mode with speed intervention, using autopilot with autothrottle. This is a standard configuration allowing dynamic airspeed control when the aircraft is coupled to a lateral and vertical RNAV approach path. After pairing, the PF continually monitored speed commands on the AGD and set them into the aircraft’s Mode Control Panel until reaching the PTP.

Upon pairing, the FIM avionics can command large speed changes to achieve gross spacing adjustments. It will also react to Target deceleration nearing the PTP, wind shift or shear, or unintended Target speed variation. This can generate relatively high magnitude and high frequency speed changes, including fast/slow speed reversals. Higher magnitude deceleration commands, as well as some speed reversals, were observed during the Demonstration and were managed using the techniques described below.

Speed commands that dictate deceleration can be particularly challenging, as aircraft energy levels on approach may preclude authoritative deceleration using only thrust reduction. Drag devices (i.e. speedbrakes, flap settings beyond approach, and landing gear) may be required sooner on approach, or must be used more assertively, than is typical in line flying. Techniques used in the Demonstration included frequent use of speedbrakes and intermediate flap settings, with assertive power adjustments. The IM pilot operational acceptability of the FIM speed guidance was rated 6 out of 7.

Useful HMI tools to aid the pilots in managing speed control include inverse-video flashing of the speed command when a new speed is issued, and a Fast-Slow Indicator to indicate deviation from ASG and to assess spacing trend information. In addition, 10-knot speed command quantization has been observed to mitigate a high speed command update rate versus 5-knot quantization used in prior engineer flight tests.

During the Demonstration, deceleration to PFAS was accomplished upon crossing the PTP using thrust reduction, as well as speedbrakes for some approaches. Landing flaps were selected when allowed by flap speed limitations. Each approach was flown to its go-around point, at which time the autopilot was disconnected and a go-around was hand-flown in accordance with previously coordinated procedures.

Pilot Perspective – Target Aircraft Flying the ILS as the Target was completely

normal. The only difference was published speeds vs. speeds assigned by ATC. The published speeds actually reduce workload on the flight crew since they know what to expect in advance vs. being given speed assignments from ATC without advanced notice.

There are two procedural items that are new from the Target flight crew’s perspective. The first is a speed deceleration window. With the goal to be at a published speed by a certain point, a published autothrottle (AT) speed is needed, which is not new,

Figure 15. EFB Showing FIM Application (left) and Honeywell Pilot Flying During Trial #1 (right). AGD (displaying “190”) Can Be Seen at Forward Edge of the Glareshield.

Honeywell Internal

but since an AT speed does not specify when deceleration is to begin, a point to begin speed reduction “no earlier than” is desired.

The second procedural item is slowing to final approach speed at the PTP. Normal operations today typically have crews planned to be at approach speed by 1,000 ft above field elevation (AFE). While it is common to be decelerating at the FAF/PTP, deceleration typically is slower than desired for Paired Approach. Guidance will need to be published for crews to begin deceleration at the PTP and to slow to final approach speed as soon as practical.

One other consideration is related to the offset approach, which can be flown by either the IM aircraft or a Category B Target aircraft (see Experimental IAPs section). There should be consideration given to the human factors associated with this approach since it does not align with the runway until 0.5 NM prior to the threshold. Although lateral and vertical guidance is available to the runway, when the aircraft breaks out of the weather, a flight crew following incorrect visual cues could lead them to line up with a parallel taxiway.

Next Steps The Paired Approach Flight Demonstration was

sponsored by the FAA’s NextGen Office and the results will go into a technical transfer package intended for the FAA Surveillance Broadcast Services (SBS) Office. The Demonstration results will inform the on-going work for development of the operational concept using the data collected and lessons learned during the demonstration. Standards development of the proposed operational concept is currently on-going in RTCA Special Committee 186 (SC-186) Working Group 4 (WG-4).

References [1] H. P. Stassen et al., “Paired Approach Concept

of Operations V 1.0,” The MITRE Corporation, Sep. 2013.

[2] D. A. Domino, David Tuomey, H. Peter Stassen, and Anand Mundra, “Paired approaches to closely spaced runways: Results of pilot and ATC simulation,” in 33rd IEEE/AIAA Digital Avionics Systems Conference (DASC), 2014, pp. 1B2-1.

[3] RTCA SC-186 WG-4, “DO-361 Minimum Operational Performance Standards (MOPS) for

Flight-deck Interval Management (FIM),” Sep. 2015.

[4] J. Scharl, “ATD-1 Avionics Phase 2 Flight Test: Post-Flight Data Analysis Report,” Boeing, Seattle, WA, NASA/CR-2017-219645, Jul. 2017.

[5] K. Leiden, S. Priess, P. Harrison, R. Stone, P. Strande, and M. Palmer, “Paired Approach Flight Demonstration: Planning and Development Activities,” in 2018 Integrated Communications, Navigation, Surveillance Conference (ICNS), 2018, pp. 3G4-1.

[6] RTCA, “Safety, Performance, and Interoperability Requirements for Airborne Spacing - Flight-deck Interval Management (ASPA-FIM), DO-328B (DRAFT).” .

Acknowledgements The authors would like to express their sincere

gratitude to the personnel at SFO ATCT, Northern California TRACON (NCT), Oakland Center TMU, Oakland Center CWSU, TUS ATCT, Tucson TRACON (U90), and Albuquerque Center CWSU for their considerable support during both the planning and execution phases of the PA Flight Demonstration.

The authors would also like to thank Steph Priess of The MITRE Corporation for developing the ASG tool that was used to calculate the ASGs for each of the runs in the PA Flight Demonstration as well as the engineering flight tests conducted at TUS.

Lastly, the authors would like to thank Dana Clont of Honeywell for her assistance in preparing the performance analysis section of this paper.

2019 Integrated Communications Navigation

and Surveillance (ICNS) Conference April 9-11, 2019