978-1-4673-1900-3/12/$31.00 ©2012 IEEE 3D4-1

AIRBORNE 4-DIMENSIONAL TRAJECTORY MANAGEMENT Bohumil Honzík and Martin Herodes

Honeywell, Advanced Technology Europe Brno, Czech Republic

Abstract

The paper describes a prototype of an airborne Flight Management System (FMS), supporting initial four-dimensional (I-4D) flight operations. The I-4D operations are based on airborne computed predictions that are used in ground systems. The goal is to establish far in advance a sequence for all aircraft converging to a merging point in a congested area. After coordination between the ground systems and aircraft, a time constraint at a merging point is assigned to each aircraft. As compensation, aircraft are allowed to fly their optimum profile up to that point, without any vectoring instructions from controllers. The prototype is based on Airbus A320 FMS software which has been enhanced by the I-4D features like Required Time of Arrival (RTA) in descent flight phase, reliable RTA window computation, and data link enhancements. Example of operational scenario is presented. The prototype has been tested with actual hardware, in an Airbus cockpit simulator and in flight as well.

Introduction The I-4D capable Flight Management System

(FMS) prototype has been developed within the framework of the Single European Sky Air Traffic Management Research (SESAR) Programme, work package 9.1 [1]. The SESAR is a private public partnership developing the future Air Traffic Management (ATM) systems for Europe and has been nearing a midpoint of its duration. Honeywell has been an active member in the SESAR since the program has begun in 2009. Leading or contributing up to 30+ projects, the I-4D has been one of the biggest projects in the portfolio. Honeywell also plays a significant role in co-ordinating SESAR and its FAA driven counterpart, NextGen.

There are up to 27,000 flights crossing a European airspace every day and the number of passengers is expected to double by the year 2020. It

would be impossible to support such demand of growth without improvements of the current infrastructure.

In the future of air traffic management environment defined by the SESAR, aircraft will need to behave in a more predictable way. In addition to following a trajectory cleared by an air traffic control, aircraft will need to fly over points of airspace at accurate times. Thus, the essential objective of the SESAR concept has been to move from constraining flights towards optimizing flights. This way, aircraft would progress in four dimensions, sharing accurate airborne predictions with ground systems. Aircraft would be able to meet time constraints at specific waypoints with high precision, when required by the increased traffic density. This would allow for better sequencing of traffic flows and would enable continuous descent operations in airport terminal areas. For example, an operation when an aircraft descends continuously with nearly idle thrust, thus avoiding a level off as much as possible prior to final approach, significantly decreases fuel consumption.

For airlines, the I-4D would allow an aircraft, with knowledge of all constraints, to plan and fly the most optimal, cost-efficient profile of its scheduled flight. Thanks to an optimized management of arrival flows, the benefit would be more capacity, better punctuality, flight efficiency as well as lower emissions, and noise reduction.

4D Trajectory Four dimensions referred to within this paper are

latitude, longitude, altitude, and time. The 4D trajectory is a sequence of waypoints described by these parameters. From an aircraft’s perspective, the 4D trajectory management is a capability to build, guide, predict, and communicate the aircraft’s 4D trajectory with an air traffic control center.

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Principle of Operation A typical operational scenario is presented in

Figure 1. In pre-flight, a pilot enters a flight plan and weather forecast data into the FMS. During the flight, the FMS is continuously calculating predicted trajectory from the aircraft’s current position until a destination airport. Such predictions are very precise, especially in comparison with predictions that may be performed on ground by an air traffic control system.

The airborne FMS has accurate data needed for such computation (current aircraft weight, model of aircraft dynamics and its parameters, weather at current position etc.). The only information available on the ground, which may be of better quality compared to the airborne side, is current weather situation in front of the aircraft and forecast. However, even after take-off, it would be possible to uplink up-to-date weather information in order to guarantee the best prediction precision possible.

Figure 1. I-4D Flight Operational Scenario

During the flight, an aircraft composes a 4D trajectory report, containing important information about the planned path. Besides the predicted aircraft state at particular waypoints described by latitude,

longitude, altitude, speed, and estimated time of arrival (ETA), the aircraft composes reliable RTA window information as well. The reliable RTA window is defined by an earliest and latest time at

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which an aircraft would be able to arrive with defined reliability at a given waypoint (also referred to as minimum/maximum estimated time of arrival, or ETA min/max). This information allows the pilot to quickly asses the RTA feasibility. Several factors define a position and size of the reliable RTA window, mainly the aircraft performance envelope, altitude, speed, altitude and speed constraints, as well as weather conditions along the planned trajectory. At a specific waypoint the reliable RTA window depends on distance from the aircraft’s current position as well; the closer the waypoint is, the smaller the window. Since there would be less time, the aircraft would be able to fly faster/slower.

The 4D trajectory report is sent at regular time intervals to the ATC. The ground tools gather such reports from all aircraft in the assigned area, helping the controller to manage the air traffic in an optimal way. If a need for trajectory correction arises, it would be done by a suitably selected time constraint, rather than vectoring. By choosing a required time of arrival at the waypoint in question, from within the corresponding reliable RTA window, the chance the time constraint will be met is very high. Moreover, the aircraft is able to optimize its flight profile in the most economical and environmentally friendly way. Downlinking the reliable RTA window prevents the ATC from requesting inappropriate RTA, thus reducing probability of rejection by the pilot and simplifying the RTA negotiation.

During the progress of the flight, the airborne systems continue sending the 4D trajectory report to the ground, even if the RTA has already been entered in the FMS. Therefore, the controllers would ensure that the ground and air expectations would be the same.

If the weather forecast changes in the area where the aircraft is flying, it is possible to uplink up-to-date information. The airborne flight management system would then take over the new data and reflect it immediately in the flight plan predictions. Nonetheless, unexpected weather changes captured neither by the airborne system nor ground weather model may occur. In such case, the RTA algorithm running in the FMS would be ready to compensate the predicted difference between the required and predicted time of arrival by an appropriate change of aircraft speed.

FMS Enhancements The I-4D prototype is based on the Airbus A320

FMS manufactured by Honeywell. The certified software has been extended in order to meet the SESAR WP 9.1 project specifications. The following areas have been modified: internal weather model, RTA algorithm, aircraft vertical control, data link, and human—machine interface (HMI).

RTA Algorithm In fact, the certified A320 FMS software already

contains the RTA functionality. The RTA may be entered at any waypoint along planned trajectory. The RTA algorithm constructs the aircraft trajectory in order to meet the RTA at the point in question. However, no speed adjustments are possible in a descent flight phase, therefore the FMS is unable to compensate for unpredicted weather changes. However, since the RTA accuracy is set to ±30 seconds, even an open loop control can still maintain the RTA.

In the I-4D prototype, the RTA speed adjustments are allowed in descent as well. The RTA may be set with resolution of seconds either as at, at or before, or at or after. Required accuracy may be set to ±10, or ±30 seconds. Based on the required time of arrival for a given fix, the FMS computes time error for this fix, i.e., ETA – RTA. Based on this time error the RTA algorithm searches for an appropriate speed to achieve the RTA.

The aircraft altitude and speed change quickly during descent, making speed adjustments in this flight phase a difficult task especially, if altitude and/or speed constraints become involved as well. On the other hand, with decreasing altitude the aircraft speed envelope becomes wider.

If the operational scenario is maintained and the RTA is selected from within the reliable RTA window, the algorithm has been designed to meet an accuracy of ±10 seconds with probability of 95%. Nonetheless, several challenging situations exist, which may prevent it:

RTA Entered Too Far from RTA Waypoint Steady wind/temperature error could lead to

speed saturation before the RTA waypoint. Time error compensation is limited.

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Strong Wind/Temperature Error Close to RTA Waypoint

The aircraft may not be able to react fast enough.

Flight Plan Change During RTA Operation Vectoring or entering of speed/altitude

constraint changes the altitude and speed profile. The resulting jump in predictions may not be compensated.

FMS Switched Off Managed Mode Without managed mode no automatic speed

adjustments are done by the RTA algorithm. Pilot would have to control the speed manually.

Weather Modelling Quality of the 4D trajectory management

strongly depends on the trajectory prediction error. There are many sources of inaccuracies involved, such as errors in performance and physics modelling, simplification assumptions, sensor errors, numerical errors etc., [2]. However, the influence of these factors is negligible. The source of error with the biggest impact is weather modelling.

The weather model errors are caused by sensing errors, limited geographic measurement, latency in data collection and processing, and forecasting errors. The guaranteed combined accuracy of temperature and wind forecast used in this project has been less than 12 kts with 95% probability.

In order to minimize inaccuracies on the airborne side, the granularity of the internal FMS weather model, used in the descent flight phase has been increased. Thus, the upgraded model is able to reflect better local weather deviations from vertical perspective, improving flight plan predictions accuracy in descent. Another fact with positive impact on the prediction precision has been weather data uplink, which became a part of the I-4D operation initiation procedure. Therefore, before an RTA is inserted in the FMS, i.e., during the RTA feasibility assessment process performed by the pilot, the internal weather model is filled with fresh weather forecast data.

HMI Updates The above-mentioned enhancements of the FMS

functionality also required modifications of the human—machine interface, e.g., the display of the reliable RTA window and RTA error, and screens related to increased resolution of the internal weather model.

Data Link Enhancements The I-4D prototype has enhanced the FMS in

data link area as well, because the certified software used supports only FANS-1/A 1

Data Link Standards

data link standard, which has not been sufficient for the I-4D operations.

The data link airborne communication is established between ground systems and on-board systems to replace voice communication. In order to understand each other, implementation of both sides has to be based on the same data link standards.

Today FANS-1/A(+) [3] is the most important present

data link standard, originally developed for oceanic operations on the ACARS 2 network including the CPDLC3 messages and ADS-C4

FANS-2/B is a present data link standard validated by a European programme Link 2000+ and mandated in the EU area according to the EC regulation No. 29/2009 for domestic operations on ATN

.

5

Future

network. Only the CPDLC messages are included.

FANS-2/B+ is a near future data link standard, being implemented according to the en-route European mandate. This data link standard would only be used for domestic operations on the ATN

1FANS [Future Air Navigation System] 2ACARS [Aircraft Communications Addressing and Reporting System] 3CPDLC [Controller Pilot Data Link Communications] 4ADS-C [Automatic Dependent Surveillance-Contract] 5ATN [Aeronautical Telecommunication Network]

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network and only the CPDLC messages would be included.

FANS-3/C is a future data link standard, which is still under development in standardization group RTCA SC-214/EUROCAE WG78 [4]. This standard would be used for oceanic and domestic operations on the ATN network. The CPDLC messages and ADS-C would be included.

The I-4D prototype has only been developed for domestic operations so far, but should be a part of the future FANS-3/C. The ATN network has been used for flight testing of the I-4D operations. For this purpose the airborne and ground systems had to be updated.

CPDLC Enhancements The certified software implements a subset of

FANS-1/A CPDLC messages. Some of these messages are intended to be used for the RTA negotiations during oceanic operations. For this purpose they include the RTA with resolution in minutes, which has not been sufficient in terminal areas.

The I-4D prototype keeps the FANS-1/A messages for backward compatibility and implements a new set of enhanced CPDLC messages including a RTA with resolution in seconds and RTA tolerance definition, as described in Table 1.

Crossing Constraints The new CPDLC messages from Table 1

including crossing constraints allow the controller to put a RTA with resolution in seconds at, at or before, or at or after a waypoint from active flight plan with accuracy of ±10, or ±30 seconds (default value is ±30s). A flight level can be specified in advance.

Route Modifications The new CPDLC messages from Table 1

including route modifications allow the controller to change active flight plans of aircraft using route clearances. The route clearances are complex and cover many scenarios. For example, a waypoint or sequence of waypoints could be inserted before, or after a specific waypoint from within an active flight plan. Alternatively, without specification of the waypoint, an arrival airport or runway could be

changed, etc. The I-4D prototype allows specification of the RTA with resolution in seconds and RTA tolerance of a waypoint within a route clearance. It would allow the controller to change an active flight plan of an aircraft and set the RTA in one uplink message.

Request Clearance There is only one new CPDLC downlink

message in the I-4D prototype (see Table 1). This message requests transmission of a new route clearance specified in the downlink message. The RTA with resolution in seconds and RTA tolerance can be included in this route clearance.

Table 1. Enhanced CPDLC Messages

Label

Format

Uplink – Crossing Constraints:

51R

CROSS [position] AT [RTAtimesec]

52R

CROSS [position] AT OR BEFORE [RTAtimesec]

53R

CROSS [position] AT OR AFTER [RTAtimesec]

58R

CROSS [position] AT [RTAtimesec] AT [level]

59R

CROSS [position] AT OR BEFORE [RTAtimesec] AT [level]

60R

CROSS [position] AT OR AFTER [RTAtimesec] AT [level]

Uplink – Route Modifications:

79R

CLEARED TO [position] VIA [routeClearanceEnhanced]

80R

CLEARED [routeClearanceEnhanced]

83R

AT [position] CLEARED [routeClearanceEnhanced]

Downlink – Request Clearance:

24R

REQUEST CLEARANCE [routeClearanceEnhanced]

- [position] is a waypoint from active flight plan - [level] is a flight level (altitude) - [RTAtimesec] is a RTA in seconds + RTA tolerance

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definition 10 or 30 seconds - [routeClearanceEnhanced] is a route clearance (e.g.

sequence of waypoints) including [RTAtimesec]

ADS-C Enhancements The certified software uses the ADS-C defined

in FANS-1/A [3] while the I-4D prototype implements the SESAR requirements for the 4D trajectory reporting, based on latest documents from standardization group SC-214/WG78 [4].

ADS-C Functionality in I-4D 1. The flight management system

periodically sends ADS data including the 4D predicted trajectory to a data link unit in an aircraft.

2. The air traffic controller initiates an ADS contract with the aircraft. This contract can be periodic, on event, or on demand (see Table 2).

3. After the ADS contract is established, the data link unit reports selected set of ADS data according to the SESAR specifications to the ground (see Table 3 and Table 4). A lot more data are transmitted to the ground in contrast with the FANS-1/A.

Table 2. Uplink Requests

Periodic Contract:

Reporting rate

EPP window

ETA min/max waypoint

Event Contract:

EPP window

EPP event type

Demand Contract:

EPP window

ETA min/max waypoint

Uplink Requests ADS contracts established via uplink requests

are divided according to type of contract (see Table 2). Periodic contract including reporting rate is able to establish periodic reporting, event contract reports on event, and demand contract on demand. These types of contracts have already been defined in the FANS-1/A; however the I-4D prototype implements different parameters.

Extended Projected Profile (EPP) window is used for an uplink of a number of waypoints or a time interval. This EPP window defines how many waypoints would be reported in a downlink report.

ETA min/max waypoint includes fix name and position option (latitude/longitude) of a waypoint from within an active flight plan for which the ETA min/max would be reported.

EPP event type specifies an event – waypoint sequenced, waypoint inserted, waypoint deleted, new waypoint in horizon, RTA changed, or RTA missed. Reporting would be triggered each time when an event occurs.

Table 3. Downlink Reports

Periodic Report:

Basic data

ETA min/max

EPP data

Event Report:

Basic data

EPP data

Demand Report:

Basic data

ETA min/max

EPP data

Table 4. EPP Data

EPP Header Data:

Current gross mass

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Predicted gross mass at top-of-descent

Speed schedule

Min/max speed schedule

Trajectory intent status data

EPP Segment Data for up to 128 Waypoints:

Latitude, Longitude

Level

Time

Speed

Fix name

RTA

Vertical type

Lateral type

Level constraint value

Speed restriction value

Downlink Reports

The data link unit in the aircraft reports to ground a part of the ADS data periodically coming from the FMS. Three types of downlink reports exist: periodic, event, and demand (see Table 3). These reports already exist in the FANS-1/A, but the I-4D prototype includes a lot more data in its reports. Mandatory basic data including a current aircraft position are used in the FANS-1/A, but other optional parameters have been newly created. An ETA min/max is reported for requested ETA min/max waypoint and EPP data are reported for requested EPP window. Periodic, event, or demand report containing EPP data described in Table 4 is called an EPP report. This type of report is very important as it includes a 4D trajectory for up to 128 waypoints from an active flight plan.

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Figure 2. Flight Trial in February 2012

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Validation The FMS I-4D prototype has been validated on

real hardware in Honeywell validation facility, in Airbus testing laboratory, and during a flight test. The validation has involved not only the RTA function, but also the data link and communication functionality.

Flight Trial In order to validate prototype functionality, a

flight trial was carried out in February 2012. Its progress has been illustrated in Figure 2. The test aircraft, Airbus A320-200 had started from Toulouse. During the climb to FL340, an ADS-C connection with an ATC had been established, and up-to-date wind and temperature data had been uplinked. The test aircraft had flown through the Eurocontrol Maastricht Upper Area Control Centre (MUAC) airspace.

The first RTA had been established at a waypoint in cruise flight phase in North-West Germany. The second time constraint the airborne and ground systems agreed on had been KUBIS, a merging point close to the Copenhagen airport. The flight had continued into Danish airspace to demonstrate an optimized descent to Copenhagen. After KUBIS had been reached, the aircraft had again climbed to its cruise flight level from which it had negotiated a third time constraint at a merging point SA620 close to Stockholm. Both the KUBIS and SA620 RTAs had been in descent. All three time constraints had been made with time error of +4, -1, and 0 seconds, respectively. The evolution of wind error, which had to be compensated for by the RTA algorithm (i.e., the unpredicted wind) is shown in Figure 3. It presents the headwind component, including the wind and temperature effect. Figure 4 shows total headwind error together with a CAS target evolution, generated by the RTA algorithm, for the last RTA defined at waypoint SA620.

Figure 3. Total Wind Error Including Wind and Temperature Effect (Headwind Component)

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Figure 4. Headwind Error and Corresponding Speed Adjustments (RTA at SA620)

Conclusion The flight test was the very first live

demonstration of an I-4D flight under operational conditions. It has successfully verified automated 4D data exchange between the aircraft flight management system and ground automation systems through data link. It was shown that the aircraft is capable to comply with time constraints with high precision.

The I-4D flight operations are the first step towards full 4D operations, which has been the main goal of the Europe’s modernization programme. In full 4D, airlines and air navigation service providers would agree before a flight on a 4D trajectory covering the whole flight, significantly enhancing the traffic predictability and optimizing management of air traffic networks. Such 4D trajectory may be updated, or revised by data link during a flight, in order to take into account various unpredictable events.

The I-4D flight trial has been a part of a complete validation campaign in the framework of the SESAR release process. Another set of flight trials and further validations aiming at validating both

technical and operational aspects has been planned over the next two years.

References [1] SESAR Joint Undertaking web pages, http://www.sesarju.eu/programme/workpackages

[2] Michael R.C. Jackson, 1997, Sensitivity of Trajectory Prediction in Air Traffic Management and Flight Management Systems, PhD thesis, University of Minnesota, pp. 44-45

[3] RTCA Incorporated, 2005, Interoperability Requirements for ATS Applications Using ARINC 622 Data Communications (FANS 1/A Interop Standard), Washington, DC

[4] RTCA SC-214 / EUROCAE WG-78 Standards for Air Traffic Data Communication Services, http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/atc_comms_services/sc214/

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Acknowledgements The authors would like to thank Petr Tieftrunk,

Erwan Paricaud and Michal Polansky for helpful comments and suggestions.

Activities developed to achieve the results presented in this paper were created by Honeywell Advanced Technology Europe for the SESAR Joint Undertaking within the frame of the SESAR Programme co-financed by the EU and EUROCONTROL.

Disclaimer The consortium includes Airbus and Thales also

but the opinions expressed herein reflect the author’s view only. The SESAR Joint Undertaking is not liable for the use of any of the information included herein.

31st Digital Avionics Systems Conference October 14-18, 2012