Summary

This paper focusses on the procedures prescribed by appropriate ATS authorities regarding the separation of departing aircraft from arriving aircraft, i.e. when a take-off clearance is based on the position of an arriving aircraft.

Introduction

1.1  Definition

A Cut-off Point (CoP) may be defined as a determined point along an instrument flight procedure from which the required minimum separation between a departing aircraft and an arriving one is considered to no longer exist, unless another form of separation is established. It is not a Standard ICAO definition thus it will be used in this WP for the sole purpose to give nomenclature to such point.

1.2  Separation Standards

It is the responsibility of the appropriate ATS authority to prescribe the required separation standard to be enforced in controlled airspace to facilitate the safe navigation of aircraft based on the provisions of ICAO PANS – ATM, Doc 4444.

The methods used to achieve separation are varied and complex, depending on the phase of flight and relative trajectories of the aircraft involved which follow Instrument Flight Procedures (IFP) that are designed according to ICAO Doc 8168 PANS – OPS criteria.

1.3  Airport Capacity

Airline expansion and airports already operating at a high capacity under stringent noise abatement procedures and political barriers for expansions have led ATS authorities to further reduce CoP separations. This reduction is basically reducing the possibility to maintain this kind of separation when unusual circumstances, such as missed approaches and non-standard approach speeds occur.

Discussion

2.1  Determining Cut-off Points

The implementation of CoPs follows the prescription to apply a separation between departing aircraft and arriving aircraft as stated in ICAO Doc 4444 §5.7. Logically, this means that the definition of CoPs must ensure separation is not infringed between a departing aircraft on a SID and an arriving aircraft. In an extensive way, it would even provide the necessary spacing from the arriving aircraft which will commence a missed approach.

The establishment of a CoP can also ensure safe and efficient flow of traffic between departing and arriving aircraft so as to avoid impeding the flow and hence airport capacity by the application of otherwise excessive separations. The development of CoPs must therefore be determined with due regard to all relevant factors/requirements to ensure vertical or horizontal separation is provided as stated in ICAO DOC4444:

(ICAO PANS-ATM (Doc 44444), 16th Edition (November 2016), Chapter 5.2 “Provisions for the separation of controlled traffic”)

2.2  The importance of an Instrument Flight Procedure (IFP) Design

Before determining a CoP on an Instrument Flight Procedure (IFP), it is important to understand the design requirements of an IFP. These requirements and design criteria will apply when defining a CoP in order to either ensure lateral separation by displacing the flight trajectories of the missed approach and SID, or by achieving vertical separation between flight trajectories by constraints or climb gradients. As long as one flight trajectory does not overlap with another flight trajectory, or vertical separation is achieved at a specific point, they are considered to be separated and safe. However, only once all the factors are considered during the design process of the IFP, it becomes possible to see the solutions in determining a practical and safe CoP.

2.3  Conceptual IFP Design – Data Collection

IFPs are mainly designed to ensure obstacle clearance. However, each flight trajectory depending on the phase of flight will have a primary and secondary protection area to also ensure an appropriate tolerance for the safe navigation of aircraft.

These trajectories can also be altered to achieve a required separation, to ensure an efficient flow of air traffic or to clear an obstacle. This depends on the need and requirements of the stakeholders. Pilots are expected to fly the IFP as published to the required navigation accuracy to ensure obstacle clearance and also clearance from other flight trajectories.

The procedure designer is responsible for the conceptual design. The major key essentials for procedure designing are:

1. The use of appropriate design criteria, together with
2. Data which meets the “quality requirements” (Quality requirements are addressed in terms of accuracy, resolution and the integrity of the data).

The IFP design criteria are according to ICAO Doc 8168 and the procedure design data can be categorized as follows:

1. ATS Requirements: This involves ATC, Users (Operators), Airspace design constraints and environmental constraints.
2. Aerodrome Data: Thresholds, Airport Reference Point, etc., weather statistics and magnetic variation and its rate of change.
3. Aeronautical and Nav-aids data: Airspace structure/classification, airway routes, transition altitudes/levels, nav-aid coordinates, coverage etc.
4. Terrain and Obstacle data: Cartographic maps and obstacles.

This data is then verified and validated as an important quality characteristic of the procedure design to ensure proper horizontal and vertical safety margins required by the procedure design criteria.

2.4  ATS requirements/Operational Data

According to IFATCA Technical and Professional Manual:

(IFATCA TPM – AAS 1.13 – determining operations readiness on new ATM systems)

This policy should, as far as possible, also apply to the implementation of new operational procedures such as CoPs to ensure their compatibility with existing ATS procedures.

Operators must also be involved jointly with ATC for the need to shorten trajectories, enhance the navigation guidance (lateral and/or availability of vertical guidance), lower the minima and enhance flyability.

Airspace design must be reviewed to address the existing constraints and requirements for a restructured airspace along with danger/prohibited areas.

Environmental constraints are also to be reviewed to avoid populated areas, sensitive areas and to regard noise abatement procedures as applicable.

All ATC and Operators requirements (operational data) is then collected for subsequent use in the design phase. The more input from the data collection, the more likely the conceptual design will correspond to stakeholder’s expectations.

Exchanges between ATC and Operators at this stage is recommended to increase efficiency as well as continuous coordination with interested/affected stakeholders, as envisaged by ICAO Doc 4444 §5.7.2.1 b) 2). As for other ATM domains, ATCOs should be involved in the design of such CoPs.

2.5  Creating the Conceptual Design

Once the conceptual design is created and documented, it is then used as a baseline for discussion with all stakeholders.

Its primary objective is to provide a forum in which all stakeholders can reach a consensus. All necessary inputs are gathered, discussed and agreed upon for the final conceptual design. Once it is formally approved, the agreed final conceptual design will be used as a baseline for the detailed design phase.

A CoPs should be established for homogeneous traffic. In mixed traffic environments, like helicopter arriving vs jet departing, or slow turboprop departing while a fast jet is arriving (or vice versa) CoPs might not be suitable for the actual definition of the desired spacing. In such cases a controller’s best judgement and training remain the only applicable ways.

A CoP is not to be used as a separation standard. The reductions in separation minima in the vicinity of the airport still have to be considered.

As from PANS ATM (Doc 4444):

Provided that a CoP may be defined as a tool to help ATCOs in performing safe air traffic management, it should not be considered as a mandatory decisional fix (“Go or No-Go”) or a standard procedure for the provision of separation.

The possibility to have aircraft with different performances would require a flexible CoP to suit every different situation. CoP is still to be considered a useful tool for ATCOs but not a separation standard or procedure.

Anytime the authority would ever mandate the application of such CoP for particular reasons (noise abatement procedure, risk assessment follow up etc.), ATCOs shall be trained and user informed accordingly.

2.6  Detailed Design

To enhance the integrity throughout the design process, the use of automated or semi-automated software tools are used under international design procedure criteria detailed in Doc 8168 (PANS-OPS). To verify the design, a technical report is further issued to describe the applied criteria.

The technical report should include the following:

• Applicable design criteria,
• Data sources and input data used,
• All calculation details,
• All design parameters used (speeds, bank angles, wind velocity, temperature, climb/descent gradients, etc.)
• Any deviation from selected criteria,
• Design assumptions and constraints, and
• Any other pertinent points of interest.

Detailed design output must be made accessible and be traceable to be used in the implementation of a new or modified Flight Procedure Design (FPD) and for the maintenance of the IFP.

Hence, once a modification is required to an IFP; e.g. to create a CoP, all relevant parties can return to the technical report and modify as appropriate the applied criteria to create a new detailed design.

2.7  Safety Assessment

A system can only be considered to be safe for operational use if its inherent risks have been identified, assessed and agreed upon to be below predefined limits.

The implementation of a procedure, the complexity and the workload imposed on ATC, cockpit workload, flyability etc. are the focus of the safety assessment not specifically the FPD itself.  In general, the impact of implementing a new procedure to the existing ATM system.

The FPD therefore must not be assumed “safe” as validated by the authority because the safety assessment is still part of the procedure approval process.

The ATM system includes Equipment (Nav-aids, surveillance, communication etc.), Procedure (ATC, Maintenance etc.) and Human interfaces (ATCOs, Engineers etc.) and so any modifications to these is subject to a safety assessment. This would require feedback from operators and ATCOs to identify hazards and implement mitigation means; reactive or preventive to ensure the risk is acceptable.

After the safety assessment is carried out, a ground validation should take place, i.e. an independent FPD review is performed to confirm correct application of criteria and data accuracy. ATC and pilots as specialists must be involved. If it is not compliant with requirements the validation step is not possible.

Once the validation is accomplished, all the stakeholders should be consulted to agree on the final proposed procedure from which it must be approved by the state prior to publication.

A system must be implemented to continuously get feedback from stakeholders about the operational implementation of the procedure (especially Operators and ATCOs).

The advice of data houses, ATCOs and Pilots actually using the procedure is particularly relevant as any new data may impact the procedure and require modification.

Conclusions

3.1  Cut-off Points should not be considered as a procedure nor a separation standard unless otherwise prescribed by the Authority. A CoP is a tool which ATCOs might use in homogeneous operational environment to expedite the flow of traffic.

3.2  The use of CoPs does not exclude the possibility to apply the existing standard ICAO-provisions (e.g. the so-called “reduced separations minima in the vicinity of the aerodrome”); ATCOs should be involved in the design of CoPs.

References

• ICAO PANS-ATM Doc 4444, Sixteenth Edition, November 2016
• Doc 9906, Vol 1
• Doc 8168