ATS – Study Restrictions in the Enroute Environment

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ATS – Study Restrictions in the Enroute Environment

50TH ANNUAL CONFERENCE, Amman, Jordan, 11-15 April 2011

WP No. 91

ATS – Study Restrictions in the Enroute Environment

Presented by TOC

Summary

In November 2007 ICAO introduced Amendment 5 to Doc 4444, which contained new procedures and phraseologies for aircraft following a SID or a STAR. This implementation was not as successful as expected. IFATCA studied the operational use of these procedures and phraseology and concluded that discrepancy exists between restrictions on SIDs and STARs and restrictions in the en-route environment. This paper will study this issue to ensure compatibility and consistency between the TMA environment and en-route procedures.

This paper proposes new Policy on en-route restrictions as well as amendments on IFATCA Policy on SIDs and STARs.

Introduction

1.1  In November 2007 ICAO introduced Amendment 5 to Doc 4444, which contained new procedures and phraseologies for aircraft following a SID or a STAR. This implementation was not as successful as expected.

1.2  IFATCA studied the operational use of these procedures and phraseology and presented a working paper at the Punta Cana Conference in 2010. The paper concluded that many problems arise from inconsistent SID and STAR design. A separate working paper on this subject will be presented at conference under agenda item B.5.4.

1.3  Further study concluded that, since the introduction of Amendment 5, discrepancy exists between restrictions on SIDs and STARs and restrictions in the en-route environment. This paper will study this issue to ensure compatibility and consistency between the TMA environment and en-route procedures.

Discussion

2.1 Background

2.1.1 In PANS-ATM (Doc 4444), ICAO has laid down two different concepts for dealing with level restrictions:

Chapter 6 – Separation in the Vicinity of Aerodromes

6.3.2.4 CLIMB CLEARANCE ABOVE LEVELS SPECIFIED IN A SID

When a departing aircraft on a SID is cleared to climb to a level higher than the initially cleared level or the level(s) specified in a SID, the aircraft shall follow the published vertical profile of a SID, unless such restrictions are explicitly cancelled by ATC.

6.5.2.4 DESCENT BELOW LEVELS SPECIFIED IN A STAR

When an arriving aircraft on a STAR is cleared to descend to a level lower than the level or the level(s) specified in a STAR, the aircraft shall follow the published vertical profile of a STAR, unless such restrictions are explicitly cancelled by ATC. Published minimum levels based on terrain clearance shall always be applied.

Chapter 11 – Air Traffic Services Messages

11.4.2.6.2.5 Level restrictions issued by ATC in air-ground communications shall be repeated in conjunction with subsequent level clearances in order to remain in effect.

2.1.2 The ICAO Air Navigation Commission, after becoming aware of the difficulties with implementation of the new procedures and phraseologies for aircraft following a SID or a STAR, requested the Secretariat to investigate the situation. In this regard, State letter AN 13/2.1-09/25, dated 3 April 2009, was circulated to determine the extent and severity of the difficulties. By the end of July 2009, 52 replies had been received from 49 Contracting States and 3 international organizations. Analysis of these replies identified two primary causes of flight crew confusion:

  1. Inconsistent implementation of the PANS-ATM provisions across the States as well as within some States. Because many States apply the PANS-ATM procedures while others do not, flight crews are left uncertain as to whether the SID/STAR altitude restrictions continue to apply or are cancelled when Air Traffic Control (ATC) assigns a new level.
  2. The differing operational concepts postulated by ICAO for en-route and terminal airspace, those being:
    • level restrictions issued to en-route aircraft must be repeated with any new level assignment if they are to remain in effect, while conversely;
    • in terminal airspace restrictions which are published on SID/STAR charts remain in effect with any new level assignment unless specifically cancelled by ATC.

This analysis by ICAO is not completely accurate. ICAO itself has placed the provisions for restrictions on SIDs and STARs in the chapter “Separation in the Vicinity of Aerodromes”, while the principle for level restrictions is placed in a chapter with general validity. It would therefore appear that all level restrictions issued by ATC have to be repeated UNLESS those restrictions are published on SIDs and STARs. It is not clear, however, whether level restrictions that are published but NOT part of a SID or STAR have to be repeated each time a clearance is amended, or remain in force unless explicitly cancelled.

2.1.3  The resulting confusion has led to increased communication as flight crews and ATC clarify the vertical parameters of the clearance and, in a few instances, has resulted in aircraft operating on a different vertical path than anticipated by ATC.

2.1.4  At the Punta Cana Conference in 2010, TOC presented a working paper regarding the revised procedures and phraseologies regarding SIDs and STARs. The directors present – acknowledging that the ICAO concerns were correct, but also noting the ambiguities regarding published en-route restrictions – valued the suggestions and adopted the following IFATCA Policy [ATS 3.28]:

Phraseology and corresponding message sets should be developed to easily indicate whether published vertical restrictions and requirements are to be followed or not.

All level change clearances for aircraft on SIDs and STARs shall explicitly indicate whether published vertical restrictions and requirements are to be followed or not, provided that phraseology is used which does not increase controller workload beyond an acceptable level.

 


2.2 Phraseology

2.2.1  Current Phraseology

2.2.2  ICAO Doc. 4444 states in Chapter 12:

… clearance to cancel level restriction(s) of the vertical profile of a SID during climb

z) CLIMB TO (level) [LEVEL RESTRICTION(S) (SID designator) CANCELLED (or) LEVEL RESTRICTION(S) (SID designator) AT (point) CANCELLED];

… clearance to cancel level restriction(s) of the vertical profile of a STAR during descent

aa) DESCEND TO (level) [LEVEL RESTRICTION(S) (STAR designator) CANCELLED (or) LEVEL RESTRICTION(S) (STAR designator) AT (point) CANCELLED].

This phraseology is complicated and cumbersome to use, especially when ATC wants to cancel some restrictions on a procedure but leave others in place.

2.2.3  Proposed Phraseology

2.2.4  At a joint meeting in Las Vegas in 2009, the IFALPA ATS Committee and the IFATCA Technical and Operational Committee reviewed new phraseology suggested as a solution to the inconsistent application of PANS-ATM. The suggested phraseology was presented by EVPT at the 2010 Annual Conference.

2.2.5  Various industry bodies including IFATCA, IFALPA and CANSO are in the process of discussing the joint IFALPA/IFATCA proposal with the aim of submitting a joint recommendation to ICAO by April 2011. An update will be provided at Conference.

2.3.1  The difference in dealing with level restrictions published on SIDs and STARs and other level restrictions (both published and issued through air-ground communications) increases the potential for error. Even the newly revised phraseology proposed for amending the PANS-ATM does not take away that distinction.

2.3.2  TOC therefore proposes to apply the same concept that ICAO recommends for the terminal environment also to published restrictions in the enroute environment.

Conclusions

3.1  The concept that ICAO recommends for SIDs and STARs should also apply to all other published level restrictions.

3.2  The problems arising from the inconsistent implementation of the PANS-ATM provisions for SIDs and STARs across the States as well as within some States could be resolved by the new phraseology proposed by IFALPA and IFATCA.

3.3  The intention of the proposed phraseology is to not repeat published level restrictions on SIDs and STARs verbatim but instead simply to instruct the aircraft to comply with the vertical profile.

3.4  Current ICAO phraseology to cancel restrictions is cumbersome.

3.5  IFATCA policy on SIDs and STARs has to be amended to be aligned with the recommendations for en-route restrictions. This amendment does not change the intention of the Policy on SIDs and STARs.

Recommendations

It is recommended that;

4.1  IFATCA Policy is:

Published level restrictions remain valid unless explicitly cancelled by ATC.

and is included in the IFATCA Technical and Professional Manual

4.2  IFATCA Policy is:

Phraseology and corresponding message sets should be developed to easily cancel published level restrictions.

and is included in the IFATCA Technical and Professional Manual

4.3  IFATCA Policy on page 3 2 3 28 of the IFATCA Technical and Professional Manual:

All level change clearances for aircraft on SIDs and STARs shall explicitly indicate whether published vertical restrictions and requirements are to be followed or not, provided that phraseology is used which does not increase controller workload beyond an acceptable level.

is amended to read:

For aircraft on SIDs and STARs, all level change clearances shall explicitly indicate whether the published vertical profile is to be followed or not, provided that controller workload does not increase beyond an acceptable level.

4.4  IFATCA Policy on page 3 2 3 28 of the IFATCA Technical and Professional Manual:

Phraseology and corresponding message sets should be developed to easily indicate whether published vertical restrictions and requirements are to be followed or not.

is amended to read:

Phraseology and corresponding message sets shall be developed to easily indicate whether a published vertical profile is to be followed or not.

Last Update: September 30, 2020  

April 19, 2020   137   Jean-Francois Lepage    2011    

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