50TH ANNUAL CONFERENCE, Amman, Jordan, 11-15 April 2011
WP No. 90
ATS – Study SID and STAR Design
Presented by TOC
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 many problems arise from inconsistent SID and STAR design. This study investigates current ICAO regulations and local implementations as well as inconsistencies with aircraft avionics equipment.
This paper proposes a new work item for the TOC work programme 2012.
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. IFATCA studied the operational use of these procedures and phraseology and concluded that many problems arise from inconsistent SID and STAR design. Policy was accepted at the 2010 Punta Cana conference stating that SID and STAR design should be globally harmonized.
1.2 In addition to the Continuous Descent Operations Manual (reviewed under Agenda Item B.5.9), the Continuous Climb Operations Manual is being developed. Both documents will have consequences on SID and STAR design.
1.3 At the 2010 Annual Conference in Punta Cana, some questions indicated that the expectation of the questioners was that all SIDs and STARs could be flown safely (that is without any additional instructions, such as cancelling requirements to permit climb on a SID). As this is not the case at some major airports, this issue will be specifically considered. Likewise the notion of endorsing SIDs with “or higher as assigned by ATC” was questioned as to its appropriateness.
1.4 In its Statement on the Future of Global Air Traffic Management, IFATCA expects service providers to move from route based flight data processing systems to geographical based ones. This could have an impact on SIDs and STARs.
1.5 The issue of compatibility and consistency with En-route procedures is the subject of a separate Working Paper presented by TOC under Agenda Item B.5.5 at this Annual Conference.
2.1.1 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 in 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.
2.1.2 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.
2.1.3 ICAO has approached all stakeholders in an attempt to harmonise the radiotelephony phraseology concerned. Among others, at a joint meeting in Las Vegas in 2009, the IFALPA ATS Committee and the IFATCA Technical and Operational Committee have reviewed new phraseology suggested as a solution to the problem. The suggested phraseology was presented by EVPT at the 2010 Annual Conference. The directors present – acknowledging that the ICAO concerns were correct – valued the suggestions and adopted the following policy statements [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.
2.1.4 Directors also felt that, ultimately, replacing phraseology was addressing the symptoms rather than actually curing the disease. Consequently, directors also adopted the following policy statement [ATS 3.28]:
|SID and STAR design and use should be globally harmonized.|
2.2 ICAO Documentation
2.2.1 ICAO has a lot of guidance material about Standard Instrument Departure Routes and Standard Instrument Arrival Routes. The concept of SIDs and STARs is laid down in Annex 11 18.104.22.168., which in turn makes reference to PANS-OPS Doc. 8168 Volume I „Flight Procedures“ (aimed at Aircraft Operators) and Volume II “Construction of Visual and Instrument Flight Procedures” (aimed at procedure designers) as well as to the Air Traffic Services Planning Manual Doc 9426. The following excerpts are from PANS-OPS Volume I and II.
Standard instrument arrival (STAR). A designated instrument flight rule (IFR) arrival route linking a significant point, normally on an ATS route, with a point from which a published instrument approach procedure can be commenced.
Standard instrument departure (SID). A designated instrument flight rule (IFR) departure route linking the aerodrome or a specified runway of the aerodrome with a specified significant point, normally on a designated ATS route, at which the en-route phase of a flight commences.
2.2.3 From Section 1 Chapter 1:
1.2 OBSTACLE CLEARANCE
1.2.1 Obstacle clearance is a primary safety consideration in the development of instrument flight procedures. The criteria used and the detailed method of calculation are covered in PANS-OPS, Volume II. However, from the operational point of view it is stressed that the obstacle clearance applied in the development of each instrument procedure is considered to be the minimum required for an acceptable level of safety in operations.
2.2.4 From Section 7, Chapter 2:
2.2 NOISE PREFERENTIAL ROUTES
2.2.1 Noise preferential routes are established to ensure that departing and arriving aeroplanes avoid over-flying noise-sensitive areas in the vicinity of the aerodrome as far as practicable.
2.2.3 In establishing noise preferential routes, the safety criteria of standard departure and standard arrival routes regarding obstacle clearance climb gradients and other factors should be taken into full consideration (see PANS-OPS, Volume II).
2.2.4 Where noise preferential routes are established, these routes and standard departure and arrival routes should be compatible.
2.2.5 From Section 3 – Departure Procedures, Chapter 1 – General Criteria for Departure Procedures:
22.214.171.124 The criteria in this section are designed to provide flight crews and other flight operations personnel with an appreciation, from the operational point of view, of the parameters and criteria used in the design of instrument departure procedures. These include, but are not limited to, standard instrument departure (SID) routes and associated procedures (see Annex 11, Appendix 3).
Note.— Detailed specifications for instrument departure procedure construction, primarily for the use of procedures specialists, are contained in PANS-OPS, Volume II, Part I, Section 3.
126.96.36.199 These procedures assume that all engines are operating. In order to ensure acceptable clearance above obstacles during the departure phase, instrument departure procedures may be published as specific routes to be followed or as omnidirectional departures, together with procedure design gradients and details of significant obstacles.
1.3.1 Design considerations
The design of an instrument departure procedure is, in general, dictated by the terrain surrounding the aerodrome. It may also be required to provide for air traffic control (ATC) requirements in the case of SID routes. These factors in turn influence the type and siting of navigation aids in relation to the departure route. Airspace restrictions may also affect the routing and siting of navigation aids.
1.4 OBSTACLE CLEARANCE
1.4.1 The minimum obstacle clearance equals zero at the departure end of the runway (DER). From that point, it increases by 0.8 per cent of the horizontal distance in the direction of flight assuming a maximum turn of 15°. 1.4.2 In the turn initiation area and turn area, a minimum obstacle clearance of 90 m (295 ft) is provided.
1.4.3 Where precipitous and mountainous terrain exist, consideration is given by the procedures designer to increasing the minimum obstacle clearance (see also PANS-OPS, Volume II, Part I, Section 2, Chapter 1, 1.7).
1.5 PROCEDURE DESIGN GRADIENT (PDG)
1.5.1 The procedure design gradient (PDG) is intended as an aid to the procedures designer, who adjusts the route with the intention of minimizing the PDG consistent with other constraints.
1.5.2 Unless otherwise published, a PDG of 3.3 per cent is assumed.
1.5.3 The PDG is not intended as an operational limitation for those operators who assess departure obstacles in relation to aircraft performance, taking into account the availability of appropriate ground/airborne equipment.
2.2.6 From Section 3 Chapter 4 – Published Information for Departures:
4.1.11 Departure procedures may be developed to procedurally separate air traffic. In doing so, the procedure may be accompanied with altitudes/flight levels that are not associated with any obstacle clearance requirements but are developed to separate arriving and departing air traffic procedurally. These altitudes/flight levels shall be charted as indicated in Table I-3-4-1. The method of charting altitudes/flight levels to correctly depict the designed procedure may differ between avionics manufacturers.
2.2.7 From Section 4 – Arrival and Approach Procedures, Chapter 8 – Charting/Aeronautical Information Publication (AIP):
In some cases it is necessary to designate arrival routes from the en-route structure to the initial approach fix. Only those routes that provide an operational advantage are established and published. These routes take local air traffic flow into consideration.
2.2.8 From PANS-OPS Volume 2, Section 3 Departure Procedures, Chapter 1 – Introduction to departure procedures:
A departure procedure may also be required for air traffic control, airspace management or other reasons (e.g. noise abatement) and the departure route or procedure may not be determined by obstacle clearance requirements alone. Departure procedures should be developed in consultation with the operators, ATC and other parties concerned. (See Volume I, Part I, Section 7 for noise abatement considerations.)
PANS-OPS Volume 2 also contains a whole Appendix that provides guidance on environmental issues.
- SIDs are primarily established for terrain and obstacle clearance but may also be designed to facilitate ATC service.
- STARs are established ONLY to provide an operational advantage.
Note: SIDs start at the end of the departure runway while STARs terminate at the initial approach fix of an instrument approach procedure. These approach procedures also account for terrain and obstacle clearance.
- There is no mention whether or not an SID may have an open vertical design. In other words, an altitude or flight level published as part of an SID is not necessarily considered to be the maximum level on the SID.
2.3 Real World SID and STAR design
2.3.1 ICAO has laid down a concept for a transition from the en-route environment to the beginning of an instrument approach procedure respectively from the end of an instrument departure procedure to the en-route environment:
Cruise → STAR → Instrument Approach Procedure → Airport → Instrument Departure Procedure/SID → Cruise
Note the inconsistency between the arrival and departure sectors: In the arrival phase, ICAO talks about STARs and Instrument Approach Procedures as separate procedures, while on the departure side Standard Instrument Departures are a subset of Instrument Departure Procedures. Logically, both should exist separately to create a mirror image of the arrival side.
2.3.2 In the Real World, states deviate from this scheme to varying degrees. One example:
Los Angeles, U.S.: Cruise → Transition → STAR → Transition → Instrument Approach Procedure → Airport → SID → Transition → Cruise
2.3.3 Even within SIDs and STARs the actual designs show the various approaches of the subject by various states. There are two extremes:
- Basic SIDs for terrain and obstacle clearance, providing lateral guidance only, and no STARs. Usually in place in low density airspace.
- Extremely long and complex SIDs and STARs that effectively already create 3D trajectories. In these cases states rely on a heavily systemized route structure to procedurally separate different traffic streams in order to maximise airspace use. One prime example of this is the London TMA. Rule of thumb: The higher the demand on the airspace, the more complex the route structure.
In SID and STAR design, safety and efficiency (for example to achieve continuous climb or descent are often counterparts.
2.3.4 Some SIDs contain an “initial cleared” altitude or flight level.
SID MEVEL out of Düsseldorf, Germany. The initial altitude is 5000 ft, while later on, the minimum cruising altitude of the SID changes to 6000 ft due to a restricted area:
Figure 1: MEVEL SID out of EDDL
According to ICAO Doc 4444 the initial altitude in cases such the MEVEL SID is a means to facilitate coordination between aerodrome and approach control:
188.8.131.52 COMMUNICATION FAILURE
184.108.40.206.1 Clearances for departing aircraft may specify an initial or intermediate level other than that indicated in the filed flight plan for the en-route phase of flight, without a time or geographical limit for the initial level. Such clearances will normally be used to facilitate the application of tactical control methods by ATC, normally through the use of an ATS surveillance system.
Where so laid down in a Letter of Agreement, TWR can assign SIDs without requesting clearance from APP, while APP protects the SID up to the initial altitude in case of a loss of communication right after departure, e.g. by descending other traffic only to 1000 ft above the initial altitude. In such cases, any clearance to climb above the initial altitude would not require ATC to cancel the initial altitude restriction. This view is backed up by the fact that, in the example shown, aircraft are required to climb to 6000 ft (ie. above the initial altitude of 5000 ft) in case of loss of communication before 28.2 DME RKN to remain clear of a restricted area right under the SID path.
2.3.5 Modern day SIDs and STARs do much more than simply connect an aerodrome to the en-route structure. Nowadays SIDs and STARs are increasingly used to impose a trajectory on aircraft from cruising level to the aerodrome and vice versa. Examples can be found in figure 2.
2.3.6 It is important to note that obstacle clearance and the separation of traffic flows are different goals of an SID or STAR. Obstacle clearance always means to cross a fix ABOVE a certain level. In case of separating traffic flows it can be either to cross BELOW or ABOVE a fix at a specified level. In a climb clearance, a restriction to cross ABOVE is usually achievable by the flight crew anyway so there is no need for ATC to cancel it. Restrictions to cross below a certain level are exclusively for ATM reasons so are much more likely to be cancelled if traffic permits.
2.3.7 The more systemised and detailed a procedure, the easier it is for aircrews and controllers to make operational errors, especially when it comes to lifting part or all of the restrictions, even if the proposed phraseology is accepted. On the other hand, in high-density airspace, heavily systemised procedures are the only option to keep controller workload at an acceptable level without hurting capacity. During peaks, published level and speed constraints should assist controllers managing air traffic, while in times of low traffic, controllers would have time available to individually lift restrictions and therefore optimise flight profiles.
Figure 2: Examples of complicated SIDs and STARs
2.4 Trajectory Concept
2.4.1 The concept of trajectories is one of the most debated subjects in today’s ATM. IFATCA makes reference to trajectory management in the Future Statement on Air Navigation. IFATCA expects service providers to move from route based flight data processing systems to geographical based ones. Eventually, IFATCA expects aircraft to fly on individual trajectories, mutually agreed to by ATC and the aircraft, and modified on a tactical basis by controllers. While such trajectories might still be constructed out of predefined segments similar to today’s published routes, the trajectory would be made up out of 3D coordinates with no need for generally published altitude restrictions. Consequently, there would be no room for interpretation as to what the flight path of any aircraft should be. TOC believes that the new proposed phraseology will suffice to bridge the gap between today’s ambiguities when dealing with published procedure restrictions and the introduction of trajectory based ATM.
2.4.2 Trajectory based air traffic management requires standardised on board equipment. So far, however, ATM has not yet made clear to aircraft operators and avionics manufacturers what its requirements are with respect to future aircraft navigation capabilities, for example turns (fly over, fly by, fixed turn radius) and other requirement like altitude, speed, or Time Over constraints. In fact, even in today’s ATM environment there is a mismatch between aircraft capabilities on one hand and ATM requirements and regulations on the other. See also agenda item B.5.7 “Study the Operation of Aircraft Flight Management Systems”.
2.5 SID/STAR naming
2.5.1 There have been incidents in the USA, Ireland and the Netherlands, where pilots have become airborne and flown a SID other than that expected even though they have been cleared on a different one. The incidents highlight another area where ATC requirements and avionics capabilities do not match.
2.5.2 Some FMS systems only accept a 4-letter name + alphanumeric code, so each procedure is named differently on the ATC Flight Plan from the Operational Flight Plan (OFP).
2.5.3 Avionics-wise, an interim solution to this problem is to adopt a standard procedure (ARINC 424 naming convention), so that, for example, an SID called DONAD1A is listed in the Company Flight Plan and FMS as DONA1A). Some FMS equipment only allows the entry of a 5-letter name; aircraft manufacturers and operators might be able to exert some influence on the choice of FMS equipment to address this problem.
2.5.4 These and maybe other issues relating to the naming of SIDs and STARs do not fall within the scope of this paper. However the subject seems a great issue. It is therefore recommended that this issue is subject of a separate study conducted by TOC.
3.1 SID design takes into account obstacle and terrain clearance. Additionally, ICAO allows for SIDs and STARs to be designed for separation or environmental purposes.
3.2 States have been driven to introducing more complex terminal procedures to accommodate growing traffic volumes and address environmental issues.
3.3 An initial level in a departure clearance exists mostly to facilitate ATC coordination and is not considered a published level restriction.
3.4 SID and STAR naming conventions are not adequately addressed by some FMS designs or by the ARINC 424 standard.
3.5 Future avionics systems need to be designed in accordance with ATM requirements. ATM has not yet formulated all such requirements.
It is recommended that;
4.1 Naming of SIDs and STARs shall be added to the TOC work programme for 2012.
Last Update: September 30, 2020