Study Advanced Offset Procedures

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Study Advanced Offset Procedures

48TH ANNUAL CONFERENCE, Dubrovnik, Croatia, 20-24 April 2009

WP No. 92

Study Advanced Offset Procedures

Presented by TOC


A number of mid-air collisions and close encounters have raised concern in the aviation community that aircraft navigation has become so accurate that collision risk mitigation is required. Strategic offset procedures – where aircraft do not fly on the centreline of a route or airway but offset to the right – have been in use in oceanic- and remote continental airspace for several years. This working paper analyses possible benefits and drawbacks of introducing similar procedures to high density airspace with Air Traffic Services surveillance. The paper proposes to amend IFATCA Provisional Policy.


1.1  The possibility of extending the use of current Strategic Lateral Offset Procedures (SLOP) into airspace that is provided with Air Traffic Services (ATS) surveillance systems and services has been discussed within IFATCA. The subject generates concern with many controllers, especially those that have the responsibility of separating aircraft in high density airspace.

1.2  Provisional Policy has been accepted at the 2008 Arusha conference, which indicates that further research is required before any offset procedure can be supported in airspace where ATS surveillance is provided. Subsequently, the Technical and Operations Committee (TOC) was tasked to further study advanced offset procedures.

1.3  The mid-air collision over Brazil, and the fact that current SLOP provisions can only be applied in oceanic- and remote continental airspace have prompted the ICAO Separation and Airspace Safety Panel (SASP) to develop an advanced strategic offset concept.



2.1.1 IFATCA has the following Provisional Policy on SLOP, which was adopted at the annual IFATCA conference in March 2008:

“IFATCA endorses Strategic Lateral Offset Procedures (SLOP) in oceanic- or remote continental airspace where there is no ATS surveillance service provided.

Further research is required before IFATCA can endorse any offset procedure in airspace where there is ATS surveillance service provided.”


2.1.2  It is felt by IFATCA that a detailed analysis of issues that could possibly be affected by SLOP needs to take place before it is allowed to use SLOP in airspace provided with ATS surveillance systems and services.

2.1.3  Deviating from a route by the use of a concept like SLOP has an impact on certain controller tools. At the 2005 Melbourne Conference, IFATCA has adopted Policy on Route Conformance Monitoring Systems (RCMS):

“A ROUTE CONFORMANCE MONITORING SYSTEM (RCMS) is a function of an Automated ATS System that monitors the position of an aircraft to detect when it deviates from its route. An RCMS is considered to be a Controller Tool.

A ROUTE DEVIATION ALERT (RDA) is an alert provided to a controller to notify that an aircraft’s position is displaced outside the tolerances defined within RCMS.”



2.2.1 IFALPA Policy is (IFALPA RAC-A, Section 3):


4.1.7 The airborne navigation system should include the capability to navigate along lateral offset tracks so as to reduce the collision risk in the case of possible loss of vertical separation on reciprocal tracks. Airborne navigation systems should have the capability to accept pilot selected offset values in increments of 0.1 NMs (up to 99.9NMs). GNSS referenced airborne navigation systems should include an embedded default lateral offset, as specified in IFALPA RAC-A, Section 3, Sub-Section 16, para. 16.4.2 (see also IFALPA Annex 2, Chapter 3, para. 3.6.3) The functions specified in and are required for the certification of RNAV equipment for use in Reduced Vertical Separation Minima (RVSM) Airspace. Wherever feasible, offsets should be automatically and gradually applied when climbing through FL 150 and should be similarly removed when descending through FL 200. Alternatively, the offsets could be applied and removed according to airspace requirements coded into the navigation database (e.g. no offset in TMA /automatic offset outside of TMAs). The offset status should always be enunciated to the pilots. The ATC route structure should allow for offset operation whenever possible.”

2.3 ICAO

2.3.1  Currently ICAO Annex 2 Rules of the Air states:

“ Unless otherwise authorized by the appropriate ATS authority, or directed by the appropriate air traffic control unit, controlled flights shall, in so far as practicable:

a) when on an established ATS route, operate along the defined centre line of that route; or

b) when on any other route, operate directly between the navigation facilities and/or points defining that route.”

2.3.2  ICAO has however recognised the need for SLOP by putting provisions developed by SASP into ICAO Doc 4444 PANS-ATM. These procedures are used exclusively in Oceanic and Remote Continental Airspace in order to provide an additional safety margin and mitigate the risk of traffic conflict when non-nominal events such as aircraft navigation errors, height deviation errors and turbulence induced altitude-keeping errors do occur. Collision risk is significantly reduced by application of these offsets. They are also used to mitigate against wake turbulence incidents between aircraft on the same track.

2.3.3  The following is an excerpt from the 2005 ICAO “NORTH ATLANTIC MNPSA OPERATIONS MANUAL”:

“Strategic Lateral Offset Procedure (SLOP)

ATC clearances are designed to ensure that separation standards are continually maintained for all traffic. However, the chain of clearance definition, delivery and execution involves a series of technical system processes and human actions. Errors are very rare but they do occur. Neither pilots nor controllers are infallible. Gross Navigation Errors (usually involving whole latitude degree mistakes in route waypoints) are made, and aircraft are sometimes flown at flight levels other than those expected by the controller. As previously indicated, when such errors are made, ironically, the extreme accuracies of modern navigation and height keeping systems themselves exacerbate the risk of an actual collision. Within an SSR environment the controller is alerted to such errors and can therefore intervene in a timely fashion. In Oceanic airspace, such as the North Atlantic, in which the controller’s awareness of traffic disposition is reliant largely upon pilot voice position reports, this is not the case. Consequently, it has been determined that allowing aircraft conducting oceanic flight to fly lateral offsets will provide an additional safety margin and mitigate the risk of traffic conflict when non-normal events such as aircraft navigation errors, height deviation errors and turbulence induced altitude-keeping errors do occur. Collision risk is significantly reduced by application of these offsets. This procedure is known as “Strategic Lateral Offset Procedure (SLOP)”.

2.4.1 This procedure provides for offsets within the following guidelines:

  • along a route or track there will be three positions that an aircraft may fly: centreline or one or two miles right; and
  • offsets will not exceed 2 NM right of centreline.

2.4.2 Distributing aircraft laterally and equally across the three available positions adds an additional safety margin and reduces collision risk. This is now a standard operating procedure for the entire NAT Region and pilots are required to adopt this procedure as is appropriate. In this connection, it should be noted that:

  • Aircraft without automatic offset programming capability must fly the centreline.
  • Operators capable of programming automatic offsets may fly the centreline or offset one or two nautical miles right of centreline to obtain lateral spacing from nearby aircraft. (Offsets will not exceed 2 NM right of centreline and offsets left of track centreline must not be made). An aircraft overtaking another aircraft should offset within the confines of this procedure, if capable, so as to create the least amount of wake turbulence for the aircraft being overtaken.
  • Pilots should use whatever means are available (e.g. TCAS, communications, visual acquisition, GPWS) to determine the best flight path to fly.
  • For wake turbulence purposes, pilots should also fly one of the three positions shown above. Pilots should not offset to the left of centreline nor offset more than 2 nm right of centreline. Pilots may contact other aircraft on the air-to-air channel, 123.45 MHz, as necessary; to co-ordinate the best wake turbulence mutual offset option. (Note. It is recognised that the pilot will use his/her judgement to determine the action most appropriate to any given situation and that the pilot has the final authority and responsibility for the safe operations of the aeroplane. See also Chapter 11, paragraph 11.5.) As indicated below, contact with ATC is not required.
  • Pilots may apply an offset outbound at the oceanic entry point and must return to centreline prior to the oceanic exit point.
  • Aircraft transiting radar-controlled airspace mid-ocean should remain on their already established offset positions.
  • There is no ATC clearance required for this procedure and it is not necessary that ATC be advised.

Voice Position reports should be based on the waypoints of the current ATC clearance and not the offset positions.”

2.3.4  As introduced in paragraph 1.3, SASP has developed an advanced strategic offset concept. It is designed to mitigate the increased risk of collision due to GNSS navigation accuracy in global enroute airspace. In other words: aircraft navigation capability nowadays has become so accurate that aircraft on the same ATS route and at the same altitude have the potential to coexist in the same location in space and time. ICAO considers implementation of such a concept an urgent requirement. At the moment the concept proposed by SASP is currently the only known application of advanced offset procedures, and is referred to as Advanced Strategic Offset Concept (ASOC). For the purpose of this working paper, the use of the acronym “ASOC” will refer to the generic concept as well as the specific ICAO one.

2.3.5  Based on input received from various organisations, SASP at its recent working group meeting has adopted “ASOC” as the interim working title for the planned mitigation measure and has developed the following draft ASOC:

  • Aircraft using GNSS as navigation source should be allowed or even required to flyrandomly selected “micro offset” right of track in tenths of a Nautical Mile (NM) up to a maximum of 0.5 NMs (value to be confirmed by scientific study); the maximum value should ensure that the sum of the actual navigation performance plus the micro offset does not exceed RNP1 accuracy requirements (RNP1 is considered to be the most stringent navigation specification for enroute operations).
  • To be used outside of terminal airspace, i.e. in the enroute phase of flight after a Standard Instrument Departure (SID) termination point until the first waypoint on a Standard Arrival Route (STAR). Distance from the aerodrome reference point might be a suitable alternative for aerodromes where no SIDs and STARs are promulgated.
  • Initial implementation by pilots input; very few aircraft already have the ability to select offsets in steps of tenths of a NM; all other aircraft navigation systems that allow offsets only in full NM should be upgraded to allow tenths of NMs as soon as practical. This is also reflected in the IFALPA policy quoted in 2.2.1.
  • Eventually the micro offset would be automated; the navigation system would select the random value and apply it automatically after leaving terminal airspace, removing it again before entering terminal airspace of the destination.
  • The procedure would be absolutely transparent to ATC, as the small amounts of deviation from route centrelines would hardly be noticeable on ATS surveillance displays and the requirement to assure that all aircraft would still stay within the required navigation performance should satisfy all airspace management requirements.
  • While a potential retrofit of aircraft navigations systems to allow the input of lateral offsets in tenths of a nautical mile should be relatively easily achievable with minimal cost, the final solution of automatic micro offsets (ASOC) would entail more complex software changes and database implications.

2.4 The SASP ASOC draft mainly deals with cockpit automation and aircraft navigation capabilities. Several ATC and airspace design issues have been identified and research is being done. Some of the most relevant issues in the view of TOC are:

2.4.1  Effect of ASOC on RCMS

Some service providers, e.g. NATS in the UK, have already implemented or are in the process of implementing closely spaced routes and associated procedures. This includes Precision Area Navigation (P-RNAV) routes and SID and STAR transitions, for example the developments in the new London Terminal Control North development, where National Air Traffic Services (NATS) is planning to introduce a 5 NM P-RNAV route spacing standard. The safety argument for those operations is based around the assumption that aircraft will be operating on the centrelines of these routes.
An RCMS is required to monitor the operation on such closely spaced routes and the conformance parameters may be set very tightly. Any ASOC (even micro ones) could result in nuisance alerts being generated by an RCMS.
Some States, for example Germany and Switzerland, also have routes that are located very close to areas with military activity. It is common that the routes are spaced 2.5 NM from such areas and examples are known where the routes are even only spaced 1 NM from such areas. In such cases the flights might also be monitored with RCMS with very tight buffers set for warning controllers of any deviation.

2.4.2  Globally Harmonized Application

There is a possibility that there will be areas where ASOC is allowed and areas where ASOC is not allowed. One Flight Information Region (FIR) might be designed to accommodate ASOC, while the next one might not. This would introduce the need to indicate on charts where ASOC is allowed and where ASOC is not allowed. This applies to individual routes and procedures and may also apply to defined airspace and sectors. Similarly there may be a need to add to the Flight Management System (FMS) route/procedure description whether ASOC is allowed or not. Points where ASOC starts to be allowed and points where ASOC ceases to be allowed would then have to be defined. There might also be a need for transition procedures to be implemented, similar to the procedures when vertical separation minima change from feet to meters and vice versa. Any risks associated with aircraft continuing ASOC on the approach phase of flight would need to be eliminated. This shows that individual applications of ASOC are not acceptable and a globally harmonised approach has to be achieved.

2.4.3  Airspace design

As stated above, some service providers have already implemented or are in the process of implementing closely spaced routes and associated procedures. Does the artificially introduced aircraft displacement affect the mathematical models that define the current route spacing- and separation standards? If it does, all ATS airspace must be designed in such a way that the artificial aircraft displacement because of ASOC does not take an aircraft outside the safety buffers currently laid down in airspace separation standards. This might result in routes having to be moved further apart from one another and/or from restricted areas. An undesired side effect might be a necessary reduction in the number of routes and/or sector capacity.

One of the areas where offset procedures are not acceptable, is the approach and departure phase. Approach and departure procedures are designed to lead aircraft to a landing runway, and to provide terrain and obstacle clearance. Any kind of offset during an Area Navigation (RNAV)-based approach or departure procedure might result in an undesirable inaccuracy in the final phase of flight, leading to issues with obstacle and terrain clearance, or – in the case of parallel runways – alignment with the wrong runway. Therefore, approach and departure procedures must be explicitly excluded from ASOC.

2.4.4  Human Factors

Having designated areas where ASOC is allowed poses the ever-present problem of mixed mode operations, with pilots having to constantly monitor whether they are currently allowed to use ASOC or not, and what value offset is expected of them. Even with symbols for ASOC being embedded into the FMS display, there is always a risk of making a mistake. The more complicated the system gets, the easier it is for humans to err. What happens if a pilot forgets to turn an offset off before commencing his or her approach towards the destination airport? At some stage in the future, ASOC may be an automatic feature requiring no pilot intervention at all, but as the introduction of ASOC is possible without fully automated avionics support this approach must be considered as well.

Most of the above are pilot-related Human Factor (HF) issues, while some, such as the establishment of ASOC transition airspace and procedures, could result in additional tasks for controllers. However, in order to not increase controller workload, ASOC should not require controller intervention at all. Associated risks must be addressed at the level of ATM System/Airspace design.

2.5 Alternatives

2.5.1 ASOC will only mitigate the risk of aircraft flying on identical same or identical reciprocal tracks at the same altitude/height. It does not mitigate the risk of aircraft on crossing or converging tracks at the same altitude/height. If ASOC is not completely transparent to controllers, then another variable is introduced into the system. Is the benefit of reducing the risk of mid-air collisions worth the increased risk from incorrectly applying ASOC? IFATCA feels that alternatives to implementing ASOC in ATS surveillance airspace should also be explored. Other methods may also be effective in managing the risk. Examples would be the elimination of bi-directional routes, and the establishment of separate TMA entry and exit points.

2.6 ASOC and the controller

2.6.1  Regardless of the scope of ASOC implementation, controllers must not be used as a mitigating factor when assessing the safety of the concept. For example, they must not be required to monitor whether the aircraft is applying ASOC or not, simply because the ASOC offset value could be so small that the controller may not be able to detect it on his radar/situation display.

2.6.2  In airspace where ASOC is implemented controller tools such as RCMS should be in place to alert controllers of significant navigation errors. Parameters of such conformance monitoring systems should be set in such a way that normal operation of ASOC will not generate route conformance warnings. Incorrect application of ASOC could however result in navigation errors of such a magnitude that route conformance warnings would – and should – be generated.


3.1  IFATCA recognises concepts such as ASOC as a potential way to reduce collision risk between aircraft in high density airspace. Studies including collision risk modelling need to be undertaken to quantify any safety benefit. Such studies shall not consider the controller a mitigating factor.

3.2  ASOC will only mitigate the risk of aircraft flying on the identical same or identical reciprocal track. An alternative worth considering would be the elimination of bi- directional routes. Further research is required to investigate alternative solutions that reduce the collision risk.

3.3  If an advanced strategic offset concept is introduced, airspace design criteria must allow for the deliberate aircraft displacement. This might result in routes spaced further apart from other routes and/or restricted airspace. A redesign of airspace could lead to reduced ATC sector capacity.

3.4  If an advanced strategic offset concept is introduced, route conformance monitoring systems must take it into account.

3.5  Any advanced strategic offset concept is only acceptable if it does not require controller intervention.

3.6  If an advanced strategic offset concept is introduced, then it must be globally harmonised.


It is recommended that;

4.1 IFATCA Provisional Policy on page 3 2 3 17 of the Technical and Professional Manual:

Further research is required before IFATCA can endorse any offset procedure in airspace where there is ATS surveillance service provided

Is replaced by:

IFATCA only supports an advanced strategic offset concept provided that;

  • Studies conclude that the concept enhances safety;
  • The concept is globally harmonised;
  • The concept is taken into account in airspace and procedures design;
  • ATC surveillance systems accommodate the concept; and
  • The concept is transparent to ATC, requiring no controller intervention at all.

And is included on page 3 2 3 17 of the IFATCA Technical and Professional Manual as Policy.

Last Update: September 29, 2020  

April 14, 2020   333   Jean-Francois Lepage    2009    

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