41TH ANNUAL CONFERENCE, Cancun, Mexico, 15-19 April 2002
WP No. 92
The Use of Lateral Offsets
Presented by SC1
1.1 The subject of Lateral Offsets was discussed at some length in WP88 which was presented to the conference in Geneva. The paper relied heavily on IFATCA experience relating to the North Atlantic region (NAT), ICAO guidance published in State Letter AN13/11.6-00/96, and IFALPA concerns about vertical collision risk when using highly accurate navigational systems.
1.2. The conclusion of WP88 stated:
“IFATCA supports the view that the use of lateral offsets must be considered in the light of safety benefits to be gained; that current separation minima take account of navigational accuracy and that any deviation has the potential to erode the safety margins. Nonetheless, it is accepted that the risks associated with either gross (vertical) navigational error or loss of planned vertical separation are increased with the use of highly accurate navigational systems. It is proposed that the IFATCA policy be extended beyond the NAT to include other airspace environments.”
1.3. The aim of this paper is to consider developments about the use of lateral offsets that have taken place mainly in NAT SPG and SASP over the past year with a view to extending IFATCA policy.
2.1. Although there is a desire to extend the use of lateral offsets globally, it is clear that there are differing issues and problems when comparing, for example, non- radar oceanic airspace and the radar TMA airspace. Consequently, the paper will look at these two types of airspace separately.
2.2 IFATCA policy for NAT RVSM airspace states:
|“Subject to the appropriate Collision Risk modeling, an off-set procedure is to be adopted in the NAT region. Prior to aircraft establishing on tracks that they establish to the following off-set procedure:
2.2. An agenda item at the North Atlantic Air Traffic Management Group (NAT ATMG) considered the development of procedures to accommodate lateral offsets to mitigate risk of collision due to a loss of vertical separation as a result of the high accuracy of navigation systems in maintaining the centre line. The lateral offset issues as seen by the North Atlantic Mathematicians’ Implementation Group (NAT/MIG) are quoted as follows (NAT ATMG/19 -WP/5 dated 3 May 2001):
1. It was reported by the MWG at NAT SPG/36 that the vertical collision risk in RVSM airspace due to operational errors was greater than the TLS (by a factor of at least 2 and possibly 3, depending on the value of the lateral overlap probability adopted). In fact, for 2 out of the 3 years for which RVSM vertical risk has been assessed, the risk due to operational errors has been greater than the TLS.
2. At MIG/5 results presented which should that as usage of GPS navigation by aircraft increases then the lateral overlap probability (a linear multiplier in the risk equation) also increases. Full GPS equipage would result in both operational and technical risk by an order of magnitude over the risk without GPS. This increase would push even the technical risk, normally considered a minor component of the risk, over the TLS. The longitudinal risk would also increase by the same factor. (Currently, it is estimated that GPS aircraft constitute approximately 30% of the NAT fleet.)
3. The MIG feels, because of the above, that it is a matter of urgency that measures are taken to mitigate the effects of increasing lateral overlap probability. The only practical way of achieving this that the MIG is aware of is the introduction of some form of lateral offset procedure as part of the normal operating practice for the NAT. There may of course be other solutions not considered by MIG.
4. It was recognised that offset procedures should reduce both technical and operational risk. Some offset procedures effectively reduce only one of these however. For example procedures that require aircraft (either GPS only or all aircraft) to fly an offset in a particular direction according to whether it is at an even or odd flight level do not reduce operational risk. The reason is that an operational error that results in an aircraft flying at an incorrect level in the belief by the pilots that it is the correct level, would fly the appropriate offset for that level it could then still collide with any aircraft which were correctly at that level with the appropriately offset.
2.3. The paper then proposed the following Lateral Offset procedure:
Lateral Offset Procedure:
This proposed procedure provides for random offsets, within the following guidelines. The intent of this procedure is to reduce risk by distributing aircraft across the three available options. Along a route or track there will be three positions (Centreline, One or two miles, right) that an aircraft may fly and remain within two miles of centreline. This procedure is applicable in NAT RVSM airspace to include WATRS The procedure is as follows:
- Operators capable of programming offsets may fly the centreline or an offset one or two nautical miles right of centreline, but no more than 2 nautical miles.
- Aircraft without offset programming capability must fly the centreline.
- Operators shall use whatever means available (e.g. TCAS, communications, Visual acquisition, to include aircraft lighting/contrails, GPWS, ADS-B) to ensure that they are not directly above or below another aircraft, and should offset laterally, not to exceed 2nm right of centreline.
- There is no ATC clearance required for this procedure nor should ATC be advised.
- Offsets for wake turbulence should not be made left of centreline nor more than 2 nm right of centreline
2.4. The IFATCA policy was based on the premise to reduce the effects of wake turbulence and false TCAS alerts. It did not consider the collision risk due to the loss of vertical separation as a result of the high accuracy of the navigation systems although the wake turbulence and TCAS problems were part of the same problem. Also, IFATCA policy states clearly the correlation between flight level and the offset. The NAT/MIG asserts that “there should be no link between offsets and flight levels”. Therefore IFATCA policy may have an inherent weakness in that does not reduce operational risk in the manner that was anticipated. Significantly, the ICAO guidance (State Letter AN13/11.6-00/96 Attachment A para 8.1) also stated “the direction of the offset is applied to the right of the centre line relative to the direction of flight”.
2.5. One of the objectives of the ICAO guidance was to ensure that offset procedures were applied in a uniform manner throughout the world and that the ICAO regions harmonize any such procedure. The guidance was developed in respect of oceanic airspace where significant lateral and longitudinal separations exist. However, it recognised that the use of lateral offsets in high-density airspace would require further investigation and that these issues were to be examined by SASP. Although IFATCA is not opposed in principle to the use of lateral offsets, it is clear that their use in TMA airspace, for instance, raises a number of problems that need resolving. The idea that an aircraft can fly a 1 nm offset without reference to ATC in RNP 1 or even RNP 5 airspace does not appear to be feasible where routes can be separated by little as 6 nm. Of course, it is axiomatic that the lower the RNP value, the more accurate the navigation system must be and arguably the higher the collision risk in the event of vertical separation error. However, the use of random lateral offsets may increase the risk of collision in the lateral dimension.
2.6. Whilst random offsets may be acceptable to controllers in fixed, parallel, route structures which have few crossing situations, they may not be acceptable in high density airspace using RNP 1 or 5 values. Controllers use various tactics and methods to separate and expedite the traffic in TMA airspace; primarily with radar vectoring although ATC applied offsets are useful in certain circumstances. The difficulty appears to be that in order to reduce or eradicate one problem with lateral offsets, it may cause other problems particularly in relation to the controllers’ function. The available evidence is not sufficient to assure that the establishment of random 1 nm lateral offsets in high density airspace reduces the collision risk arising from operational or technical risk. It may be possible to correlate the size of the lateral offset in direction relationship to the RNP value however it would appear that current FMS cannot offset below 1 nm.
3.1. Evidence has been provided by the NAT/MIG to indicate that there is a safety benefit in applying random lateral offsets up to 2nm in NAT airspace. The safety benefit accrues from the reduction of collision risk in the event of gross vertical errors. IFATCA policy is prescriptive in that it correlates flight levels to the direction of the offset. Which runs counter to the NAT/MIG assertion that there should be no link between the magnitude of offsets and flight level. Also, IFATCA policy was developed to address the problem of wake turbulence and false TCAS alerts but not specifically, collision risk from vertical errors therefore IFATCA policy should be reviewed and amended based on the evidence from the NAT/MIG.
3.2. IFATCA is not opposed to the extension of the lateral offset concept to all other oceanic/remote airspace in a global context however it is concerned that there are serious issues that have to be addressed before it can formulate policy. Far more substantive evidence will be needed before IFATCA can accept random lateral offsets in RNP 1 or 5 airspace. It is not within IFATCA resources to obtain such assurances independently therefore it will have to work closely with both ICAO and IFALPA to further the argument.
3.3. The dichotomy of more accurate aircraft systems having the potential to increase collision risk needs to be resolved therefore the subject should be retained on the work programme and accorded a certain degree of priority.
3.4. During the interim period until IFATCA or ICAO produce policy on lateral offsets, IFATCA will accept the ICAO guidelines in the draft attachment “Guidelines on the use of lateral offsets and the affect on airspace safety”.
It is recommended that:
4.1. IFATCA policy on Lateral Offsets in the NAT should be withdrawn.
4.2. The following items should be taken into account when developing further guidelines for lateral offsets;
a) Transparency to controllers;
b) Global applicable procedures;
c) Consistent in all regions and applicable in oceanic/remote airspace.
Last Update: September 29, 2020