32ND ANNUAL CONFERENCE, Christchurch, NZ, 19-23 April 1993
WP No. 90
RNAV Procedural Separation – Longitudinal Distance Standard Minima
The current Policy Statement 7.6 – RNAV Procedural Separation Minima – is operationally unrealistic in terms of existing longitudinal separation practice. It is unrealistic because it advocates a minimum time standard that in fact has already been superseded by common accepted operational practice i.e. a minimum of 5 minutes longitudinal time separation when Mach No. Technique is used.
The existing policy is not definitive because it deals with two different types of longitudinal separation. Longitudinal time separation is different both in principle and in application when compared to longitudinal distance separations. Therefore the two different separation standards should be recognised as such and specified in different policy statements.
Any separation standard is designed with the intention of providing the maximum degree of protection against the combined effects of human error, navigational error, and systems error from both airborne and ground systems.
Currently there is a proposal before ICAO that seeks to introduce a RNAV 80 NM longitudinal separation. This proposed separation is supported by the Informal South Pacific ATS Coordinating Group (ISPACG) which has indicated that longitudinal separations could eventually reduce to 30NM. SC 1 believes longitudinal distance separations of RNAV 80NM between suitably equipped aircraft should not be used until an appropriate Target Level of safety has been identified and Collision Risk Modelling established.
Whenever a separation standard is devised a level of safety must be assured. This level of safety can be expressed as a numerical value called the Target Level of Safety. The Target Level of Safety may be determined and expressed, for example , in terms of fatal aircraft accidents per 10 million aircraft flying hours. The Target Level of Safety should be dynamic so as to allow for it’s periodic review to assess current validity.
A Collision Risk assessment can be made after the actual navigation performance of aircraft operating in the area under consideration for reduced separation standards has been monitored over a period of time. Therefore before any attempt to reduce an existing separation standard is made the calculation of the collision risk inherent in the new standard must be made and compared with the Target Level of Safety.
The existing policy statement with regard to longitudinal distance or time separation standards is neither definitive nor comprehensive.
The existing 7.6 policy statement “RNAV Procedural Separation Minima” should be deleted.
A new IFATCA policy statement for RNAV Procedural Time Separation is produced in a separate working paper.
Longitudinal distance separations of less than RNAV 80NM between suitably equipped aircraft should not be used until an appropriate Target Level of Safety has been identified and Collision Risk Modelling established.
It is recommended that:
The introduction of a longitudinal distance standard less than RNAV 80NM will only be accepted once the following standards have been met :
- any change of assigned Mach No must be approved by ATC;
- facilities and training to accommodate any resultant increase in traffic levels must be in place;
- before any separation standard is reduced :
- a Target Level of Safety must be established;
- a Collision Risk Model based on distance criteria must be developed and implemented.
Last Update: September 20, 2020