APPLICATION OF SEPARATION STANDARDS

By E.O. Onasanya

INTRODUCTION

According to Wright (1901), "if one is looking for a perfect safety, he will do well to sit on a fence and watch the birds". However, as observed by ICAO (1984), "safety is no accident, it must be planned". A popular theory holds that most accidents or incidents can be traced to human failure somewhere, not necessarily the person or thing immediately involved in the occurrence. Among the factors that have impact on flight safety is the system software. This includes national and international legislation, associated orders, regulations and standard operating procedures. Separation standards published by the appropriate ATS authority, to a large extent, have the force of law and as such must be applied diligently.

SEPARATION STANDARDS

Separation standards are an essential ingredient of air traffic management. The effectiveness of an air traffic controller is generally dependent on how accurate he applies the separation minima. ICAO (1984) described separation as a "generic term to describe action on the part of ATC in order to keep aircraft, operating in the same general area, at such distances from each other that the risk of their colliding with each other is reduced". It can be effected in two planes, horizontal and vertical whereby separation in the horizontal plane can be achieved either longitudinally (by spacing aircraft each other at a specified distance, normally expressed in flying time) or laterally (by spacing aircraft side by side, but again at a specified distance from each other normally expressed in degrees). The required separation between aircraft is generally expressed in terms of minima, that is, in distances that should not be infringed.

In the recent past, many air traffic incidents have been reported within our national airspace. These were due primarily to misapplication of separation          standards. Investigations reveal that officers do not have an in-depth understanding of the separation minima. The problem area is majorly in the application of horizontal separation. This is the concern of this write-up.

According to ICAO, minima are specified in firm values of time between the moment a preceding
aircraft passes over a given point and that time when the next aircraft is allowed to pass the same point.

LATERAL SEPARATION

Lateral separation is to be applied so that the distance between those portions of the intended routes for which the aircraft are to be laterally separated is never less than an established distance to account for navigational inaccuracies plus a specified buffer.

GEOGRAPHICAL SEPARATION

This is separation positively indicated by position reports over different geographical locations as determined visually or by reference to a navigational aid. In applying this separation, air traffic controllers must ensure that there are two navigational aids between the aircraft (for positive identification of position). Generally, there is no geographical separation between aircraft in any of our TMAs. However, for instance, aircraft south of 'LAG1 VOR are geographically separated from those north of 'IBA' VOR Aircraft south of 'BDA1 VOR are separated from those north of 'KAD' VOR etc.

In these cases, aircraft are expected to be vertically separated before either crosses the nearer facility. If the two pilots are used to a particular feature e.g.River Niger or River Benue, this could be used for geographical separation purposes. Aircraft north of River Benue, for instance, are geographically separated from those south of the river. This separation is only achievable in visual conditions.

TRACK SEPARATION BETWEEN AIRCRAFT USING THE SAME NAVIGATIONAL AID OR METHOD

The track separation method generally in use is that for the VOR- which is to ensure that the tracks between two aircrafts are at least 15 degrees and one of the aircraft must be at a distance of 28km (15NM) or more from the facility. If one uses the rule of thumb in calculating the distance between the aircraft (15° at 15NM) it comes to 33/4 NM. One must realize that the radar separation under certain circumstances in some regions of the world can be reduced to 3Nm though the general radar separation minimum is 5Nm. The ability of the air traffic controller to effectively manage traffic is to a large extent dependent on picture of the traffic environment he is able to build. This is enhanced by how frequent the information or picture is updated. Using radar the information is updated every four seconds (15 revolutions per minute). For the procedural controller, the information is updated every time he scans through the strips on the bay. The accuracy of the facility in use e.g. VOR/DME is essential in meeting the required (or built in) target level of safety (generally 1:10^-7 at 95% level) for the separation minima. It is expected that the VOR/DME in use has been calibrated in accordance with the guidelines in Doc 8071-Manual on Testing of Radio Navigation Aids.

For instance, for the VOR, the schedule is three times per year (120 days). For enroute radials (those supporting airways) each e checked annually to its published full, useable distance. The interval for DME facility inspection is semi­annual (180 days) but for practical point of view, since a DME is always associated with another facility such as VOR or ILS, flight inspection of the DME should be conducted concurrently with its associated facility.

It must be emphasized that the minima cannot be less than the separation standards of ICAO. In many African States, the navigation aids are not calibrated for several years. This deficiency and the erratic power supply to these facilities affect the signal quality of the equipment. These are major factors which the appropriate ATS authority are expected to consider in arriving at an acceptable track separation minima (which in this case should be more than those specified in PANS/RAC Doc 4444- Rules of the Air and Air Traffic Services). To achieve separation minimum of 5NM, aircraft should be at least 20 degree from each other and one of the aircraft must be at a distance of at best 15Nm. Considering the difficulties highlighted above, the lateral spacing between two aircraft in our region should not be less than 5NM. Controllers must realize that there can be no lateral separation if aircraft are less than 15NM from the facility. In this case, only vertical separation is applicable.

VERTICAL SEPARATION

There seems to be no problem in the application of vertical separation minima. However the point to be made here is in passing traffic information (if traffic permits) to opposite direction traffic that are only 1000ft separated from each other. To see an aircraft in opposite direction that is as close as 0.1NM can sometimes be scary.

EFFECTIVE TRAFFIC MANAGEMENT

Concentration is an essential element in effective traffic management. ATC officers are to:

·          Scan through their strips before issuing a clearance.

·          Determine what separation standard is optimum.

·          Up date information regularly

·          Ensure that they have a total picture of the traffic and work environment at all times (this will necessarily include reading NOTAMs, log books, Met information etc)

·          Do not ever be afraid to resort to the use of vertical separation to resolve traffic problems.

CONCLUSION

The separation standards specified in Doc 4444 are the minima and must not be infringed. These should be increased by the appropriate ATS authority if there are deficiencies in ensuring the accuracy of the facilities (in use for the application of separation standards). An ATC officer is as good as his/her knowledge and application of separation standards. Every effort must be made to ensure that one has a total picture of the traffic and the work environment at all times. ATC officers should never be afraid to resort to the use of vertical separation to resolve problems or traffic conflicts.