33RD ANNUAL CONFERENCE, Ottawa, Canada, 18-22 April 1994
WP No. 101
Radar Monitoring Procedures in TMAs
At last years conference, during discussions on the MLS subject, it was decided to put radar monitoring as a separate subject on the SC 1 work programme.
This working paper reviews radar monitoring procedures only in TMA’s including precision radar monitors (PRM), lately developed and tested in the USA. Radar monitoring procedures in the en-route phase, will be dealt with in a separate working paper. Regarding radar monitoring in a SMGCS environment, IFATCA policy already exists (Tech Pol 2.2 and 2.3.).
The general principle of radar monitoring has long been established and is defined by ICAO as; the use of radar for the purpose of providing aircraft with information and advice relative to significant deviations from nominal flight path (ICAO Doc 4444 part 1).
In Doc 4444 chapter X 2.1.1. ICAO considers radar monitoring as one of the 6 functions of the general use of radar in the ATC service; maintain radar monitoring of air traffic in order to provide aircraft concerned, with information or advice relative to any significant deviations from the term of their air traffic control clearance, including cleared routes. Noted hereby is, that deviations are not considered significant until prescribed tolerance’s are exceeded.
ICAO also specially mentioned radar monitoring, in the provision of approach radar control service (Doc 4444 ch X 3.1.1.b ); provide radar monitoring of approaches made on other than radar facilities and advise aircraft of deviations from the nominal approach paths, provided that the ATC unit is equipped with a complete radar unit including at least a surveillance radar.
The principle that a controller should advise a pilot if it appears that he is deviating from the correct flight path, is reasonable, but it is considered to be in addition to the controllers primary function, which is to provide radar control service.
The purpose of this paper is to consider if radar monitoring service is now being used to permit reduction in separations regarding final approach procedures, that would not otherwise be possible.
Closely Spaced Parallel Runways
In some states radar monitoring service is now being used to make operations possible on closely spaced parallel runways. This procedure is recognised by ICAO (Doc 4444 Ch. IV 13 and circular 207 ) A spacing of 1525m (equal to 5000ft or approximately 0.82 N.M.) between the 2 parallel runway centrelines is the prescribed minimum distance that allows independent parallel approaches ( the minimum radar separation is 3nm ) . One of the provisions ( Doc 4444 Ch. IV 13.2.1.h) separate radar controllers with override capability to aerodrome control are provided to monitor the approaches to each runway.
In April 1993 , ICAO informed states and international organisations by means of state letter AN 13/34-93/51, about developments of a recommendation for minimum distance of 1035m ( equal to 3400 ft or approximately 0.55nm), between the centrelines of parallel runways for independent instrument approaches, with associated equipment requirements, procedures and guidance material based on experience gained with USA Precision Runway Monitor (PRM) system demonstration project. This project was set up by the FAA in order to enhance airport capacity.
It was concluded that in order to support reduced parallel runway spacing, several technical improvements were required to upgrade terminal surveillance systems, increased SSR azimuth accuracy, reduced SSR update period, higher resolution ATC display with automatic deviation alert. The PRM was born! The project team established a goal of 760m (equal to 2500ft or approximately 0.41nm) runway centreline spacing.
The Precision Runway Monitor (PRM) is a monopulse secondary surveillance radar system with an electronically scanned antenna and an update rate of 1 second.
In the ICAO procedures ( Doc 4444 ch 13.2.9 ) radar monitoring is required until 1nm from touchdown. Should an aircraft be threatened by an adjacent aircraft entering the Non Transgression Zone (NTZ ), (a buffer airspace between the two extended runway centrelines ) the threatened aircraft shall be instructed to alter heading to avoid the deviating aircraft the( so called escape procedure). At the moment these instructions are being issued, no radar separation exists!
In the event that instructions to alter heading have to be issued when the aircraft is descending below the minimum radar vector altitude (MRVA), this presents a problem regarding the responsibility for terrain clearance. In the relevant ICAO procedures only is noted the primary responsibility for the navigation on the LLZ course, rests with the pilot, and that control instructions and information are issued therefore only to ensure separation between aircraft. This separation is however not radar separation! Furthermore nothing is mentioned in these procedures about the responsibility of terrain clearance.
As the pilot is responsible for the navigation on the LLZ course and thus also responsible for his terrain clearance during these procedures when descending below the MRVA , the controller can only issue vertical control instructions.
Suitable escape procedures below the MRVA should be laid down, which ensures adequate terrain clearance . It should be clear to the pilot that he is conducting this escape procedure by the use of appropriate phraseology.
As distance between aircraft flying parallel approaches are very short ( at this time allowed 1525 m ( 0.82 N.M.), in the future with PRM 1036 m ( 0.55nm ) or maybe 760 m ( 0.41nm ), direct accessible communications and very quick reactions from both the controller and pilot are essential. This excludes the use of datalink for these kind of control instructions and requires a separate frequency.
Quick pilot response to controllers instructions is dependent on factors such as , the aircraft type, performance, configuration and the aircrew flying technique in IMC ( auto- pilot, flight director etc. ) and their workload. Also the degree of the turn issued in the breakout manoeuvre ( mostly a climbing turn ) can influence the time needed to reach the change in heading.
The monitoring controllers ability to observe and identify and thereafter to react to a situation when an aircraft is entering the NTZ and threatening another aircraft, is dependent on factors such as, surveillance and display equipment, workload, number of false alerts and the activity level. Especially when the activity level of the monitoring controller is low, he becomes less vigilant and boredom may occur.
MLS Advanced Procedures
When MLS curved approaches are being performed in an obstacle rich environment, it is critical that the aircraft does not deviate from the approach path, or it will lose terrain separation from the surrounding obstacles. It is also possible that there may be adjacent traffic patterns to other runways, or other airfields and a deviation from the curved approach could result in a loss of separation with an aircraft in the adjacent pattern.
One state has proposed that all curved approaches should be radar monitored up to a specific point, to ensure that the aircraft does not deviate from the flight path. The approach plate would be marked “radar required ” and the approach would only be permitted when radar was available.
The definition of radar monitoring would not appear to preclude this , but the aircraft on the approach will normally be below the minimum radar vectoring altitude (and in some situations these patterns are spaced with a distance which is less than the minimum radar separation ). This prevents the controller from issuing a radar heading to re-establish the aircraft on the curved path.
This also raises the question of the responsibility of the controller to notice the deviation from the flight path. What would be the responsibility of the controller if he did not notice the deviation until it was too late to prevent the aircraft hitting an obstacle.
There are new operations and equipment planned such GNSS and RNAV and the introduction of the RNP concept, that will change the way ATC operates. It is possible that radar monitoring could be used to allow the introduction of this equipment , or reduce the separation standards currently used. For example , would RNAV or MLS traffic patterns be closer together if radar monitoring was available ? If taken to the extreme, a complete automated ATC system could result in a situation where all the aircraft are controlled by datalink, with the controller sitting in silence and simply watching the radar screen in case somebody went the wrong way by mistake.
Tendency exists to use radar monitoring service as an integral part of approach procedures, in order to reduce separation standards for the benefit of airport capacity.
Radar monitoring is only one of the several functions performed by an approach radar controller.
Radar monitoring should not be used as the primary means of providing separation between traffic patterns, where aircraft are on their own navigation, e.g. MLS, RNAV etc. Especially in the case where those patterns and , in case of parallel approaches , those LLZ courses are spaced with a distance less than the minimum radar separation distance. Radar control service cannot take over and replace the responsibility for the navigation during the final approach procedure and the responsibility for terrain clearance below the MRVA. This is only an ATC responsibility when aircraft are under positive radar control (e.g. radar vectors , PAR or SRA), where other (increased) separation standards have to be applied.
It is not the controllers responsibility to provide terrain separation during the pilot- interpreted published instrument approach procedure. the approach procedure should be designed to ensure that terrain clearance is provided during the approach. Suitable escape procedures, and where necessary, back up equipment should be in place to ensure that the approach and any missed approach can be conducted safely.
Other issues that are to be examined related to this subject; Weather Limitations, Communications and RTF, Pilot / Controller training, Risk analysis, Airborne collision systems and Equipment failures.
It is recommended that:
Radar monitoring should not be used as the primary means of providing separation between traffic patterns where aircraft are on their own navigation.
Radar monitoring should not be used as the means of providing separation with obstacles (terrain clearance ) where aircraft are on their own navigation and below the Minimum Radar Vector Altitude (MRVA) Any escape procedures should provide adequate terrain clearance.
Last Update: September 20, 2020