A93C0130 - Loss of Separation Between Air Canada Airbus and Perimeter Airlines Inland (Ltd) Winnipeg, Manitoba 30 July 1993

Safety Action Taken

SMPE Software

The Board understands that Transport Canada (TC) has installed software to advise when the SMPE missing target phenomenon has possibly occurred. The software continually monitors the SMPE's track file capacity and causes error messages to be sent to technical and operational staff when a discrepancy exists. The effectiveness of the software cannot be validated through simulation, and since this software was installed, the phenomenon has apparently not reappeared. The Board further understands that TC is continuing its efforts with the manufacturer of the SSR to eliminate the deficiency that causes the phenomenon.

Safety Action Required

Interim Safeguards

The Winnipeg RAMP radar was certified for operational use with the SMPE missing target deficiency known, and measures were put in place to safeguard against risks of collision resulting from losses of targets. However, this investigation identified shortcomings with the existing defence mechanisms.

In the event of SSR target loss, a radar technician is required to reset the SMPE and regain SSR targets quickly. Part of the original measures to cope with the known deficiency included radar technician support over the midnight shift; however, some time prior to this occurrence, radar technician support during the night shift was eliminated. The elimination of the continuous technician support also necessitated that the "P" switch be turned on by the DSC prior to the midnight shift. As seen in this investigation, there was no procedure to safeguard against a failure of the day shift DSC to turn the "P" switch off. Moreover, the design of the SMPE reset procedure could allow errors to go undetected during the reset process.

The Board believes that there may still be latent, unsafe conditions within the measures and procedures that were designed to cope with the SMPE deficiency. If so, these conditions could again contribute to a risk of, or actual, collision of aircraft. Therefore, until the SMPE deficiency is corrected, the Board recommends that:

The Department of Transport reconsider the technical procedures and equipment associated with operating the radar with the SMPE deficiency. (A94-22)

Transport Canada's Response:

Transport Canada recognized the problem with the RAMP Radar Site Equipment (RSE) radars, characterized as ‘The SMPE Missing Target Phenomenon’ and has taken action to comply with Transportation Safety Board recommendation A94-22.

The following actions have been taken which have a direct effect on the significant contributing factors that were identified by the Safety Board.

A.  SSR Monopulse Extractor (SMPE) Software Updates:

Software upgrades were made to the SMPE in late 1993. The modified software monitors the SMPE for missing the target condition.  When the condition is detected, a forced switchcover to the standby SMPE unit occurs. This switchover is transparent to the ATS operation. Transport Canada considers the SMPE Missing Target Phenomenon to have been corrected.

B.  Use of Primary Radar Filter ('P') Switch:

The Radar Data Processing System (RDPS) R500 series software release, which sent into use in late 1993, contains enhancements that obviate the need for daily manipulation of the primary radar inhibit switch. The procedures of enabling the primary radar inhibit switch during the midnight shift is no longer practised by the Winnipeg Data Systems Coordinators (DSCs). Therefore the daily procedure of cycling the ‘P’ switch, which was identified by the TSB as a contributing factor in the occurrence, has been removed from the operational milieu.

C.  Re-enforced indication of ‘P’ Switch Selection:

An RDPS Change Proposal (CP 94-001) which provides a positive indication of the setting of the ‘P’ switch at each affected controller’s display has been approved by the Configuration Control Board. Current plans are to have this enhancement delivered in the R800 series release, which should be in mid to late 1995. When implemented, this change will reduce, to absolute minimums, the possibility of a “P” switch being inadvertently activated.

The Air Traffic Control Manual of Operations provides direction to controllers to follow in the event of the loss of radar data. These procedures would be applicable to any situation where radar data is lost. The retention of al flight data manually on the flight data strip is a redundancy feature which is in place to ensure a safe operation can be maintained, using non-radar separation techniques, in the event of any loss of radar data or radar failure. 

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