Narrative:

Aircraft was cleared for ILS 26L approach in IMC. Autoplt on, established on the localizer/GS, and cleared to land. GS suddenly jumped up and then down about 1/2 DOT with the autoplt staying connected. The oscillations were such as to alarm the crew. I reported the problem to the tower before landing and they indicated the problem might be due to crossing aircraft on the approach end of 26L. After landing I called the tower to further explain the autoplt upset and the impact on passenger. I noted to them there was an aircraft stopped inside the ILS protected area when we flew over the approach end of the runway, and that there were also several aircraft seen crossing the approach end of the runway, when we were on short final after breaking out of the clouds. The tower controller I spoke with indicated he had no idea which aircraft were on ILS approachs and which were on visual approachs during the time of the occurrence. This situation needs attention since we are only in the first month of operation with runway 26L after construction. When aircraft are cleared for an ILS in IMC conditions, the ILS protected area should be just that, protected. At the very least aircrews should be notified of any encroachment of aircraft which may affect the performance of the ILS system.

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Original NASA ASRS Text

Title: AFTER PORPOISING OF THE ACFT WHILE ON FINAL APCH A B737 PIC FILES A RPT ON THE VIOLATION OF THE ILS HOLD SHORT AREA FOR RWY 26L.

Narrative: ACFT WAS CLRED FOR ILS 26L APCH IN IMC. AUTOPLT ON, ESTABLISHED ON THE LOC/GS, AND CLRED TO LAND. GS SUDDENLY JUMPED UP AND THEN DOWN ABOUT 1/2 DOT WITH THE AUTOPLT STAYING CONNECTED. THE OSCILLATIONS WERE SUCH AS TO ALARM THE CREW. I REPORTED THE PROB TO THE TWR BEFORE LNDG AND THEY INDICATED THE PROB MIGHT BE DUE TO XING ACFT ON THE APCH END OF 26L. AFTER LNDG I CALLED THE TWR TO FURTHER EXPLAIN THE AUTOPLT UPSET AND THE IMPACT ON PAX. I NOTED TO THEM THERE WAS AN ACFT STOPPED INSIDE THE ILS PROTECTED AREA WHEN WE FLEW OVER THE APCH END OF THE RWY, AND THAT THERE WERE ALSO SEVERAL ACFT SEEN XING THE APCH END OF THE RWY, WHEN WE WERE ON SHORT FINAL AFTER BREAKING OUT OF THE CLOUDS. THE TWR CTLR I SPOKE WITH INDICATED HE HAD NO IDEA WHICH ACFT WERE ON ILS APCHS AND WHICH WERE ON VISUAL APCHS DURING THE TIME OF THE OCCURRENCE. THIS SIT NEEDS ATTN SINCE WE ARE ONLY IN THE FIRST MONTH OF OP WITH RWY 26L AFTER CONSTRUCTION. WHEN ACFT ARE CLRED FOR AN ILS IN IMC CONDITIONS, THE ILS PROTECTED AREA SHOULD BE JUST THAT, PROTECTED. AT THE VERY LEAST AIRCREWS SHOULD BE NOTIFIED OF ANY ENCROACHMENT OF ACFT WHICH MAY AFFECT THE PERFORMANCE OF THE ILS SYS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.