Narrative:

I took the local one position today; and during the relief briefing; the controller said he had received a pilot report from a falcon of fluctuations on the GS. The controller also indicated he had received 3 subsequent reports of a normal GS. I was training a developmental on this position and indicated to him to forward this pilot report to all inbound aircraft. The second inbound aircraft we talked to said he was receiving a 'large fluctuation' in the GS near casse (the outer marker). He did complete the approach and landed safely. I relayed the report to the controller in charge and the GS was reported out of service. Airway facilities (maintenance) was called out and did investigate the GS. During this entire time the GS monitors normal with a green light in the tower. During my session on local control numerous aircraft flew the GPS and localizer approaches to runway 35R in place of the ILS and all of them executed a missed approach due to low ceilings/visibility. There were calls to the tower controller in charge from both the airport and the FBO's complaining that they were losing business/customers because we had called the GS out of service. Denver TRACON also called numerous times complaining that everyone was diverting to den because of this equipment problem and that it needed to be back in service right away. They all wanted to know when it would be back; what was wrong with it; what were the reports we had received; etc. As I was getting off of position; I overheard the controller in charge telling someone he was 'going to get the GS back in service within 5 minutes.' pocc then called and said that maintenance could not find anything wrong with the GS and it was returned to service. What you must know about this situation is that we have had 3 fatal aircraft crashes; all involving the GS; and all involving bad WX (low ceilings; visibility and moisture). The same situation just occurred a week ago on the evening of 2/fri/07. Same WX conditions; same ILS; 2 pilots almost back to back had severe GS situations. (One aircraft actually was 1000 ft low on the GS and was issued a low altitude alert by our local controller indicating she was 1000 ft low!; you can hear this on our tape; I've heard it!) again; the den TRACON was furious because of the extra workload and diversions it caused. Airway facility was furious because they hadn't expected the bad WX and their technicians were not near and had to be called in after normal hours; and they couldn't find anything wrong with it. And again the GS was put back in service very quickly with no explanation as to the reason for the 'fluctuations.' after our 3RD fatal crash; we did receive a lot of media attention regarding this issue. Many pilots on the field were disturbed about the GS. Many feel it is still unsafe. Flight check spent a lot of time flying the GS; looking for any reason for these fluctuations. Unfortunately; to my knowledge; these checks were flown in daylight; dry; VFR WX conditions; and no problem was ever discovered. I believe there is something dangerously wrong with the GS during bad or moist WX. The elevation changes quite a bit along the length of the GS/localizer; there is a line of very high tension power lines that run across the localizer in this area that may give off some 'signal;' there are also 4 lane and 2 lane roads that run across the localizer where possibly a large truck could block the GS signal. I'm certainly not an expert and while I believe the maintenance people are trying to do their jobs; I believe there is a great deal of pressure by those higher up to get our GS back in service. So; consequently; it is never fully investigated during the bad WX which affects it; and I truly believe it is unsafe to the flying public in its current condition. By the way; shortly after the GS was returned to service today; a lear reported fluctuations on the GS near casse. Airway facility was listening to this transmission when the lear pilot said...it's the same fluctuation I always get in this area.

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Original NASA ASRS Text

Title: A CTLR EXPRESSED CONCERN REGARDING RECURRING ILS RWY 35R GS ANOMALIES THAT APPEAR TO OCCUR DURING INCLEMENT WX CONDITIONS.

Narrative: I TOOK THE LOCAL ONE POSITION TODAY; AND DURING THE RELIEF BRIEFING; THE CTLR SAID HE HAD RECEIVED A PLT RPT FROM A FALCON OF FLUCTUATIONS ON THE GS. THE CTLR ALSO INDICATED HE HAD RECEIVED 3 SUBSEQUENT RPTS OF A NORMAL GS. I WAS TRAINING A DEVELOPMENTAL ON THIS POSITION AND INDICATED TO HIM TO FORWARD THIS PLT RPT TO ALL INBOUND ACFT. THE SECOND INBOUND ACFT WE TALKED TO SAID HE WAS RECEIVING A 'LARGE FLUCTUATION' IN THE GS NEAR CASSE (THE OUTER MARKER). HE DID COMPLETE THE APCH AND LANDED SAFELY. I RELAYED THE RPT TO THE CIC AND THE GS WAS RPTED OUT OF SERVICE. AIRWAY FACILITIES (MAINT) WAS CALLED OUT AND DID INVESTIGATE THE GS. DURING THIS ENTIRE TIME THE GS MONITORS NORMAL WITH A GREEN LIGHT IN THE TOWER. DURING MY SESSION ON LOCAL CONTROL NUMEROUS ACFT FLEW THE GPS AND LOCALIZER APPROACHES TO RWY 35R IN PLACE OF THE ILS AND ALL OF THEM EXECUTED A MISSED APCH DUE TO LOW CEILINGS/VISIBILITY. THERE WERE CALLS TO THE TOWER CIC FROM BOTH THE ARPT AND THE FBO'S COMPLAINING THAT THEY WERE LOSING BUSINESS/CUSTOMERS BECAUSE WE HAD CALLED THE GS OUT OF SERVICE. DENVER TRACON ALSO CALLED NUMEROUS TIMES COMPLAINING THAT EVERYONE WAS DIVERTING TO DEN BECAUSE OF THIS EQUIPMENT PROBLEM AND THAT IT NEEDED TO BE BACK IN SERVICE RIGHT AWAY. THEY ALL WANTED TO KNOW WHEN IT WOULD BE BACK; WHAT WAS WRONG WITH IT; WHAT WERE THE RPTS WE HAD RECEIVED; ETC. AS I WAS GETTING OFF OF POSITION; I OVERHEARD THE CIC TELLING SOMEONE HE WAS 'GOING TO GET THE GS BACK IN SERVICE WITHIN 5 MINUTES.' POCC THEN CALLED AND SAID THAT MAINT COULD NOT FIND ANYTHING WRONG WITH THE GS AND IT WAS RETURNED TO SERVICE. WHAT YOU MUST KNOW ABOUT THIS SITUATION IS THAT WE HAVE HAD 3 FATAL ACFT CRASHES; ALL INVOLVING THE GS; AND ALL INVOLVING BAD WX (LOW CEILINGS; VISIBILITY AND MOISTURE). THE SAME SITUATION JUST OCCURRED A WEEK AGO ON THE EVENING OF 2/FRI/07. SAME WX CONDITIONS; SAME ILS; 2 PLTS ALMOST BACK TO BACK HAD SEVERE GS SITUATIONS. (ONE ACFT ACTUALLY WAS 1000 FT LOW ON THE GS AND WAS ISSUED A LOW ALTITUDE ALERT BY OUR LOCAL CTLR INDICATING SHE WAS 1000 FT LOW!; YOU CAN HEAR THIS ON OUR TAPE; I'VE HEARD IT!) AGAIN; THE DEN TRACON WAS FURIOUS BECAUSE OF THE EXTRA WORKLOAD AND DIVERSIONS IT CAUSED. AIRWAY FACILITY WAS FURIOUS BECAUSE THEY HADN'T EXPECTED THE BAD WX AND THEIR TECHNICIANS WERE NOT NEAR AND HAD TO BE CALLED IN AFTER NORMAL HOURS; AND THEY COULDN'T FIND ANYTHING WRONG WITH IT. AND AGAIN THE GS WAS PUT BACK IN SERVICE VERY QUICKLY WITH NO EXPLANATION AS TO THE REASON FOR THE 'FLUCTUATIONS.' AFTER OUR 3RD FATAL CRASH; WE DID RECEIVE A LOT OF MEDIA ATTENTION REGARDING THIS ISSUE. MANY PLTS ON THE FIELD WERE DISTURBED ABOUT THE GS. MANY FEEL IT IS STILL UNSAFE. FLIGHT CHECK SPENT A LOT OF TIME FLYING THE GS; LOOKING FOR ANY REASON FOR THESE FLUCTUATIONS. UNFORTUNATELY; TO MY KNOWLEDGE; THESE CHECKS WERE FLOWN IN DAYLIGHT; DRY; VFR WX CONDITIONS; AND NO PROBLEM WAS EVER DISCOVERED. I BELIEVE THERE IS SOMETHING DANGEROUSLY WRONG WITH THE GS DURING BAD OR MOIST WX. THE ELEVATION CHANGES QUITE A BIT ALONG THE LENGTH OF THE GS/LOCALIZER; THERE IS A LINE OF VERY HIGH TENSION POWER LINES THAT RUN ACROSS THE LOCALIZER IN THIS AREA THAT MAY GIVE OFF SOME 'SIGNAL;' THERE ARE ALSO 4 LANE AND 2 LANE ROADS THAT RUN ACROSS THE LOCALIZER WHERE POSSIBLY A LARGE TRUCK COULD BLOCK THE GS SIGNAL. I'M CERTAINLY NOT AN EXPERT AND WHILE I BELIEVE THE MAINT PEOPLE ARE TRYING TO DO THEIR JOBS; I BELIEVE THERE IS A GREAT DEAL OF PRESSURE BY THOSE HIGHER UP TO GET OUR GS BACK IN SERVICE. SO; CONSEQUENTLY; IT IS NEVER FULLY INVESTIGATED DURING THE BAD WX WHICH AFFECTS IT; AND I TRULY BELIEVE IT IS UNSAFE TO THE FLYING PUBLIC IN ITS CURRENT CONDITION. BY THE WAY; SHORTLY AFTER THE GS WAS RETURNED TO SERVICE TODAY; A LEAR RPTED FLUCTUATIONS ON THE GS NEAR CASSE. AIRWAY FACILITY WAS LISTENING TO THIS TRANSMISSION WHEN THE LEAR PLT SAID...IT'S THE SAME FLUCTUATION I ALWAYS GET IN THIS AREA.

Data retrieved from NASA's ASRS site as of January 2009 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.