Narrative:

We were vectored to intercept the ILS 28R ILS and cleared for the approach. We intercepted the localizer at 4000 MSL and tracked it. Descent made on glide slope. We did not receive the marker at 2900 as expected, but assumed it was inoperative. At 2500 MSL we received the OM signal. We were momentarily confused. Since we were 400' lower than the published altitude we broke out before we resolved our doubts. We saw the runway and continued the ILS approach (on localizer, on GS). Visibility was marginal in light snow. As the runway became clearer we realized we were low and leveled off to regain profile. Close to the runway (1/4-1/2 mi) the glide slope indicator went to full 'fly up' and the GPWS glide slope warning sounds. We were nearly on profile and our landing was uneventful. While turning in I questioned ground control if the glide slope was monitoring normal. They said yes. I described our problem including the 400' low at marker and GS warning. The air carrier flight which preceded us said they had the same problem. We observed 2 aircraft on taxiway north awaiting takeoff and another aircraft near the runway on the north side of 28R. The other aircraft was 4 or 5 blue lights back from the runway. It seemed to be north the path of the ILS antennas. On the next day, after reflecting on this problem, I spoke with the tower supervisor at pit. He stated he was not on duty during the incident, but seemed sincere and interested. He could not, however, tell me where the ILS critical area for runway 28R was. There was snow on the ground obscuring ground markings. Suggested actions: 1) all WX critical area warnings. 2) chart critical areas. 3) make ATC more aware of these areas. 4) have ATC instruct aircraft to hold outside these areas when markings are obscured.

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Original NASA ASRS Text

Title: FLT CREW OF ACR LGT CONTINUED DESCENT BELOW FAF ALT BEFORE REACHING THE FAF. GLIDE SLOPE WAS BEING INTERFERED WITH BY ACFT HOLDING FOR TKOF.

Narrative: WE WERE VECTORED TO INTERCEPT THE ILS 28R ILS AND CLRED FOR THE APCH. WE INTERCEPTED THE LOC AT 4000 MSL AND TRACKED IT. DSCNT MADE ON GLIDE SLOPE. WE DID NOT RECEIVE THE MARKER AT 2900 AS EXPECTED, BUT ASSUMED IT WAS INOP. AT 2500 MSL WE RECEIVED THE OM SIGNAL. WE WERE MOMENTARILY CONFUSED. SINCE WE WERE 400' LOWER THAN THE PUBLISHED ALT WE BROKE OUT BEFORE WE RESOLVED OUR DOUBTS. WE SAW THE RWY AND CONTINUED THE ILS APCH (ON LOC, ON GS). VISIBILITY WAS MARGINAL IN LIGHT SNOW. AS THE RWY BECAME CLEARER WE REALIZED WE WERE LOW AND LEVELED OFF TO REGAIN PROFILE. CLOSE TO THE RWY (1/4-1/2 MI) THE GLIDE SLOPE INDICATOR WENT TO FULL 'FLY UP' AND THE GPWS GLIDE SLOPE WARNING SOUNDS. WE WERE NEARLY ON PROFILE AND OUR LNDG WAS UNEVENTFUL. WHILE TURNING IN I QUESTIONED GND CTL IF THE GLIDE SLOPE WAS MONITORING NORMAL. THEY SAID YES. I DESCRIBED OUR PROBLEM INCLUDING THE 400' LOW AT MARKER AND GS WARNING. THE AIR CARRIER FLT WHICH PRECEDED US SAID THEY HAD THE SAME PROBLEM. WE OBSERVED 2 ACFT ON TXWY N AWAITING TKOF AND ANOTHER ACFT NEAR THE RWY ON THE N SIDE OF 28R. THE OTHER ACFT WAS 4 OR 5 BLUE LIGHTS BACK FROM THE RWY. IT SEEMED TO BE N THE PATH OF THE ILS ANTENNAS. ON THE NEXT DAY, AFTER REFLECTING ON THIS PROBLEM, I SPOKE WITH THE TWR SUPVR AT PIT. HE STATED HE WAS NOT ON DUTY DURING THE INCIDENT, BUT SEEMED SINCERE AND INTERESTED. HE COULD NOT, HOWEVER, TELL ME WHERE THE ILS CRITICAL AREA FOR RWY 28R WAS. THERE WAS SNOW ON THE GND OBSCURING GND MARKINGS. SUGGESTED ACTIONS: 1) ALL WX CRITICAL AREA WARNINGS. 2) CHART CRITICAL AREAS. 3) MAKE ATC MORE AWARE OF THESE AREAS. 4) HAVE ATC INSTRUCT ACFT TO HOLD OUTSIDE THESE AREAS WHEN MARKINGS ARE OBSCURED.

Data retrieved from NASA's ASRS site as of August 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.