Narrative:

I received vectors to the final approach course ILS runway 10 at bwi. WX was approximately 600 ft overcast visibility 2 mi. The reason for the missed approach was an inaccurate GS indication that caused me to descend below a normal glide path. We did not get a GPWS warning or a low altitude alert from ATC, but went around when the captain recognized that we were too low considering our distance from the runway. His situational awareness was the only warning system that worked for us. While intercepting the localizer I got an immediate intercept of the GS as well. At that moment I didn't think much of this, as a very tight vector to final with GS intercept at or prior to course intercept happens all too often at bwi. This factor -- one of reduced expectations of ATC service -- helped set the trap for me. I immediately called for landing gear and more flap extension while decelerating and starting down the GS. I had less than desirable position orientation at that point, as being tuned to the localizer frequency I did not have a bwi DME indication, and there is no longer an OM beacon on this approach. I was working quickly to get the airplane stabilized as according to the radio altimeter we were already getting close to 1000 ft AGL and we were still too fast. I knew that the approach was not going well, but other than being caught by surprise, I hadn't been able to figure out exactly why yet. As we went below about 1000 ft radio altitude the captain directed a go around, saying, 'we're too low, go around.' as we were not much below 1000 ft and indicating on the GS I was initially confused by this command, but understood why when he told me we were at least 4 mi from the runway. He confirmed that I was indicating on glide path with no red flags in view, but this information must have been false. The captain did not have a glide path indication to back mine up, as his VHF was (thankfully) tuned to bwi VOR to identify the GS crossing altitude, as I had requested. He saw that we were too low for our DME from the field. We do not know why I had an inaccurate GS indication. It may have been momentarily radiating or reflecting falsely, or more likely my GS indicator simply centered and stuck without warning flags. All the equipment worked perfectly normal on the second and completely successful approach, so this remains a mystery. Here is the point I wish to make with this report: the lack of an OM or an ILS DME is an unacceptable safety hazard. Checking altitude when crossing the marker or fix will catch an incorrect GS or altimeter. An ILS approach should preferably be equipped with both pieces of equipment, but must have at least one of them. Too many non-DME ILS approachs, such as the ILS runway 10 at bwi, have had their marker beacons removed for unjustifiable reasons. There are still many aircraft that do not have RNAV or an extra DME receiver, and we rely on the marker beacon for this check. Not doing this check seriously degrades safety. As these marker beacons disappear it is becoming common for unconcerned pilots to simply tune the localizer on both VHF navigation radios and follow the glide path, without any means to perform the GS altitude check. In fact, this is the procedure for a CAT ii/III approach, where if you are late tuning the second receiver you will not be able to complete the coupled approach. With both receivers tuned to the localizer the VOR DME cannot be received, and no warning of a false GS or an incorrect altimeter setting will be available. On a CAT I approach a concerned and aware crew can leave 1 receiver on the VOR/DME, but this is itself a less than ideal solution. With no comparison between receivers a GS error such as occurred in this incident may go undetected. The PF may not have a DME indication, and must rely on the PNF, who is busy with other tasks, for this vital xchk. Flying the approach becomes a fully 2 pilot job, rather than a 1 pilot job with the other pilot as a backup. Interestingly, our B737-200's do have an indicator that repeats the other pilot's DME readout, but the view of this readout is blocked by the copilot's arm when hand-flying. If a highly experienced crew that is aware and concerned can be caught in a situation like this, imagine what might happen to an inexperienced crew on a dark and stormy night. Callback conversation with reporter revealed the following information: the first officer stated that the captain did not question the initial descent on the false GS since he had flown quite often with the first officer and both pilots had a high level of respect for each other's competence. In the rush the PIC failed to double-check the DME readout but did get 'caught up.' the approach controllers at bwi almost always have you turned on at the final approach fix, sometimes having you intercept the GS prior to a localizer intercept. The first officer said that the quality of the ATC service at bwi is, '...less than sterling.' it seems that the controllers get into a habit of sequencing and spacing controls that are too tight. In this instance, that was not the case but it had lead to a feeling of 'expectations' that were wrong. Approach controller never said anything to the crew regarding the early descent or their DME. Reporter had some conversation with a controller at bwi and that controller said the facility is 'struggling' to cope with the traffic expansion at that airport. First officer reported the incident to their union safety department who, in turn, reported it to the airline. It isn't known if the captain also reported it to the company but he believes he did since the company policy requires that the PIC report any incident involving a missed approach or a go around.

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Original NASA ASRS Text

Title: AN IMC B737-200 FLC DSNDS ON A FALSE GS PRIOR TO THE FINAL FIX FOR RWY 10 AT BWI, MD.

Narrative: I RECEIVED VECTORS TO THE FINAL APCH COURSE ILS RWY 10 AT BWI. WX WAS APPROX 600 FT OVCST VISIBILITY 2 MI. THE REASON FOR THE MISSED APCH WAS AN INACCURATE GS INDICATION THAT CAUSED ME TO DSND BELOW A NORMAL GLIDE PATH. WE DID NOT GET A GPWS WARNING OR A LOW ALT ALERT FROM ATC, BUT WENT AROUND WHEN THE CAPT RECOGNIZED THAT WE WERE TOO LOW CONSIDERING OUR DISTANCE FROM THE RWY. HIS SITUATIONAL AWARENESS WAS THE ONLY WARNING SYS THAT WORKED FOR US. WHILE INTERCEPTING THE LOC I GOT AN IMMEDIATE INTERCEPT OF THE GS AS WELL. AT THAT MOMENT I DIDN'T THINK MUCH OF THIS, AS A VERY TIGHT VECTOR TO FINAL WITH GS INTERCEPT AT OR PRIOR TO COURSE INTERCEPT HAPPENS ALL TOO OFTEN AT BWI. THIS FACTOR -- ONE OF REDUCED EXPECTATIONS OF ATC SVC -- HELPED SET THE TRAP FOR ME. I IMMEDIATELY CALLED FOR LNDG GEAR AND MORE FLAP EXTENSION WHILE DECELERATING AND STARTING DOWN THE GS. I HAD LESS THAN DESIRABLE POS ORIENTATION AT THAT POINT, AS BEING TUNED TO THE LOC FREQ I DID NOT HAVE A BWI DME INDICATION, AND THERE IS NO LONGER AN OM BEACON ON THIS APCH. I WAS WORKING QUICKLY TO GET THE AIRPLANE STABILIZED AS ACCORDING TO THE RADIO ALTIMETER WE WERE ALREADY GETTING CLOSE TO 1000 FT AGL AND WE WERE STILL TOO FAST. I KNEW THAT THE APCH WAS NOT GOING WELL, BUT OTHER THAN BEING CAUGHT BY SURPRISE, I HADN'T BEEN ABLE TO FIGURE OUT EXACTLY WHY YET. AS WE WENT BELOW ABOUT 1000 FT RADIO ALT THE CAPT DIRECTED A GAR, SAYING, 'WE'RE TOO LOW, GAR.' AS WE WERE NOT MUCH BELOW 1000 FT AND INDICATING ON THE GS I WAS INITIALLY CONFUSED BY THIS COMMAND, BUT UNDERSTOOD WHY WHEN HE TOLD ME WE WERE AT LEAST 4 MI FROM THE RWY. HE CONFIRMED THAT I WAS INDICATING ON GLIDE PATH WITH NO RED FLAGS IN VIEW, BUT THIS INFO MUST HAVE BEEN FALSE. THE CAPT DID NOT HAVE A GLIDE PATH INDICATION TO BACK MINE UP, AS HIS VHF WAS (THANKFULLY) TUNED TO BWI VOR TO IDENT THE GS XING ALT, AS I HAD REQUESTED. HE SAW THAT WE WERE TOO LOW FOR OUR DME FROM THE FIELD. WE DO NOT KNOW WHY I HAD AN INACCURATE GS INDICATION. IT MAY HAVE BEEN MOMENTARILY RADIATING OR REFLECTING FALSELY, OR MORE LIKELY MY GS INDICATOR SIMPLY CTRED AND STUCK WITHOUT WARNING FLAGS. ALL THE EQUIP WORKED PERFECTLY NORMAL ON THE SECOND AND COMPLETELY SUCCESSFUL APCH, SO THIS REMAINS A MYSTERY. HERE IS THE POINT I WISH TO MAKE WITH THIS RPT: THE LACK OF AN OM OR AN ILS DME IS AN UNACCEPTABLE SAFETY HAZARD. CHKING ALT WHEN XING THE MARKER OR FIX WILL CATCH AN INCORRECT GS OR ALTIMETER. AN ILS APCH SHOULD PREFERABLY BE EQUIPPED WITH BOTH PIECES OF EQUIP, BUT MUST HAVE AT LEAST ONE OF THEM. TOO MANY NON-DME ILS APCHS, SUCH AS THE ILS RWY 10 AT BWI, HAVE HAD THEIR MARKER BEACONS REMOVED FOR UNJUSTIFIABLE REASONS. THERE ARE STILL MANY ACFT THAT DO NOT HAVE RNAV OR AN EXTRA DME RECEIVER, AND WE RELY ON THE MARKER BEACON FOR THIS CHK. NOT DOING THIS CHK SERIOUSLY DEGRADES SAFETY. AS THESE MARKER BEACONS DISAPPEAR IT IS BECOMING COMMON FOR UNCONCERNED PLTS TO SIMPLY TUNE THE LOC ON BOTH VHF NAV RADIOS AND FOLLOW THE GLIDE PATH, WITHOUT ANY MEANS TO PERFORM THE GS ALT CHK. IN FACT, THIS IS THE PROC FOR A CAT II/III APCH, WHERE IF YOU ARE LATE TUNING THE SECOND RECEIVER YOU WILL NOT BE ABLE TO COMPLETE THE COUPLED APCH. WITH BOTH RECEIVERS TUNED TO THE LOC THE VOR DME CANNOT BE RECEIVED, AND NO WARNING OF A FALSE GS OR AN INCORRECT ALTIMETER SETTING WILL BE AVAILABLE. ON A CAT I APCH A CONCERNED AND AWARE CREW CAN LEAVE 1 RECEIVER ON THE VOR/DME, BUT THIS IS ITSELF A LESS THAN IDEAL SOLUTION. WITH NO COMPARISON BTWN RECEIVERS A GS ERROR SUCH AS OCCURRED IN THIS INCIDENT MAY GO UNDETECTED. THE PF MAY NOT HAVE A DME INDICATION, AND MUST RELY ON THE PNF, WHO IS BUSY WITH OTHER TASKS, FOR THIS VITAL XCHK. FLYING THE APCH BECOMES A FULLY 2 PLT JOB, RATHER THAN A 1 PLT JOB WITH THE OTHER PLT AS A BACKUP. INTERESTINGLY, OUR B737-200'S DO HAVE AN INDICATOR THAT REPEATS THE OTHER PLT'S DME READOUT, BUT THE VIEW OF THIS READOUT IS BLOCKED BY THE COPLT'S ARM WHEN HAND-FLYING. IF A HIGHLY EXPERIENCED CREW THAT IS AWARE AND CONCERNED CAN BE CAUGHT IN A SIT LIKE THIS, IMAGINE WHAT MIGHT HAPPEN TO AN INEXPERIENCED CREW ON A DARK AND STORMY NIGHT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO STATED THAT THE CAPT DID NOT QUESTION THE INITIAL DSCNT ON THE FALSE GS SINCE HE HAD FLOWN QUITE OFTEN WITH THE FO AND BOTH PLTS HAD A HIGH LEVEL OF RESPECT FOR EACH OTHER'S COMPETENCE. IN THE RUSH THE PIC FAILED TO DOUBLE-CHK THE DME READOUT BUT DID GET 'CAUGHT UP.' THE APCH CTLRS AT BWI ALMOST ALWAYS HAVE YOU TURNED ON AT THE FINAL APCH FIX, SOMETIMES HAVING YOU INTERCEPT THE GS PRIOR TO A LOC INTERCEPT. THE FO SAID THAT THE QUALITY OF THE ATC SVC AT BWI IS, '...LESS THAN STERLING.' IT SEEMS THAT THE CTLRS GET INTO A HABIT OF SEQUENCING AND SPACING CTLS THAT ARE TOO TIGHT. IN THIS INSTANCE, THAT WAS NOT THE CASE BUT IT HAD LEAD TO A FEELING OF 'EXPECTATIONS' THAT WERE WRONG. APCH CTLR NEVER SAID ANYTHING TO THE CREW REGARDING THE EARLY DSCNT OR THEIR DME. RPTR HAD SOME CONVERSATION WITH A CTLR AT BWI AND THAT CTLR SAID THE FACILITY IS 'STRUGGLING' TO COPE WITH THE TFC EXPANSION AT THAT ARPT. FO RPTED THE INCIDENT TO THEIR UNION SAFETY DEPT WHO, IN TURN, RPTED IT TO THE AIRLINE. IT ISN'T KNOWN IF THE CAPT ALSO RPTED IT TO THE COMPANY BUT HE BELIEVES HE DID SINCE THE COMPANY POLICY REQUIRES THAT THE PIC RPT ANY INCIDENT INVOLVING A MISSED APCH OR A GAR.

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.