Air taxi pilot gets below GS on ILS approach to runway 15L at bwi. He had failed to note the 'off' flag. Controller issues a low altitude alert.

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Attributes
ACN	388553
Time
Date	199712
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : bwi
State Reference	MD
Altitude	msl bound lower : 1500 msl bound upper : 2500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : bwi
Operator	common carrier : air taxi
Make Model Name	PA-23-250 Aztec
Operating Under FAR Part	Part 135
Flight Phase	descent : approach
Route In Use	arrival other enroute : on vectors enroute other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : cfi pilot : instrument
Experience	flight time last 90 days : 175 flight time total : 2500 flight time type : 120
ASRS Report	388553
Person 2
Affiliation	government : faa
Function	controller : local
Qualification	controller : non radar
Events
Anomaly	aircraft equipment problem : less severe non adherence : published procedure non adherence : clearance other spatial deviation
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance controller : provided flight assist other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

I was executing the ILS/DME runway 15L at bwi, all applicable navaids were tuned and idented. I was being vectored to join the final approach course outside of slatr intersection. Both GS and localizer needles were showing normal readings and no flags. Upon interception of the GS and localizer, landing gear was lowered, and I continued down beam (GS). Approach turned me over to tower. Just after switching frequencys I noticed my GS 'flagged,' however, I did not catch it immediately. Upon checking in with tower, which was barely readable (probably due to my low altitude), they advised 'low altitude alert.' I was at 1500 ft MSL and the step-down fix descended from 2500 ft MSL to 1600 ft MSL, they advised a climb, then queried if I had DME, I advised 'affirmative' and also stated GS indication was lost. I continued the approach under localizer minimums (GS out) and landed uneventfully. Contributing factors: single pilot IFR/IMC, deteriorating WX conditions, busy terminal environment, fixation on one instrument, and the 'GS needle' remained in an 'on-glide path' indication after failing.

Original NASA ASRS Text

Title: AIR TAXI PLT GETS BELOW GS ON ILS APCH TO RWY 15L AT BWI. HE HAD FAILED TO NOTE THE 'OFF' FLAG. CTLR ISSUES A LOW ALT ALERT.

Narrative: I WAS EXECUTING THE ILS/DME RWY 15L AT BWI, ALL APPLICABLE NAVAIDS WERE TUNED AND IDENTED. I WAS BEING VECTORED TO JOIN THE FINAL APCH COURSE OUTSIDE OF SLATR INTXN. BOTH GS AND LOC NEEDLES WERE SHOWING NORMAL READINGS AND NO FLAGS. UPON INTERCEPTION OF THE GS AND LOC, LNDG GEAR WAS LOWERED, AND I CONTINUED DOWN BEAM (GS). APCH TURNED ME OVER TO TWR. JUST AFTER SWITCHING FREQS I NOTICED MY GS 'FLAGGED,' HOWEVER, I DID NOT CATCH IT IMMEDIATELY. UPON CHKING IN WITH TWR, WHICH WAS BARELY READABLE (PROBABLY DUE TO MY LOW ALT), THEY ADVISED 'LOW ALT ALERT.' I WAS AT 1500 FT MSL AND THE STEP-DOWN FIX DSNDED FROM 2500 FT MSL TO 1600 FT MSL, THEY ADVISED A CLB, THEN QUERIED IF I HAD DME, I ADVISED 'AFFIRMATIVE' AND ALSO STATED GS INDICATION WAS LOST. I CONTINUED THE APCH UNDER LOC MINIMUMS (GS OUT) AND LANDED UNEVENTFULLY. CONTRIBUTING FACTORS: SINGLE PLT IFR/IMC, DETERIORATING WX CONDITIONS, BUSY TERMINAL ENVIRONMENT, FIXATION ON ONE INST, AND THE 'GS NEEDLE' REMAINED IN AN 'ON-GLIDE PATH' INDICATION AFTER FAILING.

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.