Air carrier crew descends below GS on a rushed approach until queried by ATC.

Attributes
ACN	451802
Time
Date	199910
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : sat.airport
State Reference	TX
Altitude	msl bound lower : 2500 msl bound upper : 3000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sat.tracon
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 12r other ndb
Flight Phase	descent : vacating altitude descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : sat.tracon
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 12r
Flight Phase	descent : approach descent : vacating altitude
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : multi engine pilot : atp
Experience	flight time last 90 days : 181 flight time total : 6555 flight time type : 181
ASRS Report	451802
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : multi engine pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude altitude deviation : crossing restriction not met non adherence : published procedure non adherence : clearance other anomaly other anomaly other other spatial deviation
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued advisory flight crew : became reoriented flight crew : returned to intended or assigned course
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance Navigational Facility
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

After an AB15 am departure, my copilot and I were finishing up our 8 hour day. The time is AF15 am local and we are being vectored for an ILS runway 12R approach. Our position was just outside the OM 4 mi in trail of a B737 which was on the same approach. We were high and had a tight turn onto final. Our heading was 230 degrees. The approach controller's instructions were to turn left to a 090 degree heading to join the localizer, maintain 3000 ft until established, cleared for the ILS runway 12R approach. In order to configure and get down we elected to use a spoiler descent with a 45 degree bank turn to intercept. I joined the localizer approximately 1 mi from the 'alamo' LOM and called for gear down and the before landing checklist. While the PNF was accomplishing the tasks on the checklist, I continued to follow what I believed to be a 'live' GS indication. To my surprise it was not. The minimum approach altitude is 3000 ft and we were at 2700 ft and descending. The approach controller radioed 'say altitude.' the PNF at this point was finished with his tasks and anticipating our descent radioed 'we are through 2500 ft.' he, I am sure, believed that we were established on the ILS. I immediately saw the error and arrested the descent at 2550 ft MSL and initiated an immediate climb back to 3000 ft. Approach control broke us off the approach and re-vectored us back onto the localizer for a second attempt. This time the approach was uneventful and all GS indications were normal. Some factors leading up to this event are fatigue and poor situational awareness. The B737 ahead may have caused erroneous GS indications, I do not know. However, awareness of our position and a xchk with the GS intercept altitude would have averted this potentially hazardous situation.

Original NASA ASRS Text

Title: ACR CREW DSNDS BELOW GS ON A RUSHED APCH UNTIL QUERIED BY ATC.

Narrative: AFTER AN AB15 AM DEP, MY COPLT AND I WERE FINISHING UP OUR 8 HR DAY. THE TIME IS AF15 AM LCL AND WE ARE BEING VECTORED FOR AN ILS RWY 12R APCH. OUR POS WAS JUST OUTSIDE THE OM 4 MI IN TRAIL OF A B737 WHICH WAS ON THE SAME APCH. WE WERE HIGH AND HAD A TIGHT TURN ONTO FINAL. OUR HEADING WAS 230 DEGS. THE APCH CTLR'S INSTRUCTIONS WERE TO TURN L TO A 090 DEG HDG TO JOIN THE LOC, MAINTAIN 3000 FT UNTIL ESTABLISHED, CLRED FOR THE ILS RWY 12R APCH. IN ORDER TO CONFIGURE AND GET DOWN WE ELECTED TO USE A SPOILER DSCNT WITH A 45 DEG BANK TURN TO INTERCEPT. I JOINED THE LOC APPROX 1 MI FROM THE 'ALAMO' LOM AND CALLED FOR GEAR DOWN AND THE BEFORE LNDG CHKLIST. WHILE THE PNF WAS ACCOMPLISHING THE TASKS ON THE CHKLIST, I CONTINUED TO FOLLOW WHAT I BELIEVED TO BE A 'LIVE' GS INDICATION. TO MY SURPRISE IT WAS NOT. THE MINIMUM APCH ALT IS 3000 FT AND WE WERE AT 2700 FT AND DSNDING. THE APCH CTLR RADIOED 'SAY ALT.' THE PNF AT THIS POINT WAS FINISHED WITH HIS TASKS AND ANTICIPATING OUR DSCNT RADIOED 'WE ARE THROUGH 2500 FT.' HE, I AM SURE, BELIEVED THAT WE WERE ESTABLISHED ON THE ILS. I IMMEDIATELY SAW THE ERROR AND ARRESTED THE DSCNT AT 2550 FT MSL AND INITIATED AN IMMEDIATE CLB BACK TO 3000 FT. APCH CTL BROKE US OFF THE APCH AND RE-VECTORED US BACK ONTO THE LOC FOR A SECOND ATTEMPT. THIS TIME THE APCH WAS UNEVENTFUL AND ALL GS INDICATIONS WERE NORMAL. SOME FACTORS LEADING UP TO THIS EVENT ARE FATIGUE AND POOR SITUATIONAL AWARENESS. THE B737 AHEAD MAY HAVE CAUSED ERRONEOUS GS INDICATIONS, I DO NOT KNOW. HOWEVER, AWARENESS OF OUR POS AND A XCHK WITH THE GS INTERCEPT ALT WOULD HAVE AVERTED THIS POTENTIALLY HAZARDOUS SIT.

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.