Narrative:

The event took place after I had been cleared for the VOR-a approach to smo. According to the CDI needle of the #2 navigation radio, which was tuned to the lax VOR and the obs was set to the 007 degree right, I had just passed bevey intersection and I began my descent to the next step-down altitude (1140' MSL). I then tuned in the next step-down fix, which was the 350 degree radial. I as I passed through 2000' MSL, approach called out a low altitude alert and asked our altitude. I told the controller I was at 2000' and descending at 1000 FPM. The controller responded by saying the MDA for my approach segment was 2600' and that I should level off. I responded by telling him that I showed my position inside of bevey (the FAF). He reported my position as 2 mi outside of bevey. At this point I was level at 1800' MSL. To double check I tuned the 32 navigation obs back to the 007 degree right and it showed the CDI needle about 1/2 DOT deflection to the right, which meant I was inside bevey. The VOR had been idented prior to the approach and a strong from flag was showing. My first officer double-checked that I had the proper radial tuned in. A vot check was done 2 days prior and the #2 navigation showed a -1 degree error. The DME was not receiving smo VOR so we could not confirm the distance given by the controller. About 1 min passed when the controller called out another low altitude alert and asked our altitude. I responded by saying we were still level at 1800' MSL and that we must be having a problem with the #2 navigation because it shows me inside bevey. The controller instructed us to maintain our altitude until the airport was in sight. 1 min later we spotted the airport and proceeded visually to the runway. Because bevey is the FAF for the approach and smo VOR is the missed approach point. The terps manual allows a fix error of +/-2 mi and obstacle clearance is provided from where the fix may first be received. With this criteria it appears possible to receive the fix 2 mi outside the geographical or plotted fix and still have obstruction clearance. The safety aspect of this might not be so much for the aircraft on the approach, but for the strain it puts on the system and the controller. The controller is getting a computer generated low altitude alert for an aircraft on a normal descent profile for the approach and that takes the controller's attention away from other aircraft under his control. It also requires communication time. If this approach requires better identify of the FAF to eliminate an early descent and altitude alerts, it might be a good suggestion to make the approach a VOR-DME instead of just a VOR approach.

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Original NASA ASRS Text

Title: MSAW ACTIVATED WHEN SMT APPARENTLY DESCENDS BELOW MDA ON A VOR-A APCH.

Narrative: THE EVENT TOOK PLACE AFTER I HAD BEEN CLRED FOR THE VOR-A APCH TO SMO. ACCORDING TO THE CDI NEEDLE OF THE #2 NAV RADIO, WHICH WAS TUNED TO THE LAX VOR AND THE OBS WAS SET TO THE 007 DEG R, I HAD JUST PASSED BEVEY INTXN AND I BEGAN MY DSNT TO THE NEXT STEP-DOWN ALT (1140' MSL). I THEN TUNED IN THE NEXT STEP-DOWN FIX, WHICH WAS THE 350 DEG RADIAL. I AS I PASSED THROUGH 2000' MSL, APCH CALLED OUT A LOW ALT ALERT AND ASKED OUR ALT. I TOLD THE CTLR I WAS AT 2000' AND DSNDING AT 1000 FPM. THE CTLR RESPONDED BY SAYING THE MDA FOR MY APCH SEGMENT WAS 2600' AND THAT I SHOULD LEVEL OFF. I RESPONDED BY TELLING HIM THAT I SHOWED MY POS INSIDE OF BEVEY (THE FAF). HE RPTED MY POS AS 2 MI OUTSIDE OF BEVEY. AT THIS POINT I WAS LEVEL AT 1800' MSL. TO DOUBLE CHK I TUNED THE 32 NAV OBS BACK TO THE 007 DEG R AND IT SHOWED THE CDI NEEDLE ABOUT 1/2 DOT DEFLECTION TO THE RIGHT, WHICH MEANT I WAS INSIDE BEVEY. THE VOR HAD BEEN IDENTED PRIOR TO THE APCH AND A STRONG FROM FLAG WAS SHOWING. MY F/O DOUBLE-CHKED THAT I HAD THE PROPER RADIAL TUNED IN. A VOT CHK WAS DONE 2 DAYS PRIOR AND THE #2 NAV SHOWED A -1 DEG ERROR. THE DME WAS NOT RECEIVING SMO VOR SO WE COULD NOT CONFIRM THE DISTANCE GIVEN BY THE CTLR. ABOUT 1 MIN PASSED WHEN THE CTLR CALLED OUT ANOTHER LOW ALT ALERT AND ASKED OUR ALT. I RESPONDED BY SAYING WE WERE STILL LEVEL AT 1800' MSL AND THAT WE MUST BE HAVING A PROB WITH THE #2 NAV BECAUSE IT SHOWS ME INSIDE BEVEY. THE CTLR INSTRUCTED US TO MAINTAIN OUR ALT UNTIL THE ARPT WAS IN SIGHT. 1 MIN LATER WE SPOTTED THE ARPT AND PROCEEDED VISUALLY TO THE RWY. BECAUSE BEVEY IS THE FAF FOR THE APCH AND SMO VOR IS THE MISSED APCH POINT. THE TERPS MANUAL ALLOWS A FIX ERROR OF +/-2 MI AND OBSTACLE CLRNC IS PROVIDED FROM WHERE THE FIX MAY FIRST BE RECEIVED. WITH THIS CRITERIA IT APPEARS POSSIBLE TO RECEIVE THE FIX 2 MI OUTSIDE THE GEOGRAPHICAL OR PLOTTED FIX AND STILL HAVE OBSTRUCTION CLRNC. THE SAFETY ASPECT OF THIS MIGHT NOT BE SO MUCH FOR THE ACFT ON THE APCH, BUT FOR THE STRAIN IT PUTS ON THE SYS AND THE CTLR. THE CTLR IS GETTING A COMPUTER GENERATED LOW ALT ALERT FOR AN ACFT ON A NORMAL DSNT PROFILE FOR THE APCH AND THAT TAKES THE CTLR'S ATTN AWAY FROM OTHER ACFT UNDER HIS CTL. IT ALSO REQUIRES COM TIME. IF THIS APCH REQUIRES BETTER IDENT OF THE FAF TO ELIMINATE AN EARLY DSNT AND ALT ALERTS, IT MIGHT BE A GOOD SUGGESTION TO MAKE THE APCH A VOR-DME INSTEAD OF JUST A VOR APCH.

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.