Narrative:

I was shooting the GPS 15 approach to cgs (college park, md). I was using the reagan altimeter setting, per the plate instructions. At 2 times during the stepdown plan, I received 'low altitude' warnings from ATC. One was just outside of cajja and the other at about 3 mi from runway 15. Both times I checked my 2 altimeters and both times I was well above the stepdown minimum for that phase of the approach. The ATC altitude alarms must be set too conservatively or were out of calibration. Callback conversation with reporter revealed the following information: reporter's greatest concern was that if MSAW alert was false, then the ATC equipment needed adjustment. Reporter equipment was a garman 530, with a 10 mi radius selected. Reporter has not been back to cgs since this incident to validate his equipment or concerns. His equipment has not provided an alert on any other airport approach. Dca spokespersons advised that this was a rare issue. The cgs MSAW alert boxes are built off the dca VOR DME, 2 mi square. Adjoining the cgs airport and runway 15 final approach course, MSAW boxes vary from 500 ft to over 1400 ft. As the MSAW projects the proposed and actual route of flight, MSAW establishes an 'order of magnitude,' which faults on the conservative side, especially if the aircraft is turned on from the north, which has the higher terrain. So if ATC is issuing MSAW alerts, it's more than likely that the aircraft was inbound from the north. A spokesman advised that to completely determine the alert validity, a 'data reduction' must be accomplished. I advised that due to the nature and structure of ASRS, I would be unable to provide the data requires to validate the route of flight.

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Original NASA ASRS Text

Title: DCA CTLR ISSUES LOW ALT ALERT TO C206 ON GPS RNAV RWY 15 APCH TO CGS.

Narrative: I WAS SHOOTING THE GPS 15 APCH TO CGS (COLLEGE PARK, MD). I WAS USING THE REAGAN ALTIMETER SETTING, PER THE PLATE INSTRUCTIONS. AT 2 TIMES DURING THE STEPDOWN PLAN, I RECEIVED 'LOW ALT' WARNINGS FROM ATC. ONE WAS JUST OUTSIDE OF CAJJA AND THE OTHER AT ABOUT 3 MI FROM RWY 15. BOTH TIMES I CHKED MY 2 ALTIMETERS AND BOTH TIMES I WAS WELL ABOVE THE STEPDOWN MINIMUM FOR THAT PHASE OF THE APCH. THE ATC ALT ALARMS MUST BE SET TOO CONSERVATIVELY OR WERE OUT OF CALIBRATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR'S GREATEST CONCERN WAS THAT IF MSAW ALERT WAS FALSE, THEN THE ATC EQUIP NEEDED ADJUSTMENT. RPTR EQUIP WAS A GARMAN 530, WITH A 10 MI RADIUS SELECTED. RPTR HAS NOT BEEN BACK TO CGS SINCE THIS INCIDENT TO VALIDATE HIS EQUIP OR CONCERNS. HIS EQUIP HAS NOT PROVIDED AN ALERT ON ANY OTHER ARPT APCH. DCA SPOKESPERSONS ADVISED THAT THIS WAS A RARE ISSUE. THE CGS MSAW ALERT BOXES ARE BUILT OFF THE DCA VOR DME, 2 MI SQUARE. ADJOINING THE CGS ARPT AND RWY 15 FINAL APCH COURSE, MSAW BOXES VARY FROM 500 FT TO OVER 1400 FT. AS THE MSAW PROJECTS THE PROPOSED AND ACTUAL RTE OF FLT, MSAW ESTABLISHES AN 'ORDER OF MAGNITUDE,' WHICH FAULTS ON THE CONSERVATIVE SIDE, ESPECIALLY IF THE ACFT IS TURNED ON FROM THE N, WHICH HAS THE HIGHER TERRAIN. SO IF ATC IS ISSUING MSAW ALERTS, IT'S MORE THAN LIKELY THAT THE ACFT WAS INBOUND FROM THE N. A SPOKESMAN ADVISED THAT TO COMPLETELY DETERMINE THE ALERT VALIDITY, A 'DATA REDUCTION' MUST BE ACCOMPLISHED. I ADVISED THAT DUE TO THE NATURE AND STRUCTURE OF ASRS, I WOULD BE UNABLE TO PROVIDE THE DATA REQUIRES TO VALIDATE THE RTE OF FLT.

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.