Narrative:

After making 2 turns holding at ashie (imfr localizer backcourse 18 DME), flight began the published approach. At the 18 DME fix, I called for flaps 19 and began slowing from 200 KTS while maintaining 9000'. A few seconds later, the GPWS began sounding 'terrain, terrain.' I was expected this since there is a steeply rising mountain 7500' high near the course and have experienced the warning before. After 2 more seconds the GPWS sounded, 'whoop, whoop, pull up!' in a split second in my mind thought of all the things I could have done wrong. (Misread approach chart, altimeter, wrong DME, etc.) immediate added power and started climbing until the warnings stopped at 10500'. The aircraft coasted up to 11000', then descended to 10500' between layers. I informed approach what had happened and that I wanted to go to the holding pattern. I was instead cleared to the medford VOR, which was ok with me. I then requested a new hard altitude and one was assigned. Approach said that our company and air carrier Y had been advised of the possibility of a GPWS warning. A week later,I shot the same backcourse approach in VFR conditions with no warning (different aircraft). During the approach with the warning, I had been holding a 25 degree left drift correction. Possibly the beam pattern of the radar altimeter somehow was picking up the peak slightly left of course.

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Original NASA ASRS Text

Title: ACR MLG GOT GPWS WHILE FLYING IAP LOCALIZER DME BACKCOURSE APCH INTO MFR.

Narrative: AFTER MAKING 2 TURNS HOLDING AT ASHIE (IMFR LOC BACKCOURSE 18 DME), FLT BEGAN THE PUBLISHED APCH. AT THE 18 DME FIX, I CALLED FOR FLAPS 19 AND BEGAN SLOWING FROM 200 KTS WHILE MAINTAINING 9000'. A FEW SECS LATER, THE GPWS BEGAN SOUNDING 'TERRAIN, TERRAIN.' I WAS EXPECTED THIS SINCE THERE IS A STEEPLY RISING MOUNTAIN 7500' HIGH NEAR THE COURSE AND HAVE EXPERIENCED THE WARNING BEFORE. AFTER 2 MORE SECS THE GPWS SOUNDED, 'WHOOP, WHOOP, PULL UP!' IN A SPLIT SECOND IN MY MIND THOUGHT OF ALL THE THINGS I COULD HAVE DONE WRONG. (MISREAD APCH CHART, ALTIMETER, WRONG DME, ETC.) IMMEDIATE ADDED PWR AND STARTED CLBING UNTIL THE WARNINGS STOPPED AT 10500'. THE ACFT COASTED UP TO 11000', THEN DSNDED TO 10500' BTWN LAYERS. I INFORMED APCH WHAT HAD HAPPENED AND THAT I WANTED TO GO TO THE HOLDING PATTERN. I WAS INSTEAD CLRED TO THE MEDFORD VOR, WHICH WAS OK WITH ME. I THEN REQUESTED A NEW HARD ALT AND ONE WAS ASSIGNED. APCH SAID THAT OUR COMPANY AND ACR Y HAD BEEN ADVISED OF THE POSSIBILITY OF A GPWS WARNING. A WK LATER,I SHOT THE SAME BACKCOURSE APCH IN VFR CONDITIONS WITH NO WARNING (DIFFERENT ACFT). DURING THE APCH WITH THE WARNING, I HAD BEEN HOLDING A 25 DEG LEFT DRIFT CORRECTION. POSSIBLY THE BEAM PATTERN OF THE RADAR ALTIMETER SOMEHOW WAS PICKING UP THE PEAK SLIGHTLY LEFT OF COURSE.

Data retrieved from NASA's ASRS site as of August 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.