Narrative:

On descent into mfr, we were being worked by approach control in the descent from the north. Our company experiences several ground proximity incidents a yr on the descent into medford from the north. I asked the controller what his MVA was in terms of AGL, and he replied 1000 ft. This surprised me as the mountains north of medford rise to about 4000 ft above the field elevation of mfr. I asked the controller if they corrected MVA for nonstandard temperature and he didn't have any idea what I was talking about. I explained to him that an altimeter is only accurate at standard temperature. On a cold day, descending into medford, it will indicate higher than actual altitude and so will the mode C report from the transponder. If the temperature in medford is 20 degrees F, an airplane being vectored at the MVA will only be about 700 ft above the mountains. I thought this was the reason for the 2000 ft IFR minimum altitude in mountainous terrain and am very surprised to learn that MVA's don't include this safety factor. This is not limited to cascade approach in medford. I had the same conversation with the chinook approach controller descending into yakima, wa. We had just had a ground proximity alert at MVA in the winter and I was surprised to learn that I was being vectored at 1000 ft AGL, which on that day due to cold temperatures left us about 250 ft low. The radar altimeter still indicates correctly, hence the ground proximity alert during a high speed descent. It is my understanding that canadian controllers do raise MVA's on cold days and my question is why doesn't the united states? I feel that controllers at least need to be educated about altimetry and the inherent errors in altimeters and thus the mode C altitude reports they are seeing, as both of these controllers had no idea what I was talking about. Very few pilots are aware of the phenomena either for that matter. Some think that air data computers correct for this but they do not. It seems to me it wouldn't be that hard at all to program the radar scopes to change the MVA based on a temperature input. They already have to input barometric pressure to get an accurate altitude reading from the xponders. I think they should have to enter temperature to get an accurate reading on ground clearance.

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Original NASA ASRS Text

Title: GPWS WARNING DURING APCH INTO MFR.

Narrative: ON DSCNT INTO MFR, WE WERE BEING WORKED BY APCH CTL IN THE DSCNT FROM THE N. OUR COMPANY EXPERIENCES SEVERAL GND PROX INCIDENTS A YR ON THE DSCNT INTO MEDFORD FROM THE N. I ASKED THE CTLR WHAT HIS MVA WAS IN TERMS OF AGL, AND HE REPLIED 1000 FT. THIS SURPRISED ME AS THE MOUNTAINS N OF MEDFORD RISE TO ABOUT 4000 FT ABOVE THE FIELD ELEVATION OF MFR. I ASKED THE CTLR IF THEY CORRECTED MVA FOR NONSTANDARD TEMP AND HE DIDN'T HAVE ANY IDEA WHAT I WAS TALKING ABOUT. I EXPLAINED TO HIM THAT AN ALTIMETER IS ONLY ACCURATE AT STANDARD TEMP. ON A COLD DAY, DSNDING INTO MEDFORD, IT WILL INDICATE HIGHER THAN ACTUAL ALT AND SO WILL THE MODE C RPT FROM THE XPONDER. IF THE TEMP IN MEDFORD IS 20 DEGS F, AN AIRPLANE BEING VECTORED AT THE MVA WILL ONLY BE ABOUT 700 FT ABOVE THE MOUNTAINS. I THOUGHT THIS WAS THE REASON FOR THE 2000 FT IFR MINIMUM ALT IN MOUNTAINOUS TERRAIN AND AM VERY SURPRISED TO LEARN THAT MVA'S DON'T INCLUDE THIS SAFETY FACTOR. THIS IS NOT LIMITED TO CASCADE APCH IN MEDFORD. I HAD THE SAME CONVERSATION WITH THE CHINOOK APCH CTLR DSNDING INTO YAKIMA, WA. WE HAD JUST HAD A GND PROX ALERT AT MVA IN THE WINTER AND I WAS SURPRISED TO LEARN THAT I WAS BEING VECTORED AT 1000 FT AGL, WHICH ON THAT DAY DUE TO COLD TEMPS LEFT US ABOUT 250 FT LOW. THE RADAR ALTIMETER STILL INDICATES CORRECTLY, HENCE THE GND PROX ALERT DURING A HIGH SPD DSCNT. IT IS MY UNDERSTANDING THAT CANADIAN CTLRS DO RAISE MVA'S ON COLD DAYS AND MY QUESTION IS WHY DOESN'T THE UNITED STATES? I FEEL THAT CTLRS AT LEAST NEED TO BE EDUCATED ABOUT ALTIMETRY AND THE INHERENT ERRORS IN ALTIMETERS AND THUS THE MODE C ALT RPTS THEY ARE SEEING, AS BOTH OF THESE CTLRS HAD NO IDEA WHAT I WAS TALKING ABOUT. VERY FEW PLTS ARE AWARE OF THE PHENOMENA EITHER FOR THAT MATTER. SOME THINK THAT AIR DATA COMPUTERS CORRECT FOR THIS BUT THEY DO NOT. IT SEEMS TO ME IT WOULDN'T BE THAT HARD AT ALL TO PROGRAM THE RADAR SCOPES TO CHANGE THE MVA BASED ON A TEMP INPUT. THEY ALREADY HAVE TO INPUT BAROMETRIC PRESSURE TO GET AN ACCURATE ALT READING FROM THE XPONDERS. I THINK THEY SHOULD HAVE TO ENTER TEMP TO GET AN ACCURATE READING ON GND CLRNC.

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.