ARTCC RADAR CTLR CONCERN REGARDING THE POSSIBLE INADVERTENT WIND DIRECTION ERROR GIVEN TO PLTS BASED ON THE TRUE RATHER THAN THE MAGNETIC DIRECTION.

Narrative

EVER SINCE THE TRANSITION TO THE METAR WX RPTING FORMAT THERE HAVE BEEN SOME INCONSISTENCIES REGARDING HOW WIND DIRECTION IS RPTED. THE AIM FIGURE 7-1-16 DESCRIBES WIND DIRECTION REF TRUE N FOR AVIATION ROUTINE WX RPTS. FIGURE 7-1-2 ALSO INDICATES TRUE N FOR ASOS RPTS; HOWEVER; THE NOTE AT THE END OF PARAGRAPH 7-1-11D(3) SAYS: 'WIND DIRECTION BROADCAST OVER FAA RADIOS IS IN REF TO MAGNETIC N.' THIS AGREES WITH 7110.65 2-9-3 REGARDING THE USE OF MAGNETIC WINDS IN ATIS BROADCASTS. THERE IS NO OTHER REF IN THE 7110.65 ON THIS SUBJECT. SO IT WAS NECESSARY TO DO AN EXPERIMENT TO FIND OUT HOW WINDS ARE RPTED ON ZDV'S RADAR CONSOLES. AN ASOS SITE WAS CALLED ON THE TELEPHONE DURING AN UPDATE ON A RADAR CONSOLE; AND THE WIND DIRECTIONS DIFFERED BY THE AMOUNT OF MAGNETIC VARIATION. APPARENTLY; CTR CTLRS ARE BROADCASTING WINDS REF TRUE N IN CONTRADICTION WITH THE NOTE IN THE AIM. THERE ARE INSTANCES LIKE EMERS AND TIMES WHEN WX IS NOT UPDATED AUTOMATICALLY THAT FLT DATA CALLS ON ASOS SITE TO GET WX; AND THE INFO IS ENTERED INTO THE COMPUTER WITHOUT MAKING A CONVERSION. THEREFORE; IT IS POSSIBLE THAT A CTLR CAN BE BROADCASTING WINDS TRUE OR MAGNETIC WITHOUT KNOWING WHICH ONE IS BEING USED. AN INFORMAL SURVEY OF AIRLINE PLTS REVEALED THAT THEY ALL EXPECT WINDS TO BE MAGNETIC; MORE BECAUSE THAT'S THE WAY IT HAS ALWAYS BEEN; AND BECAUSE TRUE WINDS ARE OF NO USE; THAN BECAUSE THEY HAVE READ ANYTHING PROVING IT. AFTER TALKING TO THE ASOS TEAM AT THE PLT'S GROUP CONVENTION IN LONG BEACH; CA; IT WAS CONFIRMED THAT WINDS ARE BROADCAST OVER THE RADIO AND TELEPHONE AS MAGNETIC; BUT DISTRIBUTED TO THE WX COLLECTION NETWORK IN TRUE. THEY DID NOT KNOW FOR SURE; BUT THEY ALWAYS ASSUMED THAT THE WINDS WERE CONVERTED BACK TO MAGNETIC WHEN PRESENTED TO CTLRS. THE PROB HERE IS THAT EVERYONE ASSUMES THAT SURFACE WINDS ARE RPTED AS MAGNETIC; EVEN THOUGH ALMOST EVERY PUB SAYS THEY ARE TRUE. IT IS VERY DIFFICULT IN FACT TO FIND ANY PROOF THAT THEY ARE MAGNETIC IN CERTAIN INSTANCES. THIS LACK OF KNOWLEDGE RESULTS IN BOTH BEING PRESENTED TO CTLRS IN ZDV WITHOUT A WAY TO DIFFERENTIATE. IF THERE IS SOME GOOD REASON FOR CTRS TO USE TRUE WINDS; THAT DIFFERENCE SHOULD BE STATED IN THE WX PHRASEOLOGY. EACH PUB THAT DESCRIBES WINDS AS TRUE SHOULD MENTION THE EXCEPTIONS WHERE THEY ARE MAGNETIC; SUCH AS ASOS AND ATIS; RATHER THAN HIDE THIS FACT IN OBSCURE NOTATIONS. IN SOME PARTS OF THE COUNTRY THIS DISCREPANCY IS MINOR; WHEREAS IN OTHERS IT IS MORE SIGNIFICANT. SIMILARLY SMALL ACFT ARE LESS AFFECTED THAN FULLY LOADED TURBOJETS WITH STRICT PERFORMANCE CRITERIA. ADDITIONALLY; THE CTLR'S HANDBOOK SHOULD INCLUDE INFO ON WX FORMAT; RATHER THAN REQUIRE USE OF THE AIM.