AN ACR MLG CLBED IN RESPONSE TO A TCASII COMMAND CAUSED BY 2 MIL TANKERS.

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IFR FLT PLAN; ASSIGNED 7000 FT; L DOWNWIND FOR RWY 3 AT GEG. TALKING TO APCH CTL; APCH ADVISED US THAT 2 MLT'S WOULD BE DEPARTING FAIRCHILD TOWARD US; BUT WOULD LEVEL AT 6000 FT. WE EXPECTED A VISUAL APCH AFTER THEY CLRED UNDER US. MADE VISUAL CONTACT AND TCASII CONTACT AS THE MLT'S CLBED THROUGH 4000 FT. AS BOTH ACFT REACHED 6000 FT; WE GOT THE YELLOW 'TFC' WARNING. MLT #2 LEVELED AT 6000 FT WHILE #1 CONTINUED TO CLB. AS #1 CLBED TO 6300 FT; TCASII WARNED 'DO NOT DSND.' AS #1 CLBED TO 6500 FT TCASII INSTRUCTED 'CLB; CLB.' WE STARTED AN IMMEDIATE CLB UNTIL TCASII INSTRUCTED ALL CLR AND WE SAW THE MLT DSNDING BACK TO 6000 FT. WE STOPPED OUR CLB AT 7800 FT AND DSNDED BACK TO 7000 FT. CONCLUSION: BASIC ALT 'BUST' BY MLT LEAD. TCASII PERFORMED PERFECTLY; FO RESPONDED AS TRAINED. ONLY WORRY; LIGHT ACFT WITHOUT ALT ENCODING XPONDERS OR NO XPONDERS AT ALL! SUPPLEMENTAL INFO FROM ACN 236129: WHEN WE ADVISED THE CTLR THAT WE HAD DEVIATED FROM OUR ASSIGNED ALT DUE TO AN RA FROM OUR TCASII; THE CTLR SAID THE HIGHEST READOUT HE HAD FROM THE MLT MODE C READOUT WAS 6300 FT MSL.