AFTER FLYING SINGLE ENG TESTS; UNABLE TO RELIGHT THE ENG. FLEW UNDER TCA FLOOR BUT ACCUSED OF FLYING IN THE TCA.

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WE WERE INFORMED PHOENIX APCH SHOWED US 200 FT ABOVE THE FLOOR OF THE TCA (6000 FT) FOR THE AREA N OF SDL AT APPROX XA45 LCL TIME WHICH WAS ABOUT THE TIME WE MADE OUR SUCCESSFUL AIR START. I AM NOT SURE WHETHER OR NOT WE DID IN FACT PENETRATE THE FLOOR OF THE TCA; HOWEVER; AT NO TIME DID EITHER OF THE ACFT'S ALTIMETERS INDICATE THAT WE HAD. THE ACFT HAS 2 ALTIMETERS THAT HAD BEEN CALIBRATED WITHIN THE PAST 60 DAYS; AND WERE SET ACCORDING TO THE CURRENT SDL ATIS. THE ACFT TRANSPONDER USES A BLIND ENCODER WITH A LED READOUT AND HAD BEEN CHKED WITHIN 120 DAYS. THE TRANSPONDER DISPLAY MAY HAVE CAUSED ME TO RELAY SOME BAD INFO TO APCH. ON SHORT FINAL TO FFZ WE WERE ASKED TO SQUAWK IDENT AND RPT OUR ALT. WITHOUT REALIZING THE PURPOSE OF THE RPT; I GLANCED DOWN AND GAVE THE INDICATED READOUT FROM THE TRANSPONDER. ON A SORTIE FLOWN 3 HRS LATER; THE SAME DAY; WE CHKED THE ATIS FOR 5 ARPTS THAT LAY UNDER THE PHX TCA; AND RECEIVED 5 DIFFERENT ALTIMETER SETTINGS. RANGING FROM 28.80 TO 28.73 INCHES. THIS IS TYPICAL; AND SOME DAYS THERE IS SUBSTANTIALLY GREATER DIFFERENCES. THE CTLR LATER INFORMED ME THAT IFR TFC IS BROUGHT INTO THE TCA; IN THAT AREA; AT 6000 FT WHICH IS THE FLOOR OF THE TCA IN THAT AREA. EVEN THOUGH THERE WAS NO CONFLICT IN THIS CASE; IT SEEMS TO ME; WITH THE AMOUNT OF VFR TFC NOT CTLED BY APCH OPERATING TO ARPTS UNDER THE TCA; DIFFERENCES IN THE LCL ALTIMETER SETTINGS OF 65 FT AND MORE; ADDED TO THE ALLOWABLE 125 FT ERROR IN TRANSPONDER MODE C; PLUS THE 40 FT ALLOWABLE ALTIMETER ERROR AT 6000 FT MSL; IT IS NOT GOOD PRACTICE FOR APCH TO BRING IFR TFC IN SUCH CLOSE PROX TO UNCTLED TFC OPERATING UNDER THE TCA. ESPECIALLY WHEN YOU CONSIDER THAT MOST VFR TFC OPERATING UNDER THE TCA WILL BE USING THE ALTIMETER SETTING FOR THE ARPT THEY WILL BE LNDG AT AND NOT THE SETTING PROVIDED BY PHOENIX APCH CTL.