LEAR 25 XMITS INCORRECT MODE C INFO CAUSING CONCERN FOR ACR WHOSE TCASII READS THE INCORRECT ALT.

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ACFT #1 HAD LEVELED AT FL280 FOR ABOUT 30 SECONDS. WE WERE IN RADAR CONTACT WITH ZTL. AFTER ABOUT 30 SECONDS AFTER BEING LEVEL; WE HEARD AN ACR STATING THE NEED FOR AN 'IMMEDIATE TURN.' THEIR TCASII WAS SHOWING A TARGET CLBING IN CONFLICT WITH THEIR ALT. I AM ASSUMING THIS WAS OUR ACFT. ZTL THEN ASKED FOR OUR ALT. WE TOLD THEM WE WERE AT FL280 WITH 29.92. WE HAD NO FLAGS AND NO OTHER INDICATIONS TO SUGGEST OTHERWISE. ATC TOLD US WE WERE SHOWING FL289 ON THE ENCODER. WE WERE TOLD TO TURN OFF THE ALT SQUAWK AND WE ALSO WENT TO STANDBY ON THE PRIMARY ALTIMETER -- FLEW THE REST OF THE TRIP WITH CORRECTION. AFTER REACHING BNA AND DESCRIBED THE PROB TO MAINT; THE PROB WAS NOTED. 2 OUT OF 10 LEARJETS WE OPERATE HAVE ENCODER XFER SWITCHES LOCATED IN OBSCURE PLACES. AS IT TURNS OUT; THIS SWITCH WAS ACCIDENTALLY HIT; THUS XFERRING ENCODER INFO FROM THE COPLT'S ALTIMETER; WHICH WAS READING HIGHER THAN NORMAL AS IT TURNS OUT. AT NO TIME THEN WAS THERE EVEN AN ALT PROB WITH OUR ACFT. THE ABOVE-MENTIONED COPLT'S ALTIMETER HAS BEEN REPAIRED AND A GUARD HAS BEEN PLACED ON THE XFER SWITCH. ADDITIONALLY; A MEMO HAS BEEN SENT TO ALL PLTS DESCRIBING THIS SIT; THUS PREVENTING ANOTHER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THIS IS NOT A MAJOR PROB; AND THAT THE COPLT'S ALTIMETERS ON THE LEARJETS THAT HIS COMPANIES FLIES ARE ROUTINELY NOT CONSIDERED BY THE PLTS TO BE ACCURATE AND THEY TEND TO RELY ON THE L ALTIMETER. HE ALSO INDICATED THAT THE LOCATION OF THE XFER SWITCH THAT WAS INADVERTENTLY ACTUATED IS NOT STANDARD THROUGHOUT THE FLEET AND SOME DO NOT HAVE IT AT ALL. GUARDS HAVE BEEN INSTALLED ON ALL THOSE THAT DO EXIST. THE MALFUNCTIONING ALTIMETER THAT WAS SELECTED WAS FOUND TO HAVE A STATIC LEAK AND WAS REPAIRED.