FLC OF A LEARJET WERE INADVERTENTLY XMITTING A HIGHER ALT THAN ASSIGNED DUE TO SHOWING THE COPLT ALTIMETER SETTING RATHER THAN THE CAPT'S.

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FLT FROM CLT TO BNA. THE ALT WAS FL280 AND WE WERE IN CONTACT WITH ZTL. WE WERE CLBING TO FL280 WHEN WE WERE TOLD WE HAD TFC AT OUR 11-12 O'CLOCK POS. ABOUT 30 SECONDS AFTER REACHING FL280 WE HEARD THE ACR X PLANE SAY THAT THEY NEEDED AN IMMEDIATE TURN. THEY SHOWED ON THEIR TCASII THAT WE WERE CLBING. ZTL THEN ASKED US WHAT OUR ALT WAS. WE SAID FL280 AND 29.92 WAS SET IN THE ALTIMETER WINDOW. THERE WERE NO FLAGS TO INDICATE A PROB. ATC SAID THAT WE WERE INDICATING FL289. WE WENT TO STANDBY ON THE PLT'S ALTIMETER AND FLEW THE REST OF THE TRIP BY CORRECTION. AFTER REACHING BNA AND DESCRIBING THE PROB TO OUR MAINT PERSONNEL; THE PROB WAS EVENTUALLY FOUND. SOME OF OUR LEARJETS HAVE A SWITCH THAT CAN SWAP THE MODE C TO READ FROM THE PLT'S TO THE COPLT'S ALTIMETER. ATC AND THE PLANE WERE GETTING A READING FROM THE COPLT'S ALTIMETER; WHICH IN TURN MADE US LOOK HIGHER THAN WE ACTUALLY WERE. THE PLT'S ALTIMETER WAS CORRECT AND WE WERE AT FL280. AT NO TIME WAS THERE AN ALT INCURSION. THE SWITCH IN QUESTION HAS SINCE BEEN PUT BACK IN THE PLT POS AND THE SWITCH HAS BEEN GUARDED TO PREVENT INADVERTENT MISPLACEMENT. THE SWITCH IN QUESTION MUST HAVE BEEN ACCIDENTALLY HIT BEFORE THE FLT. THAT IS WHY THE SWITCH IS NOW GUARDED. THE SWITCH IS NOT A COMMON THING ON A LEARJET AND AT NO TIME IN A CHKLIST IS THE SWITCH TO BE CHKED FOR PROPER POS. I WOULD LIKE TO ADD THAT ATC NEVER DID ADVISE US THAT WE WERE SHOWING HIGHER THAN WE WERE SUPPOSED TO BE. IT WAS NOT UNTIL ACR X SAID SOMETHING THAT THE PROB WAS NOTICED. WHATEVER ACTION ACR X TOOK TO DEVIATE WAS AT THEIR DISCRETION AND AT NO TIME WAS THERE A POSSIBILITY OF AN ALT INCURSION.