COMPANY CHK PLT ACTING AS FLYING FO OVERSHOT LEVELOFF ALT DUE TO HIS ALTIMETER READING LOWER THAN THE CAPT'S WHICH IS THE ENCODING ALTIMETER.

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I WAS ON IOE AS A NEW DC9 CAPT WITH A RELATIVELY INEXPERIENCED INSTRUCTOR IN THE R SEAT. IT WAS HIS LEG TO DO PF DUTIES. WHEN IT CAME TIME FOR LEVELOFF AT FL240; HE USED HIS ALTIMETER TO REF THE LEVELOFF. AS WE APCHED 150 FT HIGH ON THE LEVELOFF; I ASKED IF HE WAS IN FACT LEVELING. (THE AUTOPLTS ON DC9'S TYPICALLY WILL OVERSHOOT BY 100 FT OR SO AND THEN SETTLE BACK DOWN TO THE SELECTED ALT.) HE STATED HE WAS LEVELING AND I LOOKED AT HIS ALTIMETER. BOTH WERE CORRECTLY SET AT 29.92; BUT HIS WAS READING 200 FT LOWER THAN MINE. ALT ENCODING IS OFF CAPT'S ALTIMETER. AFTER DISCOVERING THIS AND HIM TAKING CORRECTIVE ACTION; ZMP CALLED AND ASKED OUR ALT. I WAS HONEST AND TOLD THEM ALT SETTINGS WERE OK; BUT THERE WAS A 200 FT DIFFERENCE BTWN OUR ALTIMETERS. WHEN I WAS IN THE R SEAT OF THE DC9 I WAS TRAINED TO LOOK AT THE CAPT'S ALTIMETER FOR LEVELOFFS. I THOUGHT A CAPT INSTRUCTOR WOULD HAVE KNOWN THIS; BUT IT HAD BEEN A WHILE SINCE HE HAD BEEN IN THE R SEAT OF A DC9 AND ONLY HAD ABOUT 3 LEGS BEFORE FLYING WITH ME. ALMOST SCARES ME TO THINK I'LL BE FLYING AS A NEW CAPT WITH NEW-HIRE FO'S AND I CAN'T EVEN BE RELAXED WITH AN INSTRUCTOR IN THE R SEAT.