A DC9 FLC ALLOWS THEIR ACFT TO FLY 100 FT BELOW THE CHART DEPICTED ALT WHILE THE FO; PF; WAS LOOKING FOR TFC ON THE RWY 30L LDA APCH AT STL; MO.

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WE WERE FLYING ON THE LOC (LDA) FOR AN APCH TO RWY 30L AT STL. THE OTHER ACFT (B737) WAS LINED UP FOR THE PARALLEL RWY AND BELOW US (APPROX 1000-1500 FT). THE CTLR KEPT CALLING OUT HIS POS; BUT WE COULD NOT MAKE HIM OUT QUICKLY. THE FO (PF) WAS TRYING TO SEE HIM ALSO; AND TRYING TO LEVEL OFF AT THE ASSIGNED ALT. BEING A LOW TIME PLT IN THE ACFT AND PROBATIONARY PLT; HE WAS FALLING BEHIND THE SIT. PLUS; HE NOTICED WE WERE GETTING CLOSE TO THE ARPT AND STILL FAIRLY HIGH. WITH THESE FACTORS COMBINED; HE LET THE ACFT DRIFT BELOW HIS ASSIGNED ALT. I NOTICED INITIALLY HE WAS A LITTLE LOW (WITHIN 100 FT) AND CALLED OUT 'ALT;' THEN PROCEEDED TO LOOK FOR TFC AGAIN. WE SAW HIM AND RPTED TFC IN SIGHT. AT THAT TIME I CALLED OUT 'ALT' AGAIN BECAUSE WE WERE BELOW OUR ASSIGNED ALT AT THAT POINT AND TOLD HIM TO GET BACK WHERE WE BELONG. WE WERE THEN CLRED FOR THE VISUAL APCH. IN THE FUTURE; I WILL TRY TO BE MORE AWARE OF THE LIMITATIONS OF A NEW FO IN A DEMANDING ENVIRONMENT. I WILL TRY TO GAUGE HIS/HER SITUATIONAL AWARENESS AND ACT ACCORDINGLY TO KEEP OUR ACFT/PAX SAFE.