DA900 CAPT REPORTS ALT DEV WHILE MANEUVERING WITH TCS BUTTON DEPRESSED. REPORTER IS NEW TO EFIS AND VERTICAL TAPE ALTIMETER AND NOT AWARE TCS BUTTON DISCONNECTS AUTOPILOT PITCH AS WELL AS ROLL.

Narrative

DURING INITIAL DSCNT AFTER 8+ HR TRANS-OCEANIC FLT; GIVEN DIRECT TO A FIX FARTHER ALONG RTE OF FLT; HAD JUST CHANGED DEST AT REQUEST OF PAX AND HAD RECEIVED ROUTING THE NEW DEST UPON ENTERING THE FIR OF THE DEST COUNTRY. WORKLOAD SOMEWHAT HIGH. UPON ENTERING FIRST FIX OF NEW RTE; (AUTOPLT ENGAGED) MOMENTARY HESITATION ENGAGING 'NAV' MODE CAUSED AUTOPLT TO TURN HDG A LITTLE MORE THAN DIRECT COURSE TO REINTERCEPT DIRECT COURSE. INTENT ON MAKING SMOOTH TRANSITION FOR THE PAX; INTERCEDED WITH 'TCS' BUTTON WHICH DISENGAGES AUTOPLT AS LONG AS IT IS HELD DEPRESSED. TCS ALSO ALLOWS FLT DIRECTOR GUIDANCE TO CONTINUE TO FOLLOW LATERAL NAV INPUTS FROM SELECTED SOURCE; (FMS IN THIS CASE) BUT ALLOWS FLT DIRECTOR TO FOLLOW ACFT PITCH AS OPPOSED TO LAST SELECTED VERT GUIDANCE (ASEL IN THIS CASE). WHILE MANEUVERING THE ACFT TO MORE SMOOTHLY INTERCEPT NEW COURSE; GAIN IN ALT UNNOTICED UNTIL ABOUT 250 FT OFF ASSIGNED. BY THE TIME RETURNED TO ASSIGNED ALT TOTAL EXCURSION ABOUT 350 FT. RELATIVELY NEW TO FULL 5 TUBE EFIS WITH VERT TAPE TYPE ALTIMETRY READOUT; BEEN FLYING ANALOG INST VERSION OF THIS ACFT FOR ABOUT 7 YRS; RECENTLY (6 MONTHS) BEGAN FLYING FULL 5 TUBE EFIS VERSION; NOW ALTERNATING BTWN THE TWO. CONTRIBUTING FACTORS; LOW EXPERIENCE WITH VERT TAPE STYLE ALTIMETER READOUT CAUSED MORE RELIANCE ON FLT DIRECTOR. HAD SELDOM PREVIOUSLY USED TCS BUTTON TO INTERCEDE IN AUTOFLT BEING MORE INCLINED TO DISENGAGE AUTOPLT AND REENGAGE AFTER CORRECTION; AND PERCEIVED THAT VERT GUIDANCE WAS STILL VALID WHILE TCS WAS ENGAGED. ENSURE FAMILIARITY WITH ALL MODES OF AUTOMATION.