COMMUTER DSNDS BELOW ASSIGNED.

Narrative

WE WERE DSNDING TO CROSS DEER PARK AT 15000 FT; ON THE BOUNO 2 ARR IN WHITE PLAINS; NY. THE CTLR GAVE US A 060 DEG HDG FOR THE DSCNT AND DSCNT FROM 17000 FT TO 15000 FT. I NOTICED RIGHT AWAY THE FO DIDN'T HIT THE ALT SELECT BUTTON ON THE AUTOPLT; BUT HE WAS STILL ROLLING THE WHEEL TO INCREASE HIS RATE OF DSCNT TO 1500 FPM. I THOUGHT HE WAS GOING TO HIT THE ALT SELECT BUTTON. ABOUT THE TIME HE TOOK HIS HAND AWAY; THE AIRPLANE YAWED TO THE L CALLING MY ATTN TO THE ENG INSTS AND THE CAUTION PANEL. BY THE TIME I REALIZED THE ENG ON THE L SIDE HAD GONE FROM ELECTRONIC FUEL SCHEDULING TO MANUAL FUEL SCHEDULING THE CTLR HAD ASKED US OUR ALT. WE RPTED 13700 FT FOR 15000 FT. THIS IS WHEN I REALIZED WE HAD BUSTED OUR ALT. I TOLD THE FO TO CLB BACK TO 15000 FT WHILE I TOOK CARE OF THE FUEL SCHEDULING PROB. ABOUT THAT TIME THE CTLR ASKED WHY WE WERE BELOW OUR ASSIGNED ALT. I TOLD HIM WE JUST GOT A CAUTION LIGHT AND HE ACCEPTED IT AS A TCASII RA. HIS ASSUMPTION OF A TCASII RA SOUNDED LIKE A GOOD IDEA TO ASSUME ON MY PART; WHICH I FEEL WAS A BAD IDEA NOW. FROM NOW ON I'LL ASK MY FO'S WHEN; ON THE AUTOPLT AND CHANGING ALT; TO PUSH THE ALT SELECT BUTTON FIRST.