ACR FLC ALLOWS ACFT TO UNDERGO A SPD DEV AND ACFT GOES THROUGH A SMALL BUFFET AFTER ENCOUNTERING INFLT TURB. ALTDEV ALT EXCURSION RESULTED.

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IT WAS MY FIRST LINE TRIP AFTER COMPLETING IOE IN THE ACFT. CLBOUT WAS PROGRESSING NORMALLY THROUGH APPROX 310 OR 320 ENRTE TO 350. THERE WERE NUMEROUS SIGMETS CONCERNING MODERATE TO SEVERE TURB; AND WE HAD EXPERIENCED TURB DURING CLB. ATC ASKED US TO EXPEDITE CLB TO 350. I SET A SLIGHT INCREASE ON THE VERT SPD CTL WHEEL; AND CHKED TO SEE THAT THE AUTOPLT HAD RESPONDED; AND THAT THE AIRSPD WAS OKAY. THEN I REACHED OVER TO UNFOLD MY MAP AND DETERMINE THE AIRWAY HDG OUTBOUND FROM THE UPCOMING VOR. I LOCATED THE VOR AND LOOKED PAST MY UNFOLDED MAP AND SAW THAT THE AUTOPLT HAD CAPTURED THE ALT AND WAS BEGINNING TO LEVEL OFF. I PUT THE MAP ASIDE AND CHKED THE AIRSPD; IT WAS DECAYING RAPIDLY THROUGH ABOUT 200; RATHER THAN INCREASING AS I EXPECTED IT TO DO; SINCE THE ALT HAD BEEN CAPTURED. I PUT THE MAP DOWN; DISENGAGED THE AUTOPLT AND ADDED PWR AS THE AIRSPD CONTINUED TO DROP AND THE ACFT BEGAN TO BUFFET. I TOLD THE FO TO GET A LOWER ALT SINCE I COULDN'T HOLD THE ALT AND REGAIN THE AIRSPD AT THE SAME TIME. ATC SAID TO MAINTAIN 350; SO I LIFTED THE NOSE SLIGHTLY AND GOT ANOTHER BUFFET. I THEN LOWERED THE NOSE TO ELIMINATE THE BUFFET AND WAS IN A VERY SLOW DSCNT AS THE AIRSPD RECOVERED. SHORTLY THEREAFTER; ATC CLRED US TO 330; THEN 310; AND THINGS RETURNED TO NORMAL. THE MAIN LESSON TO BE REITERATED IS THAT YOU CAN NEVER AFFORD TO BE DISTR AT THE WRONG TIME. I SHOULD HAVE CLOSELY WATCHED WHAT THE (NEW; THEREFORE LESS FAMILIAR) AIRPLANE WAS DOING UNTIL EVERYTHING WAS STABILIZED; THEN LOOKED AT THE MAP; OR JUST ASKED THE FO FOR THE OUTBOUND HDG. THE FO NOTICED THAT THE SPD WAS SOMEWHAT LESS THAN IT SHOULD HAVE BEEN; BUT RATHER THAN SAYING SOMETHING IMMEDIATELY; HE REACHED FOR HIS ACFT MANUAL TO CHK THE ENVELOPE AT THAT ALT AND WT. THE RAPID SPD DECAY SET IN WHILE BOTH OF US WERE DISTR DOING SOMETHING LESS IMPORTANT THAN FLYING THE AIRPLANE. SUPPLEMENTAL INFO FROM ACN 200246. AFTER OUR CLB WAS INCREASED; I NOTICED OUR AIRSPD AT 250 KTS WHICH IS OBVIOUSLY SLOWER THAN OUR NORMAL CLB SPD BUT WELL WITHIN LIMITS FOR OUR WT AND ALT. AT THIS TIME I REACHED INTO MY FLT BAG TO PULL OUT SOME ADDITIONAL CHARTS. I LOOKED UP AFTERWARD TO NOTICE US CAPTURING OUR ALT AT 35000 FT WITH AN AIRSPD NOW AT 220 KTS. I REACHED FOR THE BUFFET MARGIN CHART TO CHK THE SPD WHEN SUDDENLY THE AIRSPD DROPPED TO APPROX 190 KTS. AT THIS TIME LOW SPD BUFFETING BEGAN TO OCCUR AND THE CAPT DISENGAGED THE AUTOPLT AND INCREASED THRUST TO MAX CONTINUOUS. HE TOLD ME TO CONTACT CENTER AND REQUEST A LOWER ALT WHICH I IMMEDIATELY DID KNOWING THAT WE WOULD BE UNABLE TO STAY AT 35000 FT. WASHINGTON CENTER REPLIED 'UNABLE; MAINTAIN FL350.' I RESPONDED 'UNABLE; WE WILL TAKE A TURN IF NEEDED BUT WE NEED LOWER NOW!' AT THIS TIME WE WERE ALREADY DSNDING PAST 34500 FT WHILE THE BUFFETING CONTINUED. ATC THEN ISSUED US A CLRNC TO FL330 AND ASKED IF WE HAD A MECHANICAL PROBLEM IN WHICH I RESPONDED 'NEGATIVE.'