FLC ENTERS HOLDING PATTERN AND ACFT ENTERS STALL BUFFET.

Narrative

AFTER PASSING TQA VOR; GIVEN INSTRUCTIONS TO RETURN TO PHILS INTXN AND HOLD DUE TO WX. PROGRAMMED FMS TO ENTER HOLDING; AND SLOWED ACFT TO 253 KTS (20 KTS ABOVE CLEAN MINIMUM MANEUVERING). ACFT ARRIVED AT THE FIX REQUIRING A R 260 DEG TURN TO PARALLEL THE OUTBOUND COURSE. A 30 KT OVERSHOOTING XWIND WAS PRESENT AT ENTRY POINT. ACFT ENTERED PROPER R TURN IN APPROX 20 DEG ANGLE OF BANK. ONCE ESTABLISHED IN THE TURN I LOOKED DOWN INTO MY KIT BAG FOR APPROX 3-4 SECONDS. AURAL 'ALT ALERT' SOUNDED. I LOOKED UP TO SEE THE ACFT DSNDING THROUGH 32800 FT IN A R 38-40 DEG ANGLE OF BANK TURN. THE AFT BEGAN TO BUFFET AND THE AUTOPLT DISCONNECTED. AS THIS WAS ALL OCCURRING; I IMMEDIATELY LEVELED THE WINGS AND ADDED PWR. BUFFET CEASED AS SOON AS THE WINGS WRE LEVELED. DSCNT WAS STOPPED AT 32200 FT AND THEN CLBED BACK TO 33000 LBS. ATC WAS IMMEDIATELY NOTIFIED OF OUR ALT LOSS. THE ACFT WAS NOT OVERWT FOR NORMAL OPS AT 133000 LBS. GROSS WT WAS 139000 LBS; SPD AT ENTRY WAS 253 KTS; AIR WAS SMOOTH; OUTSIDE AIR TEMP WAS 10 DEGS ABOVE STANDARD. THE PERFORMANCE MANUAL RECOMMENDED HOLDING SPD FOR THIS WT AND ALT IS 239 KTS. PMS RECOMMENDED HOLDING SPD WAS EVEN LESS. WE CHOSE TO USE CLEAN MINIMUM MANEUVERING SPD PLUS 20 KTS -- OR 253 KTS. SLOW SPD BUFFET FOR A 1.3 G (40 DEG ANGLE OF BANK) MANEUVER PUBLISHED AS 242 KTS. AS FAR AS I CAN FIGURE; THE FMS; IN AN ATTEMPT TO REMAIN IN ITS COMPUTED ENTRY TURN PROFILE; OVERBANKED THE ACFT TO NEAR 40 DEGS WITH THE OUTSIDE AIR TEMP AT 10 DEGS ABOVE STANDARD. THIS MUST HAVE SLOWED THE ACFT JUST ENOUGH TO REACH SLOW SPD BUFFET. I'M SURE THE SOFTWARE ENGINEERS WILL SAY THE FMS CAN'T DO THIS; BUT IT DID. I HAVE SEEN THIS EXACT SAME PHENOMENON ON THE B757 ENTERING HOLDING IN LNAV. I TAKE FULL RESPONSIBILITY FOR THIS EVENT AND HAVE LEARNED A FEW LESSONS. ALTHOUGH WITHIN THE OPERATING ENVELOPE; I WON'T USE THE FMS FOR ENTRY TURNS INTO HOLDING. I WILL USE HDG SELECT TO LIMIT MY BANK ANGLE. LOOKING AWAY IN THE TURN; EVEN FOR A SHORT PERIOD; WAS A MISTAKE. I RECALLED LATER IN THE FLT I SHOULD NOT HAVE USED NAV TO HOLD IN THIS NON-EFIS ACFT. BOTH HSI'S WERE IN THE RAD MODE. IT IS MY UNDERSTANDING THAT THIS LIMITATION IS A SITUATIONAL AWARENESS ISSUE. BEING A NON EFIS ACFT SHOULD HAVE HAD NO IMPACT ON THE FMS COMMANDING A 38-40 DEG ANGLE OF BANK; BUT MAYBE I AM MISTAKEN. THE OMISSION OF THE LIMITATION WAS NOT INTENTIONAL.