Narrative:

After passing tqa VOR, given instructions to return to phils intersection and hold due to WX. Programmed FMS to enter holding, and slowed aircraft to 253 KTS (20 KTS above clean minimum maneuvering). Aircraft arrived at the fix requiring a right 260 degree turn to parallel the outbound course. A 30 KT overshooting crosswind was present at entry point. Aircraft entered proper right turn in approximately 20 degree angle of bank. Once established in the turn I looked down into my kit bag for approximately 3-4 seconds. Aural 'altitude alert' sounded. I looked up to see the aircraft descending through 32800 ft in a right 38-40 degree 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 power. Buffet ceased as soon as the wings wre leveled. Descent was stopped at 32200 ft and then climbed back to 33000 pounds. ATC was immediately notified of our altitude loss. The aircraft was not overweight for normal operations at 133000 pounds. Gross weight was 139000 pounds, speed at entry was 253 KTS, air was smooth, outside air temperature was 10 degrees above standard. The performance manual recommended holding speed for this weight and altitude is 239 KTS. Pms recommended holding speed was even less. We chose to use clean minimum maneuvering speed plus 20 KTS -- or 253 KTS. Slow speed buffet for a 1.3 G (40 degree 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 aircraft to near 40 degrees with the outside air temperature at 10 degrees above standard. This must have slowed the aircraft just enough to reach slow speed 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 heading select to limit my bank angle. Looking away in the turn, even for a short period, was a mistake. I recalled later in the flight I should not have used navigation to hold in this non-EFIS aircraft. Both HSI's were in the rad mode. It is my understanding that this limitation is a situational awareness issue. Being a non EFIS aircraft should have had no impact on the FMS commanding a 38-40 degree angle of bank, but maybe I am mistaken. The omission of the limitation was not intentional.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.