Narrative:

Holding at terri intersection, dupont 3 arrival. WX, IMC, moderate rain, ice pellets and moderate, occasionally severe turbulence. We were next to exit hold for approach to runway 9R, phl. ATC issued descent from 6000-5000 ft. When just leveling at 5000 ft we encountered severe turbulence. I elected to trip off autoplt and hand-fly aircraft for more positive control. During the transition from automatic to manual flight, I inadvertently descended to 4700 ft MSL, a 300 ft deviation. This happened during the most severe portion of shear and turbulence. I quickly corrected the deviation and re-established autoflt. There was no ATC conflict. I felt that disconnecting the autoplt would allow better controllability of the aircraft during extreme turbulence. It so happened the transition from descent to level flight, occurred at the same time we entered this apparent shear zone. I also went to manual flight at this time. I noticed the deviation first, as the captain was talking to ATC. The decision to fly the aircraft manually, or remain on autoplt in severe turbulence is the question in this event. Also, should the pilot let the autoplt control the aircraft till it disconnects itself? From past experience, I believe I made the better decision to hand fly the airplane.

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Original NASA ASRS Text

Title: B737-400 HOLDING AT TERRI INTXN 6000 FT. CLRED TO DSND TO 5000 FT AND VECTORED FROM THE HOLDING FIX. ENCOUNTERED MODERATE TURB AND ELECTED TO HAND FLY. OVERSHOT ALT 300 FT. NO COMMENT FROM CTLR.

Narrative: HOLDING AT TERRI INTXN, DUPONT 3 ARR. WX, IMC, MODERATE RAIN, ICE PELLETS AND MODERATE, OCCASIONALLY SEVERE TURB. WE WERE NEXT TO EXIT HOLD FOR APCH TO RWY 9R, PHL. ATC ISSUED DSCNT FROM 6000-5000 FT. WHEN JUST LEVELING AT 5000 FT WE ENCOUNTERED SEVERE TURB. I ELECTED TO TRIP OFF AUTOPLT AND HAND-FLY ACFT FOR MORE POSITIVE CTL. DURING THE TRANSITION FROM AUTO TO MANUAL FLT, I INADVERTENTLY DSNDED TO 4700 FT MSL, A 300 FT DEV. THIS HAPPENED DURING THE MOST SEVERE PORTION OF SHEAR AND TURB. I QUICKLY CORRECTED THE DEV AND RE-ESTABLISHED AUTOFLT. THERE WAS NO ATC CONFLICT. I FELT THAT DISCONNECTING THE AUTOPLT WOULD ALLOW BETTER CONTROLLABILITY OF THE ACFT DURING EXTREME TURB. IT SO HAPPENED THE TRANSITION FROM DSCNT TO LEVEL FLT, OCCURRED AT THE SAME TIME WE ENTERED THIS APPARENT SHEAR ZONE. I ALSO WENT TO MANUAL FLT AT THIS TIME. I NOTICED THE DEV FIRST, AS THE CAPT WAS TALKING TO ATC. THE DECISION TO FLY THE ACFT MANUALLY, OR REMAIN ON AUTOPLT IN SEVERE TURB IS THE QUESTION IN THIS EVENT. ALSO, SHOULD THE PLT LET THE AUTOPLT CTL THE ACFT TILL IT DISCONNECTS ITSELF? FROM PAST EXPERIENCE, I BELIEVE I MADE THE BETTER DECISION TO HAND FLY THE AIRPLANE.

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.