Narrative:

During cruise at FL350, we experienced occasional light chop in IMC. The only WX on our radar was approximately 40 mi ahead and slightly right of course. The captain asked and received clearance for direct vrb to provide greater distance from WX (cell) at omn. Suddenly, we entered moderate turbulence. The captain began reducing speed to .76 mach (normal cruise was .80 mach). As turbulence increased, an overspd occurred. The automatic overspd protection for the airbus 320, the autoplt disengages, and a pick-up is commanded to reduce airspeed. The pilot is unable to override pitch-up command until airspeed is below mmo/vmo. The aircraft climbed over 3000 ft in 20-30 seconds with airspeed in the barber pole until above FL380. At that time, the captain was able to control aircraft pitch. He leveled at FL380. I had made calls to jacksonville center during the uncontrolled climb. The controller answered when the aircraft leveled at FL380 (approximately 30 seconds after the aircraft climb started). Then ZJX issued clearance to FL370 and advised us that there were no other aircraft in area. As the captain leveled the aircraft to FL380, the airspeed reduced to vls and then stabilized at .76 mach. The ZJX controller reported that there was no WX within 20 mi of aircraft on his radar screen. The remainder of flight was normal. I believe that we entered a building cell without radar return, because of sudden turbulence and the rate of climb. We entered an updraft while the autoplt was holding level at FL350. Then the overspd occurred. The automatic protection along with the updraft, caused the aircraft to climb 3000 ft. We were surprised by the sudden overspd and the climb rate at FL350.

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Original NASA ASRS Text

Title: A320 FLT CREW ENTERS MODERATE TURB, AND THE ACFT EXCEEDS ITS MAX MACH OPERATING SPD (MMO), CAUSING THE AUTOFLT SYS TO ATTEMPT TO REDUCE THE SPD.

Narrative: DURING CRUISE AT FL350, WE EXPERIENCED OCCASIONAL LIGHT CHOP IN IMC. THE ONLY WX ON OUR RADAR WAS APPROX 40 MI AHEAD AND SLIGHTLY R OF COURSE. THE CAPT ASKED AND RECEIVED CLRNC FOR DIRECT VRB TO PROVIDE GREATER DISTANCE FROM WX (CELL) AT OMN. SUDDENLY, WE ENTERED MODERATE TURB. THE CAPT BEGAN REDUCING SPD TO .76 MACH (NORMAL CRUISE WAS .80 MACH). AS TURB INCREASED, AN OVERSPD OCCURRED. THE AUTOMATIC OVERSPD PROTECTION FOR THE AIRBUS 320, THE AUTOPLT DISENGAGES, AND A PICK-UP IS COMMANDED TO REDUCE AIRSPD. THE PLT IS UNABLE TO OVERRIDE PITCH-UP COMMAND UNTIL AIRSPD IS BELOW MMO/VMO. THE ACFT CLBED OVER 3000 FT IN 20-30 SECONDS WITH AIRSPD IN THE BARBER POLE UNTIL ABOVE FL380. AT THAT TIME, THE CAPT WAS ABLE TO CTL ACFT PITCH. HE LEVELED AT FL380. I HAD MADE CALLS TO JACKSONVILLE CTR DURING THE UNCTLED CLB. THE CTLR ANSWERED WHEN THE ACFT LEVELED AT FL380 (APPROX 30 SECONDS AFTER THE ACFT CLB STARTED). THEN ZJX ISSUED CLRNC TO FL370 AND ADVISED US THAT THERE WERE NO OTHER ACFT IN AREA. AS THE CAPT LEVELED THE ACFT TO FL380, THE AIRSPD REDUCED TO VLS AND THEN STABILIZED AT .76 MACH. THE ZJX CTLR RPTED THAT THERE WAS NO WX WITHIN 20 MI OF ACFT ON HIS RADAR SCREEN. THE REMAINDER OF FLT WAS NORMAL. I BELIEVE THAT WE ENTERED A BUILDING CELL WITHOUT RADAR RETURN, BECAUSE OF SUDDEN TURB AND THE RATE OF CLB. WE ENTERED AN UPDRAFT WHILE THE AUTOPLT WAS HOLDING LEVEL AT FL350. THEN THE OVERSPD OCCURRED. THE AUTO PROTECTION ALONG WITH THE UPDRAFT, CAUSED THE ACFT TO CLB 3000 FT. WE WERE SURPRISED BY THE SUDDEN OVERSPD AND THE CLB RATE AT FL350.

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.