CPR LTT ACFT LEVELING AT 8000 HAD UNCTLABLE UPDRAFT AND SUDDEN DOWNDRAFT THAT THREW THE ACFT OUT OF CTL. NO FORECAST OF TURB IN THAT AREA OVER DELAWARE.

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THE ACFT HAD JUST LEVELED OFF AT 8000 FT AND 250 KTS WITH THE FO AT THE CTLS. I (CAPT) HAD JUST CHANGED OVER THE BWI APCH WHEN I NOTICED THE ACFT DEPARTING 8000 FT IN A CLB. BOTH MYSELF AND FO ATTEMPTED TO LEVEL THE ACFT; BUT IT CONTINUED CLB TO ABOUT 8500 FT; WHEN WE ENCOUNTERED A TREMENDOUS NEGATIVE FORCE AND BOTH OF US WERE THROWN TOWARDS THE OVERHEAD PANEL. IN A MATTER OF SECONDS THE AIRPLANE WAS IN A NEAR VERT DIVE WITH NO BLUE (ABOVE HORIZON REF) SHOWING ON THE ARTIFICIAL HORIZON. OUR INITIAL ATTEMPT AT RECOVERY HAD NO EFFECT. I RELAXED PRESSURE ON THE ELEVATOR FOR A SECOND AND BEGAN USING STABILIZER TRIM TO RECOVER. THE AIRPLANE GRADUALLY RECOVERED TO LEVEL FLT AT A SPD OF 310 KTS; AT AN ALT OF 6000 FT. THE RECOVERY SEEMED TO OCCUR FROM FLYING OUT OF THE DRAFT; AS MUCH AS FROM OUR RECOVERY EFFORTS. WE WERE USING WX RADAR AT THE TIME AND SAW NO INDICATION OF TSTM ACTIVITY AND THERE WAS NO LIGHTNING PRESENT. I HAVE FLOWN APPROX 29000 HRS OVER 34 YRS AND HAVE NEVER EXPERIENCED TURB OF THIS MAGNITUDE. I BELIEVE SINCE THERE WAS NO CONVECTIVE WX VISIBLE THIS MUST HAVE BEEN FRONTAL SHEAR. I HAVE NO DOUBT IF WE HAD STAYED IN THIS DOWNDRAFT SECONDS LONGER WE WOULD HAVE HIT THE GND. AFTER THE INCIDENT I NOTIFIED BWI APCH OF OUR DIFFICULTIES BOTH BY RADIO AND LATER TELEPHONE. A NOTAM WAS FILED WITH FSS. A THOROUGH INSPECTION OF THE AIRPLANE REVEALED NO STRUCTURAL DAMAGE.