MLG ACFT IN CRUISE ON OCEANIC AIRWAY ENCOUNTERED SEVERE WX. ICING AND TURB RESULTED IN ACFT APCHING STALL SPD WHILE FLC TRIED TO CONTACT CTLR FOR LOWER ALT; BUT WERE UNABLE. FLC DECLARED EMER IN THE BLIND. COMMENCED DSCNT AND HAD TO DSND TO FL210 BEFORE THEY WERE ABLE TO REGAIN AIRSPD.

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A LARGE LINE OF TSTMS WERE XING THE RTE WITH TOPS ABOVE FL400 OVER CARPS INTXN ON AR-3. I DECIDED TO CROSS CIRCUMNAVING THE TARGETS SO WE PREPARE IN ADVANCE. PEOPLE: TAKE YOUR SEATS; FASTEN SEAT BELTS FOR TURB. ACFT: ANTI-ICE SYS ON. PENETRATION SPD (MACH .76). APCHING THE LINE; WE START A 5 NM E DEV. ABEAM CARPS WITHOUT ANY INDICATION ON RADAR THE WX STARTED TO DETERIORATE VERY FAST WITH ICE; STATIC AND TURB. THEN THE ACFT STARTS A SLIGHT SHAKE WITH THE AIRSPD DECREASING CONTINUOUSLY. WE TRY TO CONTACT CTR ASKING FOR LOWER LEVEL WITH NO ANSWER. SO WE DECLARE THE EMER DSCNT TO KEEP OR INCREASE THE MINIMUM AIRSPD AS FOLLOWS: BLIND XMISSION DONE; SQUAWK 7700; MORE DEV L OF COURSE; DSNDING 2000 FPM. THRUST ON GAR. XING FL280 STALL WARNING ACTIVATES. WE INCREASE THE RATE TO 4000 FPM. XING FL230 THE AIRSPD START TO INCREASE; THE SHAKE DISAPPEARS AND THE WX IMPROVES; AT FL210 WE LEVEL WITH CRUISE SPD AND THRUST. FINALLY WE CONTACT CTR AND CONTINUE OUR FLT WITH NO FURTHER PROBS. CONCLUSIONS: ALT CAPABILITY DEGRADED BY ICE BUILD-UP; ADVERSE SHEAR AND ANTI-ICE SYS.