MD80 CREW HAD ALTDEV.

Narrative

THOUGH FILED AT FL280; A CLB HAD BEEN ACCOMPLISHED TO FL350; PER ZID'S RECOMMENDATION; TO PENETRATE A SQUALL LINE AHEAD. WITH LIGHT-MODERATE TURB; WE PENETRATED THE TSTM SQUALL LINE (THAT REACHED BEYOND FL410) AND CAME UPON CLR AIR ON THE BACK (N) SIDE. SIMULTANEOUSLY; CTR ISSUED A CLRNC TO FL330 FOR TFC. THE DSCNT WAS UNEVENTFUL. AT FL330; THE CAPT (PF) BEGAN TO BRIEF THE PLANNED APCH AT OUR DEST (DTW). AS THE BRIEF BEGAN; I NOTICED OUR IAS OF 240 KIAS; AND THOUGHT THIS TO BE SLIGHTLY UNUSUAL; BUT TOOK NO ACTION. THE PF CHKED THE AUTOMATION TO BE 'ON' AND CHKED THE ENGAGED THROTTLES FORWARD. CONTINUING WITH THE BRIEF; I BEGAN TO REALIZE THAT THE AIRSPD WAS INSIDIOUSLY FALLING AND WAS NOW AT 230 KIAS. THOUGH ALL ENG INSTS/PARAMETERS WERE INDICATING NORMAL; I COMMENTED THAT SOMETHING WAS NOT RIGHT. AT THIS POINT THE PF DISCONNECTED THE AUTOMATION; PUSHED THE THROTTLES UP TO MAX CRUISE EPR LIMIT; BUT WAS NOT ABLE TO MAINTAIN CURRENT AIRSPD. AT 220 KIAS; THE ACFT BEGAN TO BUFFET AND THE CAPT INSTRUCTED TO REQUEST A LOWER ALT FROM ATC. UNFORTUNATELY; THE CTLR WAS RELAYING A LENGTHY FLT CONDITION RPT TO ANOTHER ACFT AND WE WERE UNABLE TO GET IN A TIMELY REQUEST. THE ONLY WAY TO MAINTAIN AN AIRSPD OF 225 KIAS; AT THIS POINT; WAS TO DSND AT LEAST 600 FPM. AT THE FIRST OPPORTUNITY; THE REQUEST TO DSND TO FL310 WAS SUCCESSFULLY XMITTED AND CLRNC WAS ISSUED TO MAINTAIN FL310. THE CLRNC CAME JUST AS WE PASSED THROUGH FL320. NO SIGN OF TFC OR CONFLICT WAS NOTED ON THE TCASII OR WITH THE CTLR. WITH THE ONLY EXPLANATION POSSIBLE; I IMMEDIATELY INITIATED ENG AND AIRFOIL ANTI-ICE SYS; BELIEVING THAT OUR PROB WAS CLR ICE ON THE ACFT. WITHIN 90 SECONDS; AT FL310; THE ACFT BEGAN TO ACCELERATE AND OPERATE NORMALLY. WE RETAINED THE ANTI-ICE SYS ON FOR AN ADDITIONAL 5 MINS; AND THEN SECURED THEM. IN THE CLR AIR; ABOVE AN OVCST DECK; THE REMAINDER OF THE CRUISE PHASE OF FLT WAS UNEVENTFUL. IT IS MY OPINION THAT A SIGNIFICANT AMOUNT OF CLR ICE WAS ACCUMULATED ON THE CRITICAL SURFACES OF OUR ACFT DURING THE BRIEF PERIOD THAT WE TRANSITED THROUGH THE SQUALL LINE. SINCE THE CLRNC TO DSND TO FL330 CAME SO SOON AFTER OUR PENETRATION; A LOWER AOA (ANGLE OF ATTACK) WAS POSSIBLE; AND THEREFORE NO APPARENT PROB EXISTED. HOWEVER; WHILE LEVEL AT FL330; WITH THE AUTOPLT/AUTOTHROTTLES ENGAGED; THE ACFT ATTEMPTED TO HOLD AIRSPD AND ALT; BUT WAS LOSING THE BATTLE AT AN EVER SO SLOWLY RATE. IT WAS NOT UNTIL THE ANTI-ICE SYS TOOK AFFECT AND A LOWER ALT WAS RECEIVED THAT CONDITIONS RETURNED TO NORMAL. LESSON LEARNED: IN ANTICIPATION OF A NECESSARY PENETRATION OF A SQUALL LINE; EVEN WHEN THE DEPTH (THICKNESS) APPEARS NOMINAL AND THE ALT ADEQUATE (BOTH FOR TURB AND ICING); APPLY ALL ANTI-ICE SYS IN ADVANCE. WE HAD TAKEN CARE OF THE ENGS; BUT HAD NOT CONSIDERED THE ACFT'S CRITICAL SURFACES VULNERABLE UNDER THE EXISTING CONDITIONS.