FO OF MD80 RPTS UNABLE TO MAINTAIN CRUISE MACH AT FL330 DESPITE BEING 2000 FT BELOW MAX PERFORMANCE MANUAL ALT.

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AFTER REACHING CRUISE ALT AT FL330 I CHKED THE ALT CAPABILITY CHART NOTED THAT WE WERE GOOD FOR FL350; AND THE CAPT MADE HIS PA. WE STAYED AT FL330 AND AFTER A COUPLE OF MINS I NOTICED THAT THE AIRSPEED WAS AT 250 KTS INDICATED AND THE PWR WAS NOT AT THE CLB LIMIT. THE T.R.I. WAS STILL SET AT CLB SETTING. I PUSHED THE THROTTLES TO THE LIMIT AND THEN THEY CAME BACK BELOW THE CLB LIMIT. I PUSHED THEM UP AGAIN; AND AT THAT TIME THE CAPT STATED TO WATCH THE AIRSPEED AND ALSO MADE SURE THAT THE PWR WAS AT THE CLB LIMIT SETTING. THE AIRSPEED CONTINUED TO STAGNATE AND STARTED TO DECREASE. WE BOTH SAID THAT WE WERE NOT COMFORTABLE; AND THE CAPT REQUESTED A LOWER ALT AND WAS TOLD TO STAND BY. AFTER THIS; THE AIRSPEED CONTINUED TO DECREASE TO 242 KTS; AND WHEN THE CTLR STOPPED TALKING THE CAPT STATED WE WERE DECLARING AN EMER; AND WE NEEDED A LOWER ALT. THE CTLR IMMEDIATELY GAVE US FL310; AND WE STARTED A DESCENT. WE NEVER GOT ANY INDICATION OF STALL OR BUFFET OR SPEED LOW INDICATIONS; AND WE WERE IN CTL OF THE ACFT DURING THE ENTIRE EVENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR EMPHASIZED THAT; ALTHOUGH THE FIRST TIME HE MANUALLY ADVANCED THE AUTOTHROTTLES TO THE CLB LIMIT THEY RETARDED TO THEIR PREVIOUS SETTING; THE SECOND TIME HE DID SO THEY REMAINED AT THE CLB LIMIT EPR. AT THAT TIME THE IAS WAS ABOUT 240 VICE THE 255 DESIRED FOR THEIR CRUISE MACH AND WITHIN 10K OF THEIR CALCULATED 1.3G LOW SPEED BUFFET. AT THAT TIME THEY WERE OPERATING IN LIGHT ROLLING TURB AND EACH WAVE SLOWED THE ACFT SLIGHTLY AND THE THRUST AVAILABLE WAS INSUFFICIENT TO REGAIN THE LOSS. HE FELT THE CONDITION CLRLY INDICATED A CONDITION ON THE BACK SIDE OF THE DRAG CURVE FROM WHICH THEY WOULD BE UNABLE TO ACCELERATE WITHOUT DESCENDING AND THAT CONTINUED EXPOSURE TO THE DEBILITATING EFFECTS OF THE TURB ON THEIR AIRSPEED WOULD CONTINUE TO EXACERBATE THE PROB. THE DSCNT TO A LOWER ALT WAS THEIR BEST AND PROBABLY ONLY OPTION. HE STATED HIS NORMAL EXPECTATION OF PERFORMANCE AT AN ALT 2000 FT LOWER THAN MAXIMUM FOR THEIR STATED WT WOULD HAVE ALLOWED NORMAL ACCELERATION FROM THEIR DEGRADED STATE. SUCH PERFORMANCE WAS NOT AVAILABLE. HE FELT THAT HAD THEY ATTEMPTED TO CLB TO FL350 (A REASONABLE EXPECTATION BY THE BOOK NUMBERS) THE PERFORMANCE DECREMENT WOULD HAVE BEEN UNSUSTAINABLE. HE SAID THE AIRPLANE JUST SEEMED TOO HEAVY TO PERFORM AS ADVERTISED.