B737-800 CAPTAIN REPORTS MOUNTAIN WAVE ENCOUNTER AT FL400 SOUTH OF RNO WITH 29 KNOT AIRSPEED LOSS.

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S OF RNO VOR WE THOUGHT WE HEARD AN ACFT ON OUR FREQ SAY SOMETHING ABOUT A 20-30 KT LOSS IN AIRSPD. I ASKED THE FO (PNF) TO QUERY ATC ABOUT THE RIDE AHEAD AND THE RPT WE HEARD. ATC SAID NEXT FREQ WILL GIVE YOU THE RIDE RPT; CONTACT THEM. WHEN WE CHKED IN; WE QUERIED THE CTLR ABOUT THE RIDE AHEAD AND HE SAID NOTHING MORE THAN LIGHT CHOP. WE WERE ABOUT TO ASK ABOUT WHAT WE THOUGHT WAS A LOSS IN AIRSPD WHEN WE DROPPED ALMOST 29 KTS OF AIRSPD. WE WERE WITHIN 1-2 KTS OF STICK SHAKER AT THAT TIME. I TOLD THE FO TO EITHER GET US LOWER 'NOW' OR WE WILL BE DECLARING AN EMER AND COMING DOWN AS WE COULD NOT HOLD AIRSPD MUCH LONGER. THE ACFT WOULD NOT ACCELERATE AND IT WAS IMMINENT WE COULD NOT HOLD ALT MUCH LONGER WITHOUT GOING INTO STICK SHAKER ACCORDING TO ACFT INSTS AND THE HUD WHICH WAS DOWN. THE FO ASKED FOR LOWER AND THE CTLR SAID STAND BY! I LOOKED AT THE FO; SAID WE ARE GOING DOWN; SQUAWK 7700; AND ABOUT THAT TIME ATC SAID CLRED FL380. I ASKED THE CTLR WHAT HIS IDEA OF LIGHT CHOP WAS AND DID THAT INCLUDE THE 30 KT LOSS; AS I WOULD NOT HAVE BEEN AT FL400 BECAUSE 30 KTS WAS OUR BOTTOM BUFFER AND 23 KTS WAS THE UPPER BUFFER. NO COMMENT FROM ATC. HE DID SAY HE THOUGHT WE HAD THE 20-30 KT LOSS IN AIRSPD RPT FROM THE PREVIOUS CTLR. WHEN YOU GET MOUNTAIN WAVE OVER 10 KTS AND 50-75 FT DEV IN AIRSPD AND ALT; IT SHOULD GO OUT TO EVERY ACFT IN THAT SECTOR AND THOSE COMING INTO THE SECTOR. IT WAS APPARENT THAT THE CTLR ASSUMED SOMETHING THAT HE SHOULD NEVER HAVE ASSUMED AND THE PREVIOUS CTLR SHOULD HAVE REITERATED THE RIDE BEFORE WE CHANGED FREQS. THE 30 KT BUFFER WAS NOT SUFFICIENT EVEN IN A POWERFUL B737-800 TO HOLD ALT MUCH LONGER THAN WE DID. COULD HAVE BEEN WORSE; BY SOME ATC ASSUMPTIONS.