MD80 FLT CREW FLYING IN PRECIPITATION AT 15000 FT RPTS WIDE FLUCTUATIONS IN FO'S AIRSPEED. FO'S PITOT HEAT CB IS FOUND TO BE POPPED.

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PASSING THROUGH 15000 FT; FO'S AIRSPEED INDICATION BECAME UNRELIABLE; FLUCTUATING FROM 300 TO 140 KTS (STANDBY AIRSPEED ALSO BECAME ERRATIC; BUT NOT TO THE EXTENT OF THE FO'S). WE WERE IN MODERATE PRECIP; MODERATE TURBULENCE; AND OAT WAS GREATER THAN 6 DEGS. PITOT HEAT SELECTOR WAS IN THE CPT POSITION. AUTOPLT AND THE AUTOTHROTTLE DISCONNECTED. NO LIGHTS ON THE ANNUNCIATOR PANEL... CHECKED AMMETER FOR PITOT HEAT... CAPT'S WAS NORMAL; FO AND AUX READ ZERO AMPS. RAN THE QRH PROC FOR AIRSPEED/MACH INDICATIONS UNRELIABLE. CONFIRMED THE AMMETER WAS ZERO FOR FO AND AUX PITOT HEAT. CHKLIST THEN DIRECTED US TO THE 'APPROPRIATE ICE ABNORMAL PROC.' TURNED TO THE ICE SECTION IN THE QRH AND NO PROC SEEMED APPROPRIATE FOR OUR CONDITIONS... SO WE RAN THE PITOT/STALL HEATER OFF LIGHT. WE FOUND FO'S PITOT HEAT CB OUT...THE AUX CB WAS IN. WE RESET IT; AMMETER SHOWED CURRENT TO BOTH THE FO AND AUX PITOT AND AIRSPEED INDICATION RETURNED TO NORMAL. FLT COMMAND WITHOUT EVENT. POINTS TO CONSIDER: 1) WE NEVER GOT THE PITOT/STALL HEATER OFF LIGHT ON THE ANNUNCIATOR PANEL; EVEN THOUGH HEATING POWER WAS LOST TO FO'S AND THE AUX PITOT TUBE. 2) THE FO PITOT CB RESTORED POWER TO BOTH THE FO AND AUX PITOT HEATING ELEMENTS (EVEN THOUGH THE AUX PITOT CB WAS IN). 3) WHEN LOOKING FOR AN UNRELIABLE AIRSPEED PROC IN THE QRH (FLT INSTRUMENTS TAB); THERE ARE 2 CHKLISTS TO CHOOSE FROM... THIS AMBIGUITY COULD CAUSE A FLT CREW TO RUN THE WRONG ABNORMAL CHKLISTS. 4) THE AIRSPEED/MACH INDICATIONS UNRELIABLE CHKLISTS REFERS YOU TO RUN THE 'APPROPRIATE ICE ABNORMAL PROC.' WHEN TURNING TO THIS SECTION IN THE QRH; NO PROC SEEMED APPROPRIATE TO OUR SITUATION; SO WE ELECTED TO RUN THE PITOT/STALL HEATER OFF CHKLIST EVEN THOUGH THIS LIGHT WAS NOT ILLUMINATED.