B737 FLT DEPARTED WITH A MALFUNCTION KNOWN ONLY TO THE FO. THE CAPT BECAME AWARE ONLY WHEN THIS BECAME A MAINT ISSUE DURING FLT.

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THE REASON WHY THE EVENT OCCURRED WAS; IN THE OPINION OF THE FO; NOT IMPORTANT ENOUGH TO BRING TO THE CAPT'S ATTN. HIS LOGIC WAS THAT THERE WAS NOTHING ON THE BOOK THAT DISCUSSED THIS ITEM. THE FO FAILED TO INFORM THE CAPT OF AN INOP ITEM. ON DEP AND CLBOUT; I WAS NOTIFIED BY THE FO THAT WE HAD A BLEED AIR DUCT PRESSURE DIFFERENTIAL INDICATION OF GREATER THAN 10 PSI. AS WE CONTINUED TO CLB; HE ALSO NOTICED THAT THE R RAM AIR DOOR WAS INDICATING OPEN. HE ALSO STATED THAT NORMALLY THIS SHOULD BE CLOSED EXCEPT ON THE GND AND WHEN THE FLAPS WERE DOWN. I HAD EXPLAINED TO HIM THAT THE DOOR WOULD ALSO MODULATE INFLT IF THE SENSOR TEMP IS TOO HIGH AND THIS WAS NORMAL. ONCE WE WERE IN CRUISE AND ENRTE TO THE DEST; THE FO ASKS IF I WOULD LIKE FOR HIM TO WRITE THE ITEM UP. AT THIS TIME I WAS TO BELIEVE THAT HE WAS TALKING ABOUT THE PRESSURE DIFFERENTIAL. I SAID OK; AND HE THEN WROTE THE ITEM UP. AFTER HE WROTE THE ITEM UP; HE INCLUDED THE RAM AIR DOOR AND STATED THAT IT WAS INOP ON THE GND. HE WROTE THAT ON THE GND THE RAM DOOR FULL OPEN LIGHT WAS EXTINGUISHED. HE THEN PRESSED TO TEST THE LIGHT AND FOUND THE LIGHT TO BE OPERATIVE. AFTER READING HIS WRITE-UP THIS WAS THE FIRST TIME I WAS INFORMED ABOUT THE RAM DOOR NOT BEING OPEN ON THE GND. THE FO FAILED TO INFORM THE CAPT OF AN INOP ITEM. I DISCOVERED THE PROB AFTER WE DEPARTED AND WAS INFORMED IN WRITING BY THE FO WHEN HE ENTERED THE DISCREPANCY IN THE LOGBOOK. INFORMED MAINT OF THE PROB. INFORM THE CAPT OR OTHER CREW MEMBER OF ANY NON-OPERATIVE COMPONENTS -- REGARDLESS OF THEIR OPINION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT ALTHOUGH THE RAM AIR DOOR WAS NOT IN THE CORRECT POSITION DURING THE FO'S PREFLIGHT; THE FO FELT THAT THIS MALFUNCTION WAS INSIGNIFICANT.