ACFT EQUIP PROB. FCU ALT AUTO-CHANGED WHEN PUSHING ALT KNOB. ALTDEV.

Narrative

I WAS THE CAPT BUT NOT FLYING AT THE TIME. CTR CLRED US TO FL220 BY 10 MI N OF PHN VOR. THE FO (PF) SET FL220 IN THE ALT WINDOW OF THE FCU BUT DID NOT IMMEDIATELY PULL OR PUSH THE KNOB AS IT WAS STILL EARLY FOR STARTING DOWN. I THEN WENT OFF FREQ TO GET ATIS AND TALK TO OUR OPS AT RDU. WHILE DOING THIS; THE FO PUSHED THE ALT KNOB TO BEGIN A 'MANAGED' DSCNT. AFTER A MOMENT OR 2; I NOTICED ON THE NAV DISPLAY WE WEREN'T GOING TO MAKE OUR FL220 AT 10 MI N OF HPW CONSTRAINT. THEN I NOTED THAT 21000 FT WAS NOW SHOWING AS THE ARMED ALT ON MY PFD. I QUICKLY GLANCED AT THE FCU ALT WINDOW; IT SAID 21000 FT ALSO. HAVING BEEN OFF FREQ AND STILL NEEDING TO BE OFF FREQ. I ASSUMED THAT WE'D GOTTEN A NEW CLRNC TO FL210 AND THEREFORE THE OLD CONSTRAINT AT 10 MI N OF HPW WAS INVALID AND THAT 21000 FT WAS WHERE WE SHOULD BE GOING. HOWEVER; WHEN I RETURNED TO THE ATC FREQ; MY FO SAID THAT THERE HAD BEEN CONFUSION ABOUT WHERE WE WERE CLRED TO; AND THAT SHE HAD READ BACK FL210 AND CTR HAD SAID WE HAD ONLY BEEN CLRED TO FL220 AND THEN HAD CLRED US TO FL210 AFTER THE FACT; SO NOW WE SHOULD STAY AT FL210. WE COULDN'T FIGURE OUT HOW IT HAPPENED -- UNTIL THE NEXT DAY. WE KEPT THE SAME PLANE AFTER OUR OVERNIGHT. TWICE; ON THE SECOND DAY WE SAW THE ALT WINDOW ON THE FCU CHANGE BY 1000 FT WHEN WE PUSHED IT TO 'MANAGE' DSCNT TO A LOWER ALT. SO WE SET THE ALT SELECTOR TO 100 FT INCREMENTS (TO AVOID 1000 FT CHANGES HOPEFULLY) AND WROTE IT UP IN THE MAINT LOG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: AS FO; PF; BEGAN DSCNT IN ORDER TO MAKE THE ATC ASSIGNED ALT CONSTRAINT OF FL220; PUSHING THE FCU ALT KNOB CAUSED THE ALT SETTING TO CHANGE TO FL210. AT THE TIME; PUSHING THE MANAGED DSCNT KNOB; THE FO DIDN'T NOTICE THE ALT CHANGE. THE CAPT WAS OBTAINING ATIS AND COMPANY COM; SO WAS OUT OF THE LOOP. WHEN THE CAPT'S ATTN WAS BACK IN THE COCKPIT; HE SAW THAT THEY WERE NOT GOING TO MAKE THE ALT CONSTRAINT AND NOTING THAT THE ALT NOW READ FL210; ASSUMED THAT A REVISED CLRNC HAD BEEN RECEIVED WHILE HE WAS 'OFF THE AIR;' AND; THAT THE CONSTRAINT WAS NO LONGER VALID. IN THE MEANTIME; THE FO HAD HEARD A COMPANY ACFT WITH A SIMILAR CALL SIGN GIVEN A DSCNT CLRNC TO FL210 AND THE FO SAID; 'I DON'T REMEMBER IF I ROGERED THEIR CLRNC BY MISTAKE OR NOT; BUT ASSUMED THE FL210 SET IN THE FCU WAS CORRECT.' THE FLC CONTINUED THE SEQUENCE IN THE SAME ACFT AND THEN NOTED THAT THE SAME THING HAPPENED WHEN SETTING THE FCU ALT AND IT CHANGING WHEN PUSHING THE KNOB. IT WAS WRITTEN UP; A CAPT RPT WAS SENT TO COMPANY; BUT RPTR HAS HAD NO FURTHER FEEDBACK FROM MAINT OR COMPANY MGMNT.