ALT EXCURSION WHEN FO ANTICIPATED CLB TO HIGHER.

Narrative

DURING CRUISE AT FL350 AND SLIGHTLY W OF THE RTE SBY J209 ORF J174 DIW WHILE BEING CTLED BY WASHINGTON CENTER AND DEVIATING OFF AIRWAYS FOR HVY TSTMS; A CRUISE CLB TO FL370 WAS REQUESTED BY MY COPLT TO AVOID WX. PER CENTER; THIS REQUEST WAS DENIED DUE TO TFC. MY COPLT AND I DISCUSSED A CLB TO FL390 (IN THE COCKPIT) BUT DISMISSED IT DUE TO LACK OF ACFT PERFORMANCE. DURING THIS PERIOD THE ALT HOLD MODE OF THE FLT DIRECTOR WAS ENGAGED AND COUPLED TO THE AUTOPLT WHICH WAS HOLDING 35000 FT (FL350). HOWEVER; MY COPLT SELECTED 39000 FT ON THE ALT SELECTOR (ASEL) ANTICIPATING A CLB FROM 35000 -- WITHOUT MY KNOWLEDGE OR DISCUSSION. THIS ASEL UNIT IN THIS ACFT IS LOCATED ON THE COPLT'S INST PANEL ABOVE THE ATTITUDE; OUT OF THE PLT'S NORMAL FIELD OF VIEW. THE ASEL SELECTED ALT DOES NOT READ OUT (NOT DISPLAYED) ANYWHERE ELSE IN THE COCKPIT. AFTER THIS CLB DISCUSSION WITH WASHINGTON CENTER AND THE ACFT WAS ON AUTOPLT HOLDING FL350; I (THE PLT) LEFT THE COCKPIT TO ASSIST A PAX FOR APPROX 8 MINS. WITHIN A COUPLE (2) OF MINS AFTER RETURNING TO THE COCKPIT; I NOTICED THAT THE ACFT WAS CLBING THROUGH APPROX 36200 FT AND IS 'PSA' (PRESELECTED ALT) MODE. I (THE PLT) RAPIDLY DSNDED THE ACFT BACK TO 35000 FT (FL350). DURING THE DSCNT THE CENTER ALSO NOTICED THAT OUR ALT WAS INCORRECT SINCE THEY ASKED WHAT OUR ALT WAS. SINCE WE WERE DSNDING VERY RAPIDLY; WE REPLIED THAT OUR ALT WAS 35200 FT AND LEVELED OFF QUICKLY AT 35000 FT. WE HAD NO FURTHER DISCUSSION WITH CENTER REGARDING THIS ALT EXCURSION (DEV) BUT WITHIN APPROX 3 TO 5 MINS; WE DID HEAR AN ACFT CHK IN ON FREQ AT FL370 BUT NEVER SAW HIM (ACFT); EVEN THOUGH WE WERE VFR AND STEADY FL350. DURING THE REMAINDER OF THE FLT; WE SAW THE ASEL MALFUNCTION AT LEAST TWICE BUT WITHOUT ALT DEV SINCE THE ASEL WAS THEN ALWAYS SET AT THE ASSIGNED ALT. THE ASEL CTLR WAS CAUSING THE FLT DIRECTORS TO AUTOMATICALLY TRIP FROM ALT HOLD MODE INTO PSA MODE WITHOUT PLT ACTION; EXPLAINING WHY THE ACFT HAD PREVIOUSLY DISENGAGED ALT AT 35000 AND HAD CLBED TO 36200 FT IN PSA MODE AIMING FOR THE ANTICIPATED ALT OF 39000 THAT THE COPLT HAD INCORRECTLY SET IN THE ALT SELECTOR. THIS WAS A COPLT PROBLEM AND AN ACFT SYS PROBLEM COMBINED. THIS OCCURRENCE WILL BE PREVENTED IN THE FUTURE BY: 1) REPAIR AND ANALYSIS OF THE FAULTY ASEL CTLR. 2) DISCIPLINARY ACTION FOR THE COPLT TO LEAVE ASEL SELECTED TO ASSIGNED ALT; NOT TARGET ON DESIRED ALT AND TO IMPROVE COCKPIT AWARENESS. 3) THE COPLT WILL ANNOUNCE ALT WHEN CHANGING ASEL ALT ON THIS ACFT. 4) DESIGNING EFIS THAT DISPLAY ASEL ALT ON BOTH PLT AND COPLT INSTS.