PRESSURIZATION CTL ON ACR MLG LOST AS ACFT REACHED CRUISE ALT OF FL350. ARTCC AMENDED ALT ASSIGNMENT TO 10000' ON REQUEST. OXYGEN MASKS DROPPED BEFORE PRESSURIZATION CTL REGAINED. ACFT RETURNED TO DEP ARPT.

Narrative

JUST AFTER LEVEL OFF AT CRS; CABIN BEGAN TO CLB AT APPROX 3000 FPM. AUTO FAIL AND STAND BY LIGHTS CAME ON. PRESSURE CTLR SET TO MANUAL. AC AND DC AND GND WERE SELECTED AND USED. VALVE INDICATOR SHOWED CLOSED ENTIRE TIME. PACKS SWITCHED TO HIGH-REDUCED CLB TO 1500'. DURING ABOVE CALLED ATC AND CLRD TO FL240; THEN 10000'; SO IT WAS NOT NECESSARY TO DECLARE AN EMER. EMER DES CHKLIST WAS ACCOMPLISHED. AT APPROX 22000' CABIN PRESSURE BEGAN TO RESPOND AND DES. MAX CABIN ALT WAS 15000-16000'. OXYGEN MASKS DEPLOYED AT 14000'. DISPATCH CONTACTED AND ACFT RETURNED TO ORD. BOTH PLTS SUSPECT MAX RELIEF VALVE OPENED AND DID NOT RESET. WITH PWR OF MLG THIS COULD HAPPEN MORE OFTEN WITH MORE USE OF MAX RELIEF VALVES. NO PROB WITH ANY OF THE ABOVE; OTHER THAN THE OXYGEN MASKS DEPLOYING SCARED THE PAX; AS WOULD BE EXPECTED. SUPPLEMENTAL INFORMATION FROM ACN 79963: WHEN IT WAS DETERMINED THE CABIN WAS NOT RESPONDING APPROPRIATELY AN EMER DES WAS ACCOMPLISHED. AT APPROX 22000' THE CABIN BEGAN TO DES AND WAS CTLABLE THEREAFTER. THIS WAS A LIGHT PLANE ON A COLD DAY. CRS ALT WAS REACHED WITH DIFFERENTIAL PRESSURE INDICATOR IN THE YELLOW BAND.