PLT STRAYS OFF ASSIGNED ALT 500 FT.

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ON TKOF CAPT WHO WAS FLYING FELT FLT CTLS WERE TOO STIFF AND NOT RESPONSIVE. HE DECIDED TO RETURN TO O'HARE (ORD). WE LEVELED AT 5000 FT AND 250 KTS AND TOLD DEP CTL. WE WERE SWITCHED TO APCH CTL AND WAS TOLD TO CALL OUR BASE AND MAKE ARRANGEMENTS. THE ACFT WAS APPROX 8-10 MI SE TO E OF ORD VOR. AS I FOUND AND TUNED THE RADIO FREQ I LOOKED UP AT THE INSTS AND NOTICED THAT WE WERE AT 4500 FT. I TOLD THE CAPT TO CLB TO 5000 FT AND HE DID. WE WERE AT 4500 FT FOR ABOUT 2 SECONDS. WE LANDED UNEVENTFULLY AND DID NOT DECLARE AN EMER. IN RETROSPECT WE SHOULD HAVE TO PROTECT US FROM CTL PROBS AND ASSOCIATED ALT LOSS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPT SAID THE ACFT TYPE WAS A 737-300. HE SENT THE RPT IN BECAUSE OF THE 500 FT ALT EXCURSION. HE BEMOANED IN PASSING THE PHENOMENA OF PLT'S RELUCTANCE TO DECLARE AN EMER. IN THIS CASE HE FELT THE CAPT WAS OVER REACTING TO STIFFNESS IN THE CTLS. HE DID NOT FEEL THE CTLS WERE STIFF. BUT; THE DECISION TO RETURN WAS NOT HIS CALL. THE MAINT COULD FIND NOTHING WRONG WITH THE AIRPLANE.