DURING ILS APCH; SHORT FINAL; IMC AND NIGHT; THE AUTOPLT DISCONNECT WARNING LIGHT CAME ONE. RPTR THOUGHT IT HAD DISCONNECTED; BUT WHEN TRYING TO CTL THE ACFT MANUALLY; WAS UNABLE TO GAIN AILERON CTL BEFORE HE DISCONNECTED MANUALLY WITH THE YOKE SWITCH AND IN THE MEANTIME HAD LOST 200 FT ON THE FINAL DSCNT.

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ON APCH TO ORD RWY 27L AT 600 FT AGL; AUTOPLT DISCONNECT LIGHT (NO WAILER) CAME ON. THOUGHT AUTOPLT HAD DISCONNECTED. WAS UNABLE TO CTL AILERONS. FELT LIKE THEY WERE IN MANUAL REVERSION. THOUGHT I HAD HYD FAILURE BUT NO INDICATION OF SUCH. THEN SAW THAT CTL WHEEL STEERING WAS ANNUNCIATED ON AFDS. THEN DISCONNECTED AUTOPLT WITH YOKE SWITCH AND AIRPLANE THEN FLEW NORMALLY. IT TOOK US ABOUT 200 FT OF DSCNT TO FIGURE OUT AUTOPLT HAD NOT DISCONNECTED COMPLETELY. DURING THAT 200 FT OF DSCNT ON GS THE ACFT WAS NOT CTLABLE IN THE ROLL MODE. IT WAS CERTAINLY NOT ACTING LIKE IT WAS IN CTL WHEEL STEERING WHICH WAS ANNUNCIATED. IN THE 9 YRS ON THIS AIRPLANE; I HAVE NEVER SEEN OR HEARD OF THIS; NOR HAVE I RECEIVED ANY TRAINING ON THIS TYPE OF MALFUNCTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT HAS SUBMITTED A RPT TO HIS COMPANY; CONTACTED THEIR CENTRAL AIR SAFETY CHAIRMAN AND HAS NOT YET RECEIVED A RESPONSE CONCERNING THE FAULTY AUTOPLT. THE AUTOPLT PARTIALLY DISCONNECTED; BUT REMAINED IN CTL WHEEL STEERING; WHICH WAS NOT IMMEDIATELY SEEN DUE TO PROX OF SCAN AND ON FINAL APCH IN IMC AT NIGHT. RPTR CAPT THOUGHT HE HAD LOST AILERON CTL OF THE ACFT; BUT WHEN DISCONNECTING THE AUTOPLT WITH THE YOKE BUTTON; HE REGAINED FULL CTL. RPTR SAYS THAT IT WAS VERY INSIDIOUS BECAUSE AN AUTOPLT LIGHT CAME ON INDICATING IT WAS DISCONNECTED; WHEN IN FACT; IT WASN'T. ALSO; NORMALLY WHEN THE AUTOPLT AUTO-DISCONNECTS; THE FLC CAN HEAR THE 'SNAP' AS THE WAFER SWITCH MOVES TO OFF. HE WAS PARTICULARLY DISTRESSED BECAUSE OF THE RELATIONSHIP TO APPARENT FLT CTL PROBS RECENTLY ON THE B737. HE HAS CONTACTED BOEING THROUGH SOME FRIENDS AND IS PARTICULARLY UPSET THAT HIS COMPANY HAS NOT FOLLOWED THROUGH WITH INVESTIGATING THE PROB AND CONTACTING HIM WITH WHAT WAS FOUND ON THE ACFT. MAINT CHANGED THE MODE CTL PANEL AND RENDERED THE AUTOPLT INOP AFTER THEY LANDED AT ORD AND THE ACFT CONTINUED ON THE NEXT FLT.