EMBRAER RJ SUFFERS ALT DEV ON APCH WHEN THE AUTOPLT ATTEMPTS TO CAPTURE A FALSE GLIDE SLOPE.

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DURING APPROACH TO RWY 28 AT ORD; WE WERE CLEARED TO MAINTAIN 5000 FT UNTIL WAVIE AT 180 KTS AND CLEARED FOR THE APPROACH. APPROACH WAS ARMED ON THE FGP; AND IT ARMED AND CAPTURED BOTH LOC AND GLIDE SLOPE. NOT LONG AFTER THIS; APPROX 5 MILES OUTSIDE OF WAVIE; THE AIRCRAFT PITCHED UP AGGRESSIVELY TO 10-12 DEGS NOSE UP. (WE BELIEVE IT WAS A FALSE GLIDE SLOPE INDICATION BECAUSE THE GLIDE SLOPE 'NEEDLE' WHICH WAS DEAD ON AT CAPTURE SUDDENLY SHOWED US WAY BELOW THE GLIDE SLOPE.) THE AIRPLANE RESPONDED AS IT THOUGHT IT SHOULD WITH A HARD PITCH UP TO RECAPTURE; WITH A SUBSEQUENT RAPID LOSS OF AIRSPEED. I IMMEDIATELY DISENGAGED THE AUTOPILOT AND ADDED NECESSARY THRUST TO RETURN US TO 180 KTS; WHILE PITCHING OVER TO RECAPTURE THE GLIDE SLOPE. (AT THE APEX OF THE PITCH UP WE HAD CLIMBED TO ROUGHLY 5800 FT; AT A VERY HIGH VERTICAL SPEED; WITH AIRSPEED RAPIDLY BLEEDING OFF.) THE ALTITUDE WAS RECAPTURED AND RE-INTERCEPTED THE GLIDE SLOPE AND CONTINUED THE APPROACH. THERE WAS NO MORE GLIDE SLOPE FLUCTUATIONS; AND EVEN IF THERE WERE; THE REST OF THE APPROACH WAS HAND FLOWN TO A NORMAL APPROACH AND LANDING. THERE WAS NO ATC COMMUNICATION TO US REGARDING THE INCIDENT.