B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem
Aircraft
Captain (CA) was Pilot Monitoring (PM). First Officer (FO) was (PF) Pilot Flying. She was on first trip on this aircraft since IOE. Aircraft was on autopilot; level at 4;000 feet on intercept heading 300 for the ILS 28C at ORD. Clear beautiful night. We had been cleared for a visual approach; speed 180 knots until SEIKA. We were on the glideslope with the localizer 'alive' when the FO selected 'approach'. The aircraft pitched up; increased power and began to climb. The FO was confused (and rightfully so) by this. I explained that it sometimes does this...'we'll talk about it later.' The FO disengaged the autopilot and descended the aircraft back to glideslope. We continued the approach and made an uneventful landing. We climbed above our altitude by about 350 feet during this anomaly.  Approach control made no call to us about the altitude excursion. This phenomenon has happened to me a handful of times; the first being on my IOE 6 years ago. It seems to happen (but not always) when the glideslope is centered and the aircraft is on an intercept heading to the localizer when selecting 'approach' mode. While most have been a relatively benign 100 to 200 feet climb; this one was the most aggressive I've seen.
B737-800 Captain described an altitude deviation involving an uncommanded climb while capturing the Runway 28C localizer at ORD with the glideslope centered. Reporter stated he has seen this anomaly in the past.