B737-800 captain described an altitude deviation involving an uncommanded climb while capturing the ...

Narrative:

Captain (ca) was pilot monitoring (pm). First officer (first officer) 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 first officer selected 'approach'. The aircraft pitched up; increased power and began to climb. The first officer was confused (and rightfully so) by this. I explained that it sometimes does this...'we'll talk about it later.' the first officer 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1239481
Time
Date	201502
Local Time Of Day	1801-2400
Place
Locale Reference	ORD.Airport
State Reference	IL
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance

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.