A319 captain reported an excursion from assigned altitude due to a failed/malfunctioning FMGC2.

Attributes
ACN	1678058
Time
Date	201908
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	A319
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 199.23 Flight Crew Total 14535.50 Flight Crew Type 5213.35
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

Cruising at [flight level] 360 with AP2 engaged (this was the first officer's leg and he was returning to his seat from a lav break). FMGC2 screen blanked and the ap commanded an aggressive descent as if it was attempting to catch a descent path. I disconnected the ap/at and leveled off at 34;800 [feet]; the first officer notified ATC of our altitude [and] we were re-cleared to climb to [flight level] 360. We reengaged AP1 and at and climbed back to [flight level] 360. FMGC2 screen blanked out numerous times after this; we considered it unreliable and continued on AP1. After determining that we were still legal for rvsm we confirmed with ATC that we could continue at [flight level] 360. We talked with the flight attendants and there were no injuries or further concerns in the cabin. Dispatch queried us [because] the aircraft taps reported severe turbulence (the ride was smooth). Dispatch and [maintenance control] were notified and then briefed verbally once on the ground.

Original NASA ASRS Text

Title: A319 Captain reported an excursion from assigned altitude due to a failed/malfunctioning FMGC2.

Narrative: Cruising at [Flight Level] 360 with AP2 engaged (this was the First Officer's leg and he was returning to his seat from a lav break). FMGC2 screen blanked and the AP commanded an aggressive descent as if it was attempting to catch a descent path. I disconnected the AP/AT and leveled off at 34;800 [feet]; the First Officer notified ATC of our altitude [and] we were re-cleared to climb to [Flight Level] 360. We reengaged AP1 and AT and climbed back to [Flight Level] 360. FMGC2 screen blanked out numerous times after this; we considered it unreliable and continued on AP1. After determining that we were still legal for RVSM we confirmed with ATC that we could continue at [Flight Level] 360. We talked with the flight attendants and there were no injuries or further concerns in the cabin. Dispatch queried us [because] the aircraft TAPS reported severe turbulence (the ride was smooth). Dispatch and [Maintenance Control] were notified and then briefed verbally once on the ground.

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.