Gulfstream V / G500 / G550
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 100; Flight Crew Total 6800; Flight Crew Type 3500
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance
Aircraft
Cleared 'Descend Via RAZRR2 Arrival; 30L Transition' in KSJC. We were in level flight on LNAV; autopilot engaged. I selected VNAV to descend via the arrival as the descent path was ahead of the aircraft. This is a normal procedure in the G-550. For some reason yet discovered; the plane went into a max performance descent; VFLCH. I corrected the premature descent using vertical speed; staying on autopilot. We lost a little over 2;000 feet during the incident/recovery. I climbed the plane back up to previous altitude; but sequenced the next waypoint with lower altitude restrictions and just complied with that. I actually never made it back up to FL240 as it was no longer a valid part of the arrival.Gulfstream is looking into this and hopefully we can understand why this happened.
G-V Captain reported excursion from cleared altitude when the aircraft went into an uncommanded max descent.