Gulfstream V / G500 / G550
Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 20; Flight Crew Total 14800; Flight Crew Type 400
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew Overcame Equipment Problem
Aircraft
This aircraft had just been accepted from Gulfstream. The crew had done a post acceptance flight and everything was working properly. When arriving [at destination] we were on vectors for left downwind to Runway XXL. We had the autopilot on and level at 4000 feet. The autopilot began to lower the nose inadvertently and we lost 300 feet prior to regaining control and getting back up to 4000 feet.
[Report narrative contained no additional information.]
G-V flight crew reported the autopilot initiated an unwanted 300 foot descent from the selected altitude.