BAe 125 Series 800
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (mon) 5; Air Traffic Control Time Certified In Pos 1 (yrs) 0
Communication Breakdown
Airspace Violation All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert
Aircraft
I was working departure when Aircraft X departed ZZZ. Soon after initial contact; once Aircraft X was clear of traffic landing ZZZ1; aircraft was instructed to climb to 2;000 [ft]. Instead of climbing; the aircraft reached about 1;000 ft. before descending as low as 600 ft. The MVA in the area is 1;800 ft. Aircraft was told to immediately climb to 2000 ft several times but stayed between 1000 ft. and 600 ft. for about 10 miles. Aircraft was informed the MVA in the area was 1;800 ft. Aircraft also did not seem to be able to maintain course. Eventually the aircraft climbed to 2;000 ft. and it seemed everything was fine with the aircraft afterwards. There was also difficulty understanding the pilot due to the pilot's accent during this time. [I have no recommendations because] I have not seen a similar event happen in the past.
TRACON Controller reported H25B failed to respond to radio communication while descending and flying erratic course; resulted in entering a higher MVA.