Globemaster (C-17)
Air Traffic Control Time Certified In Pos 1 (yrs) 8
Confusion; Situational Awareness; Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Procedure; Airspace Structure
I was training a Controller on the sector. He gave a descent to an aircraft from 4;000 feet down to 3;000 feet. When the pilot read it back multiple aircraft keyed up at the same time. One of them was the correct pilot; another was another aircraft checking in; and the third was a C17 that thought the transmission was for him to descend to 2;000 feet. The Controller gave the instruction very clearly; and the descent was to a different aircraft to a different altitude. When the pilot called later he said that he always gets a decent to 2;000 feet there so he thought that's what the call was. When [the C17] descended to 2;000 feet that dropped him into a 2;000 foot (Minimum Vectoring Altitude). The altitude was recognized but not before penetrating a 2;500 foot MVA. A climb was issued and the pilot said he could maintain his own terrain and obstruction clearance. But that was after the MVA was already violated. It was obvious when the altitude read back was happening that more than one aircraft were keying up; it wasn't obvious that 3 were. Let alone an aircraft with a very different call sign descending to a very different altitude. This was a pretty clear case of expectation bias. The pilot was made aware of the situation. He said it was a very clear VFR day so he saw all the terrain just fine; and he just thought that is where aircraft got their descent to 2;000 feet.
A Controller conducting on the job training reported multiple aircraft responded to the trainee issuing a descent clearance and one of the aircraft descended below the Minimum Vectoring Altitude.