Date: 201506
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: HS 125 Series
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Engine Control
Reporter organization: Corporate
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Human factors: Distraction; Workload; Situational Awareness; Troubleshooting
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: FO and Captain were flying 2 passengers to Airport ZZZ.  As we were approaching the ZZZ we were asked by ATC to begin a descent from flight level 390 to flight level 370.  When the Captain began to pull the throttles back to begin the descent;  it became apparent that the throttle controlling the #2 engine was stuck in the open position and would not allow a reduction in power to that engine.  The crew discussed the situation and decided to ask ATC for a lower altitude (in the lower 20;000 foot range) in case that engine had to be shut down and we needed to make a single engine approach.  ATC granted our request and gave us vectors to a more direct approach to the airport.  While in the descent; the throttle lever became active again and a normal approach and landing was made at ZZZ. During our descent we were asked several times by the ATC controllers if we had an emergency situation.  Every time we answered that it was not an emergency and once the throttle became active again on the #2 engine; we relayed that information to ATC and repeated that we were not in an emergency situation and would need no assistance at the airport upon landing.Evidently ATC took it upon themselves to activate an ARFF warning because fire and emergency equipment were at the ready when we landed at ZZZ.   We discussed the situation with the person in charge of the fire department and filled out a report with him.


Second narrative: When cruising at FL390 we (the crew) were asked to begin a descent. It became apparent that the throttle control for the #2 engine would not move from its present position to allow a reduction of power. We asked ATC to allow us to descend to a lower altitude than they originally requested. We were granted a lower altitude and given vectors towards a more direct route to our destination ZZZ. When asked by ATC what the problem was; we told them about the throttle issue. When asked if it was an emergency situation we told them that it was not.During the descent the throttle lever became active and responsive and we then relayed that information to ATC. A normal approach and landing followed at ZZZ. I do not know what caused the situation but our maintenance facility was notified and will inspect the aircraft.


Synopsis: Upon commencing descent the number 2 throttle would not respond to pilot inputs. The crew advised ATC of the situation and requested a lower altitude. However; during the descent; the throttle became responsive again and a normal approach and landing was accomplished.