Date: 201907
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: UAV - Unpiloted Aerial Vehicle
Operator: Government
Flight plan: IFR
Mission: Tactical
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Government
Reporter function: Pilot Flying; Single Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Instrument
Human factors: Communication Breakdown; Confusion; Situational Awareness; Time Pressure
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Aircraft; Manuals
Primary problem: Human Factors

Narrative: We were cleared direct to ZZZ at 26;000 feet on with Center. We were given a 20 degree right turn for traffic which we complied and then were told by Center Direct to ZZZ a few minutes later. Shortly after; we were cleared by center down to 22;000 feet. I had just read back the clearance and was about to start a descent when Center told us to maintain a 1500 FPM rate of descent down to 22;000 feet and then descend to 8;000 feet. We are unable to descend below 18;000 feet in that area due to our COA (Certificate of Authorization) with ATC. I believe the Controller was new to the area or unfamiliar with our UAS operations. I told the Controller that we were unable to comply with a descent below 18;000 feet. The Controller came back and told us we needed to maintain a 2500 FPM descent down to 22;000 feet and then pilot discretion to 8;000 feet. I did not comply with the clearance or start a descent as I told the Controller we were unable to descend below 18;000 feet and that we were a UAS. We are not able to descend below 18;000 feet until reaching the TFR which was about 100 miles away. Center said we were coming in broken and unreadable on the radio and I do not know if they heard us say unable. I did not want to start a descent until I had clarification from ATC about the clearance. ATC communicated to the other aircraft that was going to be a conflict that they needed to level off but the other aircraft was not able to understand ATC. I could tell the urgency in the Controller's voice when he told us we needed to start a 2500 FPM descent. I do not remember him ever saying immediately at anytime but I wanted to avoid a possible traffic conflict; so I manually took control and started an aggressive descent. The MQ9 is capable of a fast descent while under manual control but it increases workload dramatically and takes a little time to establish a fast descent. I was passing 25;500 feet when the Controller told us to maintain 26;000 feet if we had not started our descent. I told him we were descending and he asked us our descent rate. I was just passing 1500 FPM and reported it and he told us we were in violation of the clearance. I did not respond at this point due to radio delay and ATC not understanding us and focused on flying the aircraft. Once I was established in a steady descent; I had my [Second Officer] switch us to comm 2 and told ATC that we were descending to FL180 but unable to descend lower. ATC told us to maintain FL180 and proceed direct to ZZZ. I turned the aircraft towards ZZZ but due to the previous vectoring I did not have a new op mission set up yet and did not have time as I was still hand flying the MQ9 thought the level off at FL180. ATC told us that we were 10 degrees left of course and I turned right for a more direct route. After reestablishing all hold modes and changing the Emergency mission I got a mission set up to take us direct back to ZZZ. About 40 minutes later Center again told us to descend below 18;000 feet and again I had to again tell them unable. During the incident; myself and the [Second Officer] were extremely confused by Centers instructions as the Controller changed the clearance four if not five times every time they gave it. I did not want to start a descent until I had clarification on the clearance. I never once heard the Controller use the word immediately in the descent clearance and if he would have I would have complied immediately. As UAS operations become more numerous in the US; Controllers need to understand the limitations of UAS. There are delays with radio reception and transmission and UAS are not able to comply as fast as manned aircraft. In this situation; ATC would have been better off telling us to immediately descend if it was that urgent and I should have had my [Second Officer] switch to Com 2 as soon as we realized Center was having trouble understanding and tell ATC 'unable' loud and clear when they give us a clearance we cannot comply with instead wasting time trying to clarify.




Synopsis: A UAS pilot reported ATC issued confusing control instructions including a descent clearance below 18;000 feet even though the aircraft was not authorized to fly below 18;000 feet.