Date: 201503
Local time of day: 1201-1800
Location: ASE.Airport, CO
Flight conditions: VMC
Light: Daylight
Aircraft: Citation X (C750)
Operator: Fractional
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E ASE
Reporter organization: Fractional
Reporter function: Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Confusion
Second reporter function: Pilot Not Flying; Captain
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: During our initial descent into ASE we were given several speed changes and delay vectors. Our first delay was about 15 miles northeast of the field and was vector that lead to a left 360 degree turn to a heading of 320 at 16;000 ft. This was followed by a right turn to a heading of 360 and a speed change to 210 knots; then 170 knots; then 10 knots less and then a question of what's your speed. We said 170 and slowing to 160. Then we got a heading of 270 degrees that ended in a vector through the final approach course. At this time we were cleared direct JARGU which is the first fix inside DBL (Red Table VOR) when we received the clearance we were 6 NM outside of JARGU. The approach controller asked us if we have the field insight; we replied yes and was instructed 'cleared direct the field for the visual approach' at this time the altitude selector was already set to 13;400 feet. At that time we configured the airplane for landing and left 16;000 feet for 13;400 ft. Thinking we heard 'cleared for the visual approach' as we descended through 15;300 feet the controller queried our altitude and we read back 15;200 ft. The approach controller advised us that we were supposed to maintain 16;000 feet but that we were now in his airspace and were now cleared for the visual approach.  The rest of the approach continued without incident. After clearing the runway; ground control asked us to call the tower cab regarding a pilot deviation.I believe we maintained situational awareness throughout the entire approach into ASE. ATC was operating at maximum capacity at the time of flight. Several delay vectors and assigned speeds were given to us to accommodate their high traffic volume. The approach controller left us high inside red table VOR and it was getting close to not being able to clear us for the approach. Everything was pointing to a visual approach as it was the controller's last option. The phraseology of 'cleared direct to the field for approach' and the need to make our upcoming crossing altitude lead to hear/ interpret the we were cleared for the visual.It is our responsibility as a crew to verify any instructions issued by ATC. This situation has allowed me to re-evaluate my CRM and challenge ATC if anything seems out of the ordinary in regards to instructions.


Second narrative: The Approach Controller seemed to be operating at max capacity and we probably received 20 radio calls within a 10 minute period. A lot of the calls were back to back. We as a crew should have clarified what the controller actually wanted but the flow and sequence of the situation had everything pointing to a cleared for the visual approach clearance. A vector through final; then proceed to JARGU; do you have the field; yes; then 'cleared to the airport for the visual approach'?. It was our mistake and we should have listened closer and verified what the controller actually meant however a clearance that said 'proceed direct to the field' and expect the visual would have been less confusing. I think we heard the word cleared and just assumed the rest.


Synopsis: CE-750 flight crew experiences many heading assignments and speed changes arrival into ASE. Eventually; with the field in sight; the crew is 'cleared direct the field for the visual approach'. The words 'cleared for the approach' were not issued but the crew assumed they were and begins descent. The error is pointed out and the visual approach clearance is issued.