Date: 201306
Local time of day: 1201-1800
Location: ZDC.ARTCC, VA
Flight conditions: Mixed
Light: Daylight
Aircraft: Falcon 900
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZDC
ATC / advisory: Center ZDC
Reporter organization: Corporate
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Human factors: Confusion; Communication Breakdown
Second reporter function: Captain; Pilot Not Flying
Anomaly: ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: Level cruise at FL390; eastbound on J42; east of BKW. Washington Center issued the following clearance to us. 'Descend now to FL350; maintain a descent rate of 1500 FPM or greater and then cross DENNY at FL330.' We read back the clearance exactly as it was issued; ('descend now to FL350 at 1500 FPM or greater and cross DENNY at FL330'); and immediately began our descent at 1500 FPM. The Controller did not reply and proceeded to communicate with other aircraft on the frequency; as expected. While in the descent; we calculated that a 1500 FPM descent would allow us to level at DENNY exactly at FL330 and planned to maintain that descent rate all the way down to FL330. After approximately 60-90 seconds; while descending through approximately FL370 the Controller said to us; 'Descend to FL350 and maintain a descent rate of 1500 FPM or greater.' We immediately read back 'we are descending to FL350 with a descent rate of 1500 FPM and will cross DENNY at FL330.' The Controller again did not reply and went on to communicate with other aircraft. We figured she issued the clearance again because either 1) she did not copy our original read back or 2) she was verifying our descent rate was at least 1500 FPM. While leveling at FL330 just west of DENNY; the Controller came on and told us we were only cleared to FL350. We replied that we were cleared to descend to FL350 immediately; at 1500 FPM or greater; and cross DENNY at FL330 and that we read that back twice. We then confirmed that we were level at FL330 and asked her what she wanted us to do now. She instructed us to maintain FL330 and said there was a misunderstanding. We maintained FL330 and continued the flight without incident.In my opinion; the second clearance was not clear. It never occurred to us that she was attempting to get us to level at FL350. (It appears to be much clearer when only our transmissions are highlighted; as in this report. However; in real time on a busy frequency; we did not comprehend the Controllers' intentions) I feel the clearance would have been easily understood if the Controller added only a few extra words; such as 1) 'descend to FL350 ONLY' 2) 'descend to FL350 and delete the crossing restriction at DENNY' or another variation of this.


Second narrative: No additional information was provided in the secondary narrative.


Synopsis: Imprecise clearance phraseology and the failure to confirm understanding of the clearance resulted in the flight crew of a DA-90 descending to an altitude not intended by the Controller.