B767-300 and 300 ER
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Communication Breakdown; Confusion
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Environment - Non Weather Related
I was the Pilot Flying on aircraft; on a flight from KIAD to ZZZZ. [Copilot] was PIC; and acting as Pilot Monitoring. On climb out from KIAD; we were flying the Jacoby 3 departure; Swann transition. We were given clearance to climb to FL210 and to contact the next Sector Controller. We did so and were given the instruction to 'turn twenty degrees right for traffic'. After complying with that instruction we were level at FL210 heading 120 at approximately 300 KIAS. Our next instruction from Washington Center was to 'maintain two seven zero'. I understood this to mean to continue our climb to FL270 so I set 27;000 [feet] in the MCP and selected VNAV to initiate the climb. [Pilot Monitoring] responded by stating 'climb and maintain FL270'. We were then given an instruction to go direct BROSS intersection. I selected BROSS on the FMS and after concurrence from [Pilot Monitoring]; reenabled LNAV. As we were climbing through approximately 23500; Washington Center asked us what our assigned altitude was. [Pilot Monitoring] replied FL270 and the controller said 'no; that was an assigned speed; two seven zero knots' and gave us a new assigned altitude of FL250. I immediately enabled the speed window and slowed to 270 and reset the altitude window to 25;000 [feet]. We were then handed off to the next controller and the rest of the flight was conducted normally. Weather conditions were VFR with excellent visibility and at no time did we encounter any traffic either visually or on TCAS. I believe the cause of the event was nonstandard terminology on ATCs part and conformation bias on my part. I was expecting a climb instruction so that is what I heard.
B767 First Officer reported an altitude excursion and a speed deviation; both likely due to non-standard radio phraseology and failure to clarify ambiguity.