Date: 201604
Local time of day: 0001-0600
Location: ZZZZ.ARTCC, FO
Light: Dawn
Aircraft: B767-300 and 300 ER
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
ATC / advisory: Center ZZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I sent this message to our Chief Pilots the day I learned of this occurrence and thought I had pressed the submit as a report button; but the gate keeper advised me yesterday of my lack of filing. Here is a cut and paste of the closed Pilot to Chief report....About 24 hours ago; I received a phone call from Captain regarding a report from Oceanic of a loss of separation on my flight. [The caller] was confused as to the accusation and asked me if I remembered the clearance received from Oceanic? I answered 'yes'; quite clearly; as it was different from any I had ever received.To comment on my experience level involving international; especially trans-Atlantic experience; I have over 21 years of crossings without an incident. We all know how important separation is and surely; nothing would be attempted without a very clear message of intent from both our flight and the controlling agency. [It was] mentioned that the transcript would be available shortly; as the only way to obtain it was to petition Commercial Radio for the information; which had been ordered. I anxiously await the dialogue; as I can't imagine how this could have been an incident?The IRO and I were on duty; with the FO taking his required break; off the flight deck. As the ride wasn't perfect and the FMC advising that a higher altitude might be more efficient; I was flying and asked FO if he had an objection to a request for a climb to FL390? He thought that was a good idea and the request was sent via Controller Pilot Data Link Communications (CPDLC).Oceanic sent a clearance that was a bit different from most I had seen; but seemed urgent and we discussed the clearance before accepting. Since I don't have the transcript; I'll have to use recall; which won't be exact; but will be fairly correct...... Oceanic said we had the capability to climb to FL390; to reach [fix] at FL390. IRO and I discussed this and agreed to monitor the Position page to track our progress. Seeing that we had a 6 minute window; more or less; I opened the vertical speed window on the FMS and initiated a 400 feet/minute accent; which would produce a level-off; easily; within the required time-frame and not noticeable for the passengers. Instead of being concerned with the exact plotting of the required level-by clearance; we both agreed that it was very clearly; a level-by-[fix] clearance. Looking at the prog page/position on the FMC; I initiated a level-off at FL390 about 5 minutes later; or 19.5 degrees; within the requested window. We had armed the CPDLC to report the level FL automatically; which we verified was sent properly.I noticed the aircraft that crossed our path and it was not visible on the TCAS; but I was thinking that this must be the reason for the crossing restriction; even though it didn't appear on our 2000 feet TCAS viewing capability. Something seemed strange? A 6 minute time to climb to an altitude and an aircraft within view 5 minutes later seemed strange? No message or issue was transmitted via datalink; but it stuck in our mind that it was a bit unusual. A definite surprise having a call regarding our non-compliance!I look at this and realize it was just about sunrise and the brain was a bit tired; but not an excuse; especially with 2 of us comparing opinions. I don't believe there was a conflict; but will be anxiously awaiting the dialogue from Commercial Radio. If there was a problem; we both would have inadvertently missed it; which really makes me take notice that it can happen to any of us; as none are infallible. The only explanation would have been thinking we were Westbound instead of Eastbound in monitoring the position? I'm still in denial. I'm not sure if the recording feature is exact and doesn't have a 100% accurate measure at times? I would think that it's impossible to report level and not have an exact position stamp?Okay; now that I have gotten all the information; I did my own test; using the time stamps of the clearancereceived; clearance confirmed and the time required to climb.On a similar flight; the next sequence after my submission of the text above; I did a test of aircraft performance in an identical time scenario. With a normal autopilot pitch up and normal power/throttle advancement; plus a no wind condition (we compared no tail wind potential and then subtract for a strong tailwind/ground speed increase). With a 90 knot direct crosswind; we started a time hack and it took 1 minute and 22 seconds to cover the distance from [our current position]. I figured that without 95 knot direct tailwind; we would have had to be level in 1 minute; 6 seconds. This type of clearance is normally received with - be level by - 3 degrees; which gives a lot of time to climb. Since this was a 'stand on your tail' clearance; we both were lured in to thinking it was a degree and a half to climb; not 3/4 of a degree to climb. Our fault; no doubt; as we should have rejected it. This showed complacency on both our parts and I can only attribute it to minor fatigue; lots of stress and the sunrise that was to come thereafter.




Synopsis: B767-300 Captain reported accepting a clearance to a higher altitude that they were not able to comply with which resulted in a loss of separation.