Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Communication Breakdown; Time Pressure
ATC Issue All Types; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Requested ATC Assistance / Clarification
Human Factors; Airspace Structure; Procedure; Weather
Westbound on NAT D (North Atlantic Track D) at assigned FL390 and assigned mach .82; we experienced moderate turbulence. We slowed from mach .82 to mach .80 to improve the ride. We notified Gander via CPDLC(Controller Pilot Datalink Communications) of the change immediately; and received an acknowledgment shortly after via CPDLC. [A few minutes later]; the ride improved. We accelerated back to assigned mach .82; notified Gander via CPDLC immediately and again received acknowledgment via CPDLC. Shortly after; we received a message via CPDLC; something to the effect of Next time; make request prior to change. No further communication related to this scenario. Flight continued uneventfully to full stop landing.Change in speed was effected to improve the ride in moderate turbulence. If ride and timing permits; notification to ATC should be made in advance of speed change.
A330 First Officer reported moderate turbulence resulted in a speed reduction before making the request with ATC.