B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Communication Breakdown
ATC Issue All Types; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance
Human Factors; Procedure
Descending into LGA with Approach Control; we requested RNAV approach and thought we would get the RNAV visual for 31 at LGA. They didn't confirm our request; but were busy; so we assumed we'd get what we requested. As we were descending from 7000' to 4000' on a vector; we felt we were very close to the approach and not on a downwind vector. Captain; Pilot Flying; decided to slow; so as to be stabilized; if we were then cleared for the approach. Approach Control had other ideas as we were vectored across the approach into a right downwind. Then ATC queried us about our speed and we stated 210 to which we were admonished and told; 'You can't slow down without telling us.' they then assigned us another speed and descent into the approach. We failed to let them know of desire to slow and our decision to do so. We should query ATC before slowing down and get clarification on what approach we were to receive so as to prevent getting bitten by the expectation bias.
B-737 First Officer reported slowing from assigned speed prior to receiving approach clearance from ATC.