Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 403
Situational Awareness; Confusion
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR
Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Procedure
I was climbing through Flight Level 180 and was given a clearance to descend and maintain FL180 while the local altimeter setting was 29.77. This conflicted with FAR 91.121. The Captain queried the clearance and was told again FL180. ATC responded affirmative. The Captain states this was not a legal clearance and ATC responded it was for traffic avoidance. We were unsure if separation had been lost but there was no TCAS TA or RA and we proceeded normally after descending to FL180.
B737 First Officer reported ATC assigned them FL180 when the altimeter setting was below 29.92.