B737 first officer reported ATC assigned them FL180 when the altimeter setting was below 29.92.

Attributes
ACN	1620075
Time
Date	201902
Place
Locale Reference	ZAB.ARTCC
State Reference	NM
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 403
Events
Anomaly	ATC Issue All Types Deviation - Procedural Clearance Deviation - Procedural FAR

Narrative:

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.

Original NASA ASRS Text

Title: B737 First Officer reported ATC assigned them FL180 when the altimeter setting was below 29.92.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.