B737-800 flight crew reported descending to within 800 ft of terrain on approach to den after mis-se...

Narrative:

On descent to landing at den; the first officer (first officer) and I incorrectly set 30.37 instead of the field altimeter of 29.37 passing through FL180. While conducting the approach checklist; each of us checked the setting against each other; but neither checked the setting against the copy of ATIS; which was correct. The descent; with vectors for weather and to join the downwind; did not include a level off; which might have allowed ATC to catch our lower than cleared altitude. In visual conditions; on a vector for base leg and cleared to descend out of 9000 ft MSL to 7500 ft MSL; I started to think something was wrong; specifically; why are we cleared to an altitude that looks so close to the ground; so far from the field? The radio altimeter showed a descent through about 1200 ft AGL when I started verbalized my concerns with the first officer who was; correctly; on an inside scan. I called for a climb and 'helped' by adding power while the first officer caught up with our evolving understanding that something was wrong with the clearance. The lowest radio altimeter reading I saw was 800 ft AGL when more or less at the same time the altitude warning based on configuration activated; and approach control gave us a low altitude alert. We realized what had happened; got the correct altimeter while climbing; and made a normal visual approach and landing.

Original NASA ASRS Text

Title: B737-800 flight crew reported descending to within 800 ft of terrain on approach to DEN after mis-setting the altimeter by one inch of mercury.

Narrative: On descent to landing at DEN; the F/O (First Officer) and I incorrectly set 30.37 instead of the field altimeter of 29.37 passing through FL180. While conducting the Approach Checklist; each of us checked the setting against each other; but neither checked the setting against the copy of ATIS; which was correct. The descent; with vectors for weather and to join the downwind; did not include a level off; which might have allowed ATC to catch our lower than cleared altitude. In visual conditions; on a vector for base leg and cleared to descend out of 9000 ft MSL to 7500 ft MSL; I started to think something was wrong; specifically; why are we cleared to an altitude that looks so close to the ground; so far from the field? The radio altimeter showed a descent through about 1200 ft AGL when I started verbalized my concerns with the F/O who was; correctly; on an inside scan. I called for a climb and 'helped' by adding power while the F/O caught up with our evolving understanding that something was wrong with the clearance. The lowest radio altimeter reading I saw was 800 ft AGL when more or less at the same time the altitude warning based on configuration activated; and Approach Control gave us a Low Altitude alert. We realized what had happened; got the correct altimeter while climbing; and made a normal visual approach and landing.

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.

Attributes
ACN	1404924
Time
Date	201611
Local Time Of Day	0601-1200
Place
Locale Reference	D01.TRACON
State Reference	CO
Environment
Light	Daylight
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 163
Person 2
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

B737-800 flight crew reported descending to within 800 ft of terrain on approach to den after mis-setting the altimeter by one inch of mercury.