Air carrier captain reported descending too early on a visual approach.

Attributes
ACN	1561941
Time
Date	201807
Local Time Of Day	1801-2400
Place
Locale Reference	GTF.Airport
State Reference	MT
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	Visual Approach
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Flight Instructor Flight Crew Flight Engineer Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Total 11000 Flight Crew Type 100
Events
Anomaly	Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

Using RNAV Z 21 as a reference; descended to 4700 while lining up for visual approach to 21. I wish I had stayed higher. I didn't see the obstacles under the final approach course just outside kudey.

Original NASA ASRS Text

Title: Air carrier Captain reported descending too early on a visual approach.

Narrative: Using RNAV Z 21 as a reference; descended to 4700 while lining up for visual approach to 21. I wish I had stayed higher. I didn't see the obstacles under the final approach course just outside KUDEY.

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.