Date: 201707
Local time of day: 1201-1800
Location: OBU.Airport, AK
Flight conditions: VMC
Light: Daylight
Aircraft: Small Aircraft; Low Wing; 2 Eng; Retractable Gear
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Initial Approach
Airspace: Class E ZAN
ATC / advisory: Center ZAN
Reporter organization: Personal
Reporter function: Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Instrument; Flight Crew Lighter-Than-Air; Flight Crew Multiengine
Human factors: Confusion
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Procedure
Primary problem: Procedure

Narrative: This was my first ever time flying any approach into OBU; because of this I briefed the approach twice while enroute. I was cleared for RNAV (GPS) Rwy 09 into OBU direct KOPGE then the KOPGE TAA. I got the altimeter setting from AFM descended to 4000 ft within 30 miles of KOPGE. I was on a direct line from west (approximately 15 NM west) of the HSL VOR to KOPGE at 7000 ft. ATC cleared me for the approach 70 NM south of KOPGE (I was approximately 15 west of HSL at the time) and advised I that turn to advisory frequency. This was unusual to me as I am not usually cleared that far out and turned over to advisory that early. Later I learned it is because there is no coverage there and I cannot raise center in that area. I fixated on the JIDGA IAF and the initial approach altitude of 4000. I confirmed the route on my iPad which was up to date. Also I; as is my habit; checked the profile view on my iPad to make sure there were no terrain conflicts and none were noted. I was VMC the entire time during this event; I was flying the approach because ceilings were reported at 900 ft and I wanted to do the correct thing and fly IFR into OBU rather than 'Scud run' it into the airport.I received a relay call from an airliner that there had been an altitude alert ATC's screen for me and requested I climb to 6000 which I did. I reached KOPGE descended back down to 4000 and completed the approach. I contacted Fairbanks radio as per the relay instructions but at this time I went IMC and they asked me to take a phone number to call ATC when I can. I declined at the time because I was busy on the approach and said I would call when able. The rest of the approach went as planned; I broke out around 850 ft finished the landing to Runway 09; called FSS on my phone copied the number and called Center to at which time I realized what I had done on the approach; I descended below the IAF altitude which gave the alert. The entire event occurred when I was in VMC conditions before reaching KOPGE. I was above a layer all the way to KOPGE; then I descended at KOPGE below the clouds.




Synopsis: Light twin pilot reported failing to maintain required altitude while on approach to OBU resulting in an altitude alert from ATC.