Date: 201307
Local time of day: 1201-1800
Location: JNU.Airport, AK
Flight conditions: VMC
Weather elements / visibility: 10
Aircraft: Medium Large Transport; Low Wing; 2 Turbojet Eng
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Initial Approach
Airspace: Class E ZAN
ATC / advisory: Center ZAN; Tower JNU
Reporter organization: Corporate
Reporter function: Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Communication Breakdown; Confusion; Training / Qualification; Workload
Second reporter function: Pilot Not Flying
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Contributing factors: Environment - Non Weather Related; Procedure; Aircraft
Primary problem: Ambiguous

Narrative: After being cleared by Anchorage Center to descend and maintain 7;000 FT direct to Juneau; we were cleared for a visual approach [Runway] 26; and asked to report visual contact with Juneau. At this time we were overflying the beginning of the Gastineau Channel from the south arriving.While I was listening on VHF One Anchorage Center and waiting to report visual contact with Juneau Airport; pilot not flying was establishing a radio contact on VHF Two with Juneau Tower; during this communication he set 4;000 FT in the altitude windows and gave me a sign to descend; which I did. We were approaching the city of Juneau; still in visual contact with the ground in VMC condition; no other traffic was flying around. I was very concerned and concentrated on maintaining visual separation from obstacle looking for VFR traffic while in the Gastineau Channel; and didn't want to interrupt the communication that the pilot not flying was having on VHF 2 with Juneau Tower; while listening to VHF One. When pilot not flying was back on VHF one; at the same time Anchorage Center told him that he had an alarm because we were descending below the safe altitude; and that this was a violation and asked us to call when on the ground; which we did. Pilot not flying confirmed to Anchorage Center that we were descending in VMC conditions and explain that he understood that as we were cleared for a visual approach [and that] we were cleared to descend. We continued the visual approach for Runway 26 and landed normally. I told the pilot not flying that because he made me a sign to start descent after having set 4;000 FT in the altitude windows; I was assuming that we were cleared by Juneau Tower to descend and therefore released by Anchorage. Anchorage Center was still waiting [for us to] report visual with Juneau Airport to release us.We were both listening to different VHF. We were in extremely good VMC conditions. Insuring all the time our one visual separation to obstacle and didn't see any traffic. Juneau Tower was aware of our position and intentions since we overflew the beginning of the Gastineau Channel from the south and before we started our descent from 7;000 to 4;000 FT. The different perception came from the fact we [did not have] the same information because [we were] listening to 2 different VHF. We had both a different judgment of the situation but did not realize it until Anchorage Center advised us of the altitude violation. My decision to descend [came from] the pilot not flying's action to set 4;000 FT and giving me a sign to descend while speaking to VHF 2. I should have informed Anchorage Center we were descending as soon as we did it; even if I thought Juneau Tower took over the ATC. Miscommunication.


Second narrative: Positioning leg to JNU during day time; weather VMC with unrestricted visibility.At 7;000 FT MSL and at around 15 NM southeast closing in on our destination JNU; we descended from the 7;000 FT assigned altitude by Anchorage Radar to start our visual approach into JNU. Five minutes before we started our descent from 7;000 FT; Anchorage Radar had lost radar contact with us and told us to report field in sight. We were also talking to JNU Tower (on VHF#2) that had cleared us for a visual approach to Runway 26 through the Gastineau Channel.Debriefing the event with the pilot flying we concluded that the confusion came from the fact that Anchorage Radar had told us that they had lost radar control with us (which we interpreted as radar control terminated) and together with the fact that we were cleared by the Juneau Tower for a visual approach to Runway 26 we descended from our Anchorage Radar assigned altitude of 7;000 FT to start the visual approach; before reporting to Anchorage that we had the field in sight and therefore their release to do so.When we told Anchorage Radar we had the field in sight a couple minutes later after initiating our descent from 7;000 FT; they ask for our altitude and we reported we were at 5;500 FT descending for the visual to Runway 26. Anchorage Center told us that we had done a 'possible pilot deviation' and gave us a phone number to call upon landing which I called about 30 minutes after landing.Contributing factors were:1) Communication with 2 frequencies at the same time. 2) Although we had trained in the simulator for this specific airport during our 6 month recurrent at Flight Safety we were particularly concerned about the approach due to the combination of the terrain particulars and the numerous VFR traffics (helicopters; sea planes etc...) in the traffic pattern.


Synopsis: A Corporate aircraft crew arriving JNU reported the airport in sight and began a descent from the ATC assigned 7;000 FT before being cleared because they were busy and misunderstood the ATC terrain constraints.