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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site as of July 2013 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.