A R44 II pilot failed to completely close one of two cowling door latches allowing it to come unlatched in flight causing a pop and vibration. After an emergency autorotation into a field; the Reporter found the open latch; closed it and flew the aircraft home.

Narrative

I was flying south at 2;000 FT MSL about 6.18 NM northeast of ZZZ. During the flight; there was a bang or pop sound followed by a vibration. I looked at my instruments and the clutch light had come on. With a clutch light and a vibration; I executed an autorotation to land immediately; per my POH requirement. During my autorotation; I called out a Mayday call to ZZZ Tower. I landed the R44 in a field under the engine power. There was no damage to the aircraft; persons; or property. A few minutes later; a helicopter was seen flying in my direction. I called him on the helicopter frequency and informed him that we had landed safely. I asked the pilot to call off the emergency services. I then checked the R44 and found that a cowling door (1 of the 2 wing nuts) was unlatched. During the preflight; I may have not latched it fully. The wing nut turns 90 degrees to lock in place. What I think took place; was the front wing nut was only half way in its hole and when I turned it 90 degrees it felt snug; as if it was locked. I think the bang/pop sound was the nut coming out of its hole and the vibration was the cowling flapping around with the other wing nut in place. The clutch light was indicating the belts adjusting which is normal. After finding the cowling door was unlatched and a preflight; everything on the R44 checked out. I called my boss; we spoke for a brief time. Then I started the R44 and ran it for a short time; then pulled in to a hover -- no noise or vibration was present. After some time had passed; I flew the R44 back to the hangar. After getting the R44 in the hangar; I did another preflight -- nothing looked different. If there was any doubt about the aircraft; I would not have flown it back. Since the flight was at night; it elevated the stress; I went to the ground thinking worse case scenario. When safe on the ground and finding the problem; I did not see any reason to take further steps.