Tower Controller described a harrowing event when an inbound SVFR Helicopter became disorientated an entered a spinning descent for landing.

Narrative

The pilot entered the airspace on an SVFR flight from the West. The reported weather included an overcast at 600 feet and the time was approximately 30 minutes past sunset. As the pilot approached from 2 miles West of the airport; he entered the cloud layer with Mode C eventually indicating an altitude of 900 feet. The Mode C readout was consistent with visual observations before and after the event. In response to ATC inquiry; the pilot eventually said he could not see the ground and was in the clouds; disoriented. When the aircraft descended below the ceiling; it appeared to be spinning or rotating for about 1 revolution before being stabilized and re-oriented toward the airport. The helicopter's descent was arrested about 200 feet AGL. The pilot then landed at the airport without incident. A pilot deviation was reported. Recommendation; ensure that pilots of helicopters maintain a safe airspeed in marginal conditions and a safe altitude below the clouds. Additional training in unusual attitude recovery or unintended helicopter flight into clouds may be appropriate.