C404 pilot reports inability to maintain airspeed and and buffeting in the flight controls due to icing. Tail boot failure is suspected and and an emergency is declared. The buffeting disappears as soon as airspeed increases in the descent and ice can be seen melting off unprotected areas of the aircraft at 7;000 FT. Maintenance can find no problem with the tail deice system.

Narrative

On a routine flight I experienced what I believed at the time was a tail boot failure resulting in the accumulation of ice. After departing I was cleared direct to destination at 11;000 FT. The temperature at the surface was just below freezing. PIREP's regarding ice showed light to moderate rime at the 15;000 level. Due to the relatively short flight I planned to stay at my assigned 11;000 as a climb through the ice would slow me down and in my opinion not provide relief from icing conditions. I monitored the wings to ensure ice was being shed with the use of the installed boot system. It appeared to be working very well. Around the half way point I noticed that I was starting to have to apply more back pressure to maintain altitude. I increased power slightly to compensate. As the flight continued air speed continued to degrade to hold level flight. Normal cruise power results in 150-160 KTS IAS in level flight in this aircraft. As the flight progressed my IAS dropped as low as 120 KTS. It was at this point I began to experience buffeting in the flight controls. I immediately declared an emergency. My request was lower altitudes as they became an option. I was stepped down to 7;000 FT where I noticed the accumulated ice on the non-protected parts of the aircraft melting. Buffeting stopped as soon as a descent was started. I believe this may have been due to the increased air flow over the controls. Upon leveling out at 7;000 FT I was able to hold altitude and safe airspeed without buffeting. I commenced the VOR/DME approach and I broke out near minimums. A successful landing followed. As I exited the aircraft I check the tail for accumulated ice but did not notice abnormal amounts. The aircraft was written up for an inspection of the deice boots. The local mechanic provided the inspection and signed the aircraft off for further flight.