RV-7
Air Traffic Control Fully Certified
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Air Traffic Control Issued New Clearance; Flight Crew Landed in Emergency Condition; General Declared Emergency
Aircraft
About XA30 AM local time; I was conducting a briefing for Local Control.  As I was covering the last of the traffic we noticed a primary target about 1 NM in front of an inbound guard helicopter.  The relieving Controller questioned the target to which my reply was;'I'm not sure.'  At that moment the suspect target squawked 7700 and the Guard Helo called the Tower.  We then heard a mayday call on guard frequency and noticed a smoking inbound aircraft out the window.  The Guard helo was calling to inform us of the emergency aircraft.  The relieving Controller took control of the position and called on guard frequency to clear the emergency aircraft to land on Runway 32.  The emergency aircraft safely landed on Runway 32 with emergency personnel enroute.
Local Controller reports receiving a mayday call on guard while conducting a shift change briefing.  The aircraft is sighted on approach trailing smoke and cleared to land.