Small Aircraft; High Wing; 1 Eng; Fixed Gear
Flight Crew Private
Flight Crew Last 90 Days 15; Flight Crew Total 1800; Flight Crew Type 1500
Time Pressure
ATC Issue All Types; Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Runway
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Executed Go Around / Missed Approach; Flight Crew Took Evasive Action
Human Factors; Procedure
While on downwind we were cleared to land on 34 left; with no other planes in the pattern. On short final; with all strobes and alternate flashing (wig-wag) headlights operating; our aircraft was at 100 feet AGL and over the threshold; Tower cleared a single engine aircraft to depart on 34L in front of us. Another Tower Controller immediately came on the radio and directed the incursion aircraft to immediately halt and hold position. It complied; but was on the runway side of the hold short line. We were instructed to 'go-around' and turn crosswind at our discretion. We climbed to 1;000 feet MSL and entered downwind. The Tower then released the incursion aircraft cleared to depart 34L. We were again cleared to land 34L. PAE ground stated 'training' was being conducted.
GA pilot reported a conflict during their final approach to PAE airport when an aircraft was cleared to takeoff on same runway then stopped by Tower; but not before aircraft had incurred the runway.  Reporter was instructed to go-around. Pilot reported ATC training had been a contributing factor.