PA-28R Cherokee Arrow All Series
Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 80; Flight Crew Total 968; Flight Crew Type 291
Communication Breakdown; Situational Awareness
Conflict NMAC; Ground Incursion Runway
Flight Crew Took Evasive Action
Human Factors; Procedure
Two accidents almost happened today. My student and I were in the pattern.  We extended downwind to allow a DC-3 to land.  As we were preparing to turn base we saw a plane pass about 100 ft or less underneath us. They made no radio calls and only entered the pattern on extended base.  They continued onto final and landed shortly after the DC-3 touched down.  The DC-3 was still on the runway (approximately 2500 ft separation). He promptly left making no departure radio call but did report leaving the area to the west.
Piper PA-28 instructor reported they were on an extended downwind when another small airplane cut under them nearly causing a midair collision.