INSTRUCTOR AND STUDENT IN A C150 HAVE AN NMAC WITH AN MU2 THAT TOOK OFF ON A DIFFERENT RWY.

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I WAS WITH A STUDENT FLYING R CLOSED TFC ON RWY 33 AT DPA. THERE WERE SEVERAL OTHER PLANES IN THE PATTERN; PLUS INBOUND AND OUTBOUND TFC. IT WAS BUSY; BUT AVERAGE FOR A SUMMER EVENING AT DPA. THE TWR CTLR; AT TIMES; SEEMED TO BE HAVING TROUBLE STAYING ON TOP OF THINGS. ON SEVERAL OCCASIONS HE WOULD CALL PLANES BY THE N-NUMBER OF OTHER PLANES IN THE PATTERN. AT ONE POINT; HE CLRED AN MU2 TO DEPART RWY 10 WITH THE INSTRUCTION TO TURN R TO 270 DEGS. HE GAVE THE MU2 NO WARNING THAT THERE WAS TFC IN THE PATTERN FOR RWY 33. WE WERE ON R XWIND AT THIS TIME AND WERE NOT WARNED ABOUT THE DEPARTING MU2. I WATCHED THE MU2 DEPARTING RWY 10 WHILE THINKING ABOUT HOW TO STAY OUT OF HIS WAY. I TOLD MY STUDENT TO KEEP OUR BASE IN TIGHT; AS I DID NOT WANT TO GET IN A POS WHERE WE COULDN'T SEE THE MU2. ON BASE; I SAW THE MU2 AT THE SAME ALT; APPROX 250 FT N OF US; JUST STARTING HIS R TURN. THE TWR THEN CALLED 'DO YOU SEE THE CESSNA ON R BASE?' THE MU2 REPLIED IN A SURPRISED VOICE 'I DO NOW!' THE TWR REPLIED 'I DIDN'T EXPECT YOU TO TURN THERE.' THIS IS NOT BY ANY MEANS THE FIRST TIME I'VE FOUND THE PATTERN AT DPA TO BE DISORGANIZED AND DANGEROUS. THE CTLR IN CHARGE AT THE TIME OF THIS INCIDENT IN PARTICULAR HAS REGULARLY BEEN LAX IN GIVING ACFT INFO ON LOCATIONS OF OTHER ACFT IN THE PATTERN; AND CLARIFYING AIRPLANE'S POS IN THE LNDG SEQUENCE. HE ALSO TIES UP THE TWR FREQ WHEN THINGS ARE BUSY SCOLDING PLTS FOR XPONDER MALFUNCTIONS AND NOT GIVING ENTIRE XMISSIONS ON INITIAL CALL-UP; ITEMS THAT SHOULD BE ADDRESSED ON GND AFTER THE PLANE HAS LANDED. HE SOMETIMES DOES NOT GIVE FULL INSTRUCTIONS; EG; THE OTHER EVENING AFTER LNDG ON RWY 10; I WAS TOLD; WHILE ROLLING OUT; 'HOLD SHORT OF RWY 33.' THE FULL XMISSION SHOULD HAVE BEEN; 'TURN L ON TXWY C; HOLD SHORT OF RWY 33.' THIS ARPT SEES A HVY TFC LOAD WITH A LOT OF TRAINING FLTS. THE TWR NEEDS TO DO A MUCH BETTER JOB OF COMMUNICATING AND ACT IN A MORE PROFESSIONAL AND RESPONSIBLE MANNER OR THERE WILL ALMOST CERTAINLY BE A MIDAIR COLLISION. I CAME VERY CLOSE ON AUG/XA/99. I HAVE OVER 10 YRS OF FLYING EXPERIENCE; MOSTLY AT TWR CTLED ARPTS; AND I HAVE NEVER EXPERIENCED SUCH A POORLY CTLED AIRSPACE. TWR OPS AT DPA NEED TO BE INVESTIGATED AND IMPROVEMENTS MADE!