THREE DIFFERENT CFIS AND THEIR STUDENT PLTS TAKE THE SAME ACFT AT DIFFERENT TIMES AND FLY IT AFTER IT HAD PASSED ITS TIME FOR AN ANNUAL INSPECTION AT ZZZ; UNITED STATES.

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ON NOV/FRI/01; AND NOV/SAT/01; I INADVERTENTLY FLEW THE SAME ACFT OVER ITS ANNUAL INSPECTION. OUR TRAINING ACFT ARE ON A PHASE INSPECTION PROGRAM. THE ACFT WE WERE FLYING IS ON A 3-PHASE INSPECTION PROGRAM. MOST OF OUR AIRPLANES ARE ON A 4-PHASE INSPECTION PROGRAM. THE STATUS SHEET IN THE ACFT WE WERE FLYING IS DIFFERENT THAN THE REST OF OUR ACFT. THE STATUS SHEET IN THIS AIRPLANE HAS THE PHASES LISTED AND THE TIME IN WHICH THEY ARE DUE; AS WELL AS A DATE UP TOP WHEN THE ANNUAL IS DUE. THE DATE LISTED WAS NOV/FRI/01. THE OTHER ACFT'S DATES ARE LISTED AS JUST NOV/FRI; WHICH MEANS THE INSPECTION IS DUE AT THE END OF THE MONTH INSTEAD OF THE BEGINNING. THE 2 DIFFERENT WAYS THAT THE DATES ARE LISTED IS CONFUSING AND IS A FACTOR FOR WHY THE ACFT WAS DISPATCHED BY THE SCHOOL AND WHY WE FLEW THE ACFT. THIS PROB CAN BE PREVENTED IN THE FUTURE BY IMPLEMENTING A STANDARD SYS FOR ALL OF THE TRAINING ACFT AND RECURRENT TRAINING FOR THE DISPATCHERS AND PLTS WHO ARE USING THE SYS. NOTE: I WAS ACTING AS CFI FOR COMMERCIAL STUDENTS DURING BOTH THE INCIDENTS. SUPPLEMENTAL INFO FROM ACN 529164: ON NOV/FRI/01; MY COMMERCIAL STUDENT AND I; FLEW AN AIRPLANE THAT WAS DISPATCHED TO US; WHICH WAS ACTUALLY OUT OF ANNUAL INSPECTION. WHEN MY STUDENT EXAMINED THE STATUS SHEET FOR MAINT OF THE AIRPLANE THE ANNUAL INSPECTION WAS DUE AT XA59 ON NOV/FRI/01; AND NOT THE ACTUAL TIME IT WAS REQUIRED ON OCT/THU/01 AT XA59. THERE WAS A MISREPRESENTATION OF THE EXPIRATION OF THE ANNUAL INSPECTION ON THE STATUS SHEET WHICH IS MADE BY THE SCHOOL LCL MAINT SHOP; AND MY STUDENT AND I PROCEEDED TO FLY THE AIRPLANE FOR TFC PATTERN PRACTICE WITHOUT THE KNOWLEDGE THAT OUR AIRPLANE WAS UNAIRWORTHY; WHICH WAS NOT DISCOVERED UNTIL THE FOLLOWING DAY. THE ACFT IN QUESTION WAS XXXX; A C172RG.