91.215 "deviations"

[Bernoulli]

Minitour... If at some point during a flight you realize your Mode C is inop (or ATC alerts you to the transponder problem) then simply state to ATC that your mode C is inop and request permission to enter or transition their airspace with the inop mode C. There is no "official word for word statement" . Simply acknowledge the transponder problem and ask for permission to continue into or through their airspace. You should only fly into or through Class A, B, & C airspace with an inop transponder to reach a destination in which the transponder can be fixed. I would not go on a training flight with a known inop transponder and ask for permission... this is simply pushing the system and could lead to complications that simply are not worth it.