The aspect of S/E charter also has another twist in that the piston engine must be within a 12 year limit. That means that while an aircraft engine can have say a TBO of 2000 hours it may have only reached 1000 hours by the 12 year limit. OK, the aircraft can continue to fly up to and beyond 'on condition' but only on non pax charter ops. The 12 year limit also applies to twin pistons.
That is because Lycoming Service Instruction No. 1009BE says this;
All engine models are to be overhauled within twelve (12) calendar years of the date they first entered service or of last overhaul. This calendar year time period TBO is to mitigate engine deterioration that occurs with age, including corrosion of metallic components and degradation of non-metallic components such as gaskets, seals, flexible hoses and fuel pump diaphragms.
And under AD-ENG-4 you have to adhere to all stated manufacturer overhaul periods for charter endorsed aircraft. Unless you can show your own system of maintenance, which Lycoming also allows for.
Continental has the same 12 year limit for most of its models as far as i can see. Rotax seems to be the most complicated with 5-15 years dependent on model, not sure how many of those would be in a "charter" aircraft though.
BTW, the AD only specifies "
For aircraft in Charter operations", it does not consider passengers.