You need an ATP to be PIC of any of the following under Part 135:
- A jet airplane of any size
- An airplane with 10 or more passenger seats
- A multi-engine airplane that is used in a commuter operation (at least 5 round-trips per week on at least one route between two or more points according to a published schedule, in an airplane that has less than 10 seats and a maximum payload capacity of 7,500lbs or less)
Otherwise, a commercial certificate is sufficient, though there are experience requirements (1,200 hours - of which 500 must be cross-country, 100 hours must be a night, and 75 hours in instrument conditions, of which up to 25 may be in a simulator - to be PIC for IFR flight, 500 hours - of which 100 must be cross-country and 25 at night - to be PIC for VFR flight). There are certain specific exceptions to the experience requirements, but those are intended for flight in very rural areas (in other words, Alaska), and for specific purposes.
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