[olympus]

Not exactly, and the TapJets response doesn't fully lay out that they met all the requirements for Part 91 flying, but they mentioned non-paying passengers.

You can fly under part 91 rules commercially but there are restrictions on how you can operate. You can operate under part 91 rules non-commercially, like I do, and that basically means that my passengers can only pay for their fair share of the gas burned. I can't make any profit. If the trip only burns 20 gallons I can't ask them to pay to fill up my tanks (51 gallons). But I can let them buy me a hamburger, as long as that's something they would have done anyway if I didn't take them flying.

Every flight under part 135 is considered "commercial," meaning that pilots must have a commercial rating as well as numerous other requirements like extra maintenance, inspections, and record keeping. Even if there are no paying passengers, a flight might still be under part 135 rules, but I'm not sure about the specifics.

I believe it's legal (but I'm not a lawyer) for a plane that flies under part 135 rules one day to be operated under part 91 rules the next day. Imagine flying a chartered flight under part 135 rules to Los Angeles. While on the ramp, the 100-hour inspection comes due, but the plane is still current with it's annual inspection. The owner of the jet would still be allowed to use it under part 91 rules, but it would not be allowed to conduct another charter flight until it passed the 100-hour inspection.

[AceyMan]

Just to address the Part 91 vs 135 for commercial flight: it is legal provided there are no revenue generating cargo or passengers aboard (e.g., zero people and a couple of lost bags = nope, because bags are freight).

The carriers commonly make use of Part 91 for reposition flights to move equipment into place to resume original schedules following weather diversions or whatnot.

(me: Licensed Aircraft Dispatcher)