jrh said:
If you say, "Hey friend, I can't afford to go flying just for the heck of it, but if you want to help out by paying half, I'll take you for a sightseeing ride," that is legal.
I don't think it is. As stated in your hypothetical, your purpose is strictly to build time and have someone pay part of it. You've already seen the sights form the air. You coldn't care les about sightseeing. Your friend's purpose is sightseeing.
One of the more interesting FAA Legal opinions is a 1977 one where a pilot asks about contributing to a campaign by transporting a candidate for a share of the expenses. In saying that this was a no-go, the Counsel said:
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it is not a joint venture (you are not running for office)
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Another interesting one is the private pilot who got nailed for taking a friend's sick father to the hospital for a share of the expenses, the NTSB saying there was no common purpose for the flight.
If you happen to fly with friends a lot and consequently get financial help for flying, I don't think the FAA cares.
That's the ticket. If the FAA doesn't care, obviously it's not a problem. If the FAA has no reason to know, then it won't care. That "sick father" one is a good example. Although the NTSB decision doesn't say so, you are definitely left with the impression that this guy did something to piss someone off.
Chances are unless a Part 135 operator notices what's going on and complains or one of your "friends" decides the ride sucked and calls the local gendarmes, or have an incident and there an investigation, you are okay.
But that's about getting caught, not whether the activity is legal or not, right?