It's a recreational airplane, but we do use it for some purposeful travel. (A36-single engine piston [propeller], 2 crew seats and 4 passenger seats, ~200mph) My parents are about 700 miles away and my wife's parents are about 625 miles away, both of which are an easy non-stop flight, provided my kids can "hold it". ("Lavatories are located at either end of the flight.")
When you compare against super-discount airline airfares for 4 tickets, "our" airplane is slightly cheaper to decide to take it instead of airlines. There are many days when weather-wise it's safe to go by airline jet but unsafe by small airplane. (Of course, there's a massive overhead to that calculus, but day-of-travel is slightly cheaper.) Flight planning and pilot training is time-consuming, so it only makes sense if you already wanted to do that as your hobby. Absent a real desire to do it, it's not worth the time, cost, and risk. Take the airlines or drive.
I've never done the math prior to today. I'll make an honest attempt at it and post the answer I come up with, regardless of whether it makes me look good or bad. :)
For the trip to my parents, I have a choice to fly commercial into Myrtle Beach, to fly private into the small airport near their house, or to drive. I'll consider CO2e and a 4-passenger load.
Commercial: 760nm->1300km(693 flight plan plus 10% uplift for ground ops and APU) => 328kg CO2e + 180 miles of airport transport by car => 32 kg CO2e => 360 kg total
Private car: 30mpg, 4 pax, 844 sm => 149 kg CO2e (burning ~28 gallons). (Also is a 14 hour drive, so miserable.)
Our airplane: 616nm => 1200km (including a 5% uplift for climb and ground ops), at 11nmpg, burning about 60 gallons, which matches our actual historical fuel purchases quite closely. Assuming same CO2e/gal as a car => 320 kg CO2e Add 100 miles or so of ground transport and that puts you around 18 kg for the cars and 338 kg total. (Distance is less because we go to an airport that is closer to my home airport and much closer to my parents' house.)
So it looks remarkably close on the airplane side, if the commercial airline figures below are accurate. I'm quite confident the car and our airplane figures are ballpark close enough (absent a math error). That also requires taking a direct flight, which is not always possible (the direct is seasonal). The emissions on connecting flights are much worse than the above with twice as many ground operations. (Jets can burn ~1/3 of their cruise fuel flow while idling on the ground. Pistons are more like 1/6th.)
When you compare against super-discount airline airfares for 4 tickets, "our" airplane is slightly cheaper to decide to take it instead of airlines. There are many days when weather-wise it's safe to go by airline jet but unsafe by small airplane. (Of course, there's a massive overhead to that calculus, but day-of-travel is slightly cheaper.) Flight planning and pilot training is time-consuming, so it only makes sense if you already wanted to do that as your hobby. Absent a real desire to do it, it's not worth the time, cost, and risk. Take the airlines or drive.