I know everyone says a 172 isn't a real 4 seat plane but I've put 4 adults in one, left very little fuel behind and was absolutely legal/safe.
One day I was minding the store on a Saturday morning as the only employee at the FBO. A 172 landed, taxied up to the fuel pump, and four adults got out and came into the office. The pilot asked me to top off the tanks. I walked out, glanced in the luggage compartment and was slightly relieved that there was only a briefcase there. So, I filled the tanks.
When the pilot paid the fuel bill, I asked him if he was at all concerned about four adults and full fuel. He said, no, it had flown fine on the way from northern Minnesota that morning. I was thinking, yeah, but the air had to be a lot denser a few hours ago and hundreds of miles NW of here.
He told me they were headed to Shiocton, which is a grass airstrip with a 2,200-foot runway. I mentioned that a 172 had crashed into trees at that airport on its third takeoff attempt. He replied that there would not be four people on board when they take off there.
His takeoff appeared normal, but the rest of the day I was uneasy, and I watched the news later that evening hoping there would not be a story about an airplane crash. There wasn't.
On Monday, I related the story to my boss and asked if I should have done anything differently. He said, no, it is always the PIC's decision, and if we had not sold him the fuel, he probably would have just gone somewhere else for it.
