I have to say that I whole heartedly disagree. When one taxis to the runway, he has the intent of flight. When no takeoff is made, then there will be no landing, obviously.
My time starts the moment I start the engine (And start moving) until the engine is off and the aircraft is parked.
I count all of this as PIC time because of the original reason stated above; the fact that if something happens, I am responsable for it.
For example, I had a student not to long ago that was training for his multi engine add-on to his commercial certificate. When he flew the maneuvers, he was well within the PTS on the first try. As his flying was beautiful, his taxiing made me want to cry for a different reason. All over the place, off the taxiways, the only time we held centerline was for a split second when he was crossing it, shooting off in another direction. I ended up training him for 2.0 of time in the air and 1.1 hours on the ground.
As a requirement for his rating was taxiing, I had to train him on the ground in how to taxi the aircraft properly. Does this count as instruction? Yes it does as he needed that training to fulfil the PTS requirements for the checkride. Do I count this as flight time? Absolutely! We never got to a parking spot until we were complete with the taxi instruction that I gave him so even when we 'stopped' the aircraft was not "At rest." If I would have taxied with him before the flight I would have still logged the time as flight time on the basis that we intended to fly.
As far as the logging of the taxi and runup if there was a problem found with the aircraft. The aircraft began moving under it's own power with the intent of flight. True, so the flight time clock began. All the time taxiing and during the runup IS flight time according to the F.A.A. Once the decision was made that the takeoff could not be completed, the flight time stopped. The aircraft taxiing back to base is not flight time. However it is "Total Time."
As for the idiots taxiing around just to build time, you must have flight time in order to qualify for a certificate or rating. Since they had no intention of taking off, they had no flight time and that's why they got in trouble. Not to mention that you need no 'safety pilot' when you're on the ground. Just moronic.
How do you know when to end flight time then? I sure don't know, the airplane never landed, so therefore never came to rest, AFTER LANDING. You can whole heartedly agree with the regs all you want, but in the eyes of the FAA, your opinion doesn't matter. In the definition of flight time, I don't see anything about who is PIC, or who is responsible if anything happens. You're absolutely correct that the clock starts when the airplane start moving under its own power for the purpose of flight, but when flight cannot be accomplished, the nullifies flight time. Remember, a statement is true, only if the entire statement is true, if part of it is false, the entire statement is false. You can't pick and choose which parts of the regs you want to follow, which is exactly what you are implying, that since you would be taxiing for flight, that makes it flight time, even though you can't complete the second part of the definition. As far as your scenario about the multi student, as long as you landed, and then did all the taxi work, it absolutely would be flight time, because the airplane had not come to rest after landing yet...but if you had a separate lesson in which all you did is taxi work, and no flying, it is not flight time.
Just think about it, I'm not pushing my opinion on you, I'm pushing the FAA's opinion on you, and that is the only opinion that matters. There are many things in the regs I don't agree with, but I can't argue my point, because my point doesn't mean anything.
Onto how to charge...I agree that instructor time and airplane time do not have to be the same. If the airplane breaks, you should not have to pay for the plane (unless, of course, you own the airplane), but why should you not pay for instruction? Just because the plane broke does not mean that you did not receive any instruction, does it? I did have an experience once where we had two aborted takeoffs and did not fly. We did the run-up, and looking back on it, I should have caught the problem, carb heat on, got a drop in RPM, but when I turned it off, the RPM's didn't come back. I also trained out of a short field (2300' runway), so we are on the takeoff role, halfway fown the runway, and weren't getting good RPM's and only at about 40-45 kts, so we aborted, we tried again, same results, so we parked, let the mechanics know, and it turns out that the cable for the carb heat broke during the run-up, and it was stuck in the on position. Did I pay for my instructor, yes, because he provided instruction, especially on the aborted takeoffs, and plus, we did ground training after that. My instructor had a simple rule, you pay for the time that you book him, whether you fly or do ground (obviously weather doesn't count, or if you called him because you were sick or whatever). If you are a no show, you still pay him the next time you see him. If he is a no show, the next lesson is free. Funny how I never had to pay extra, and he never had to give a free lesson, so it really worked well. Yes I know, he was an independent contractor at the airport, the flight school did not actually employ the instructors, so at those schools it is different, but for the independent guys, it works well