One other factor is winds. The 500 and I'm sure other many other aircraft are sensitive to shutdown in high winds. The main rotor can contact the tailboom as the rotors start to droop.
Some of the places we are working in the mountains doing powerline work where there is 25-40kts of wind 300+ days of year yet light winds down on the plateau where we hangar. On some of the more windy days we will takeoff at 7am work till 7pm and never shut down, usually fly for 6-8 hours and sit for 2-4 (spread out throughout the day) The wind is often the deciding factor on weather to risk shutting down on the mountain at the LZ.
Time is not a factor for us in a billing sense. We bill off of collective hobbs. So the customer doesn't get charged for time on the ground.
- Cycles is a factor (wear on the turbine and starter)
- Risk of problems on restart is a factor. But we always have a mechanic and GPU with us.
- Wind is a factor.
- Turnaround time, e.g. time to get back out there and fulfill a need is a factor. In addition to the 3 minute cool down you risk hotstart with re-starting right away after shutdown.
- Fuel burn is a factor, albeit a minor one.
- Safety is a factor, as far as we would like to shut down at least once a day and have the mechanic give the helicopter good a look-over, most days that happens, but not always. They always give it a look-over several times a day, but sometimes its still running.
With properly trained crews the difference in risk between hot fueling a fueling shut down is negligible.
We don't have passengers to load, but even if we did, I would have no problems loading small groups of trained passengers with an escort.
As for bio breaks, that what frictions are for. with the 500 at ground idle you can pull the collective up to your arm pit and rock the cyclic and it wont fly. the blades aren't going fast enough to generate lift. So its not like some aircraft that you have to worry about a breeze rolling over, e.g. the Kmax.