For the
Stunt Flyer in Just Cause 2 achievement there is
no time limit imposed, just a cumulative amount of time that you must consecutively perform the action for. As you're not having to complete the action
within a time limit or
at a certain time the Time Sensitive flag is not applicable.
If the achievement description was instead 'Fly an airplane close to the ground for 30 seconds
within a minute of taking off.' then the Time Sensitive flag would be applicable, as there's a clear time constraint of a minute given in which you must complete the action - but this isn't the case.
The reason I mentioned the counter resetting in my previous explanation was to highlight that although the achievement is an accumulation of time, due to the requirement to perform the action in one consecutive go the Cumulative flag is also not applicable in this instance.
Thanks again for the submission, and I hope that clarifies the use of the Time Sensitive flag a little bit more.