OK, here's the deal. First, a refresher from the AIM:
5-1-11. Change in Proposed Departure Time
a. To prevent computer saturation in the en route environment, parameters have been established to delete proposed departure flight plans which have not been activated. Most centers have this parameter set so as to delete these flight plans a minimum of 1 hour after the proposed departure time. To ensure that a flight plan remains active, pilots whose actual departure time will be delayed 1 hour or more beyond their filed departure time, are requested to notify ATC of their departure time.
b. Due to traffic saturation, control personnel frequently will be unable to accept these revisions via radio. It is recommended that you forward these revisions to the nearest FSS.
Note that the minimum is 1 hour, but many Centers set the time-out parameters to 2 hours. And YES, when the NAS is busy, the Centers do run short of codes.
Getting your clearance does not automatically "re-set" the time-out clock. That's based strictly on your filed departure time. (P-time) Yes it IS true, that many ground controllers (he normally gets the strip after Clearance Delivery has issued it) will update the time if he notices it's about to time out. I'll personally update it once, IF I notice it. If you're three or more hours late taxiing though, I won't update the silly thing all day, UNLESS you call me or Clearance to ask us to. On the other hand, it's very easy for a busy ground controller to miss a clearance about to time out. If you're going to be more than an hour later than your P-time, simply flip the avionics master on, call Clearance or Ground, and ask them to update the thing. Takes 30 seconds. What else you got to do while waiting for the Boss to show?
Now, if the Wx is lousy, and we're running lots of ground delays, the Centers often re-set the time-out parameters to 3-4 hours for the day, or afternoon. If you get an Expect Departure Clearance Time, (EDCT) 2-3 hours after your P-time, you clearance **SHOULD** still be good at the EDCT time. Once in a great while, something happens and it drops anyway, but that's rare. If there's some other sort of delay program, like a reservation program in effect, and you're being delayed a substantial time, it would be a good idea to remind Clearance to keep your flight plan active.
As far as Clearance re-filing for you, I'll generally do it if it's something relatively simple and remains domestic. If it's international, forget it. You really need to re-file everything with AFSS. If you want to file something complicated or very detailed, it may be best if you re-file with AFSS.
Now, understand that how you file with FSSS and how Clearance delivery enters a "flight plan" is two separate things. The only thing Clearance can enter via an FDIO (Flight Data In-Out) terminal is :
A/C ID
Type
TAS
Dep Airport
Est Dep Time
Altitude
Route and Destination.
Note that there's no provision in FDIO for any SAR stuff like color of aircraft, fuel on board, number of bodies, alternates etc. We can enter **Simple** remarks in field 11, but it needs to be about 25 characters or less. If you want the SAR stuff filed for ANY reason, you'll need to file with FSS or DUATS again.
If you're a GA pilot about to blast off into bad/deteriorating WX, I may insist you re-file with AFSS just to get a current WX update. If you're a 121 or 135 operator, I'll figure you're a big boy or girl, and got a stack of Sigmets and CWAs from ops anyway.
Hope that helps.
