Why does ATC assign a SID and then give different instructions after takeoff?

Question

I have had many occasions at different airports flying a small single engine piston aircraft out of a Charlie or Delta and they will assign the SID, but give different instructions upon getting airborne. Is the SID assigned as a lost comms safety net? Or is there another reason why they assign the SID and then don’t actually have you fly it?

For example, flying out of KCHA, being assigned the Chattanooga Eight, and then being given completely different heading and altitude instructions open contacting departure. Same thing has happened at KAVL on the Asheville Seven.

Answer 1

A SID, among other things, is a tool to ease ATC coordinations for departing traffic. For airports with established SIDs, it is common for the tower to be allowed to depart aircraft on the established SIDs, without explicitly coordinating (as in, picking up the phone) every single flight with approach control. Approach knows to expect traffic departing on the SIDs, and will keep those areas free of arriving or other aircraft. This saves a lot of time for both tower and approach, who would otherwise need to agree specifically on the departure instructions given to every single flight prior to departure.

Once you make it airborne, approach may well take you off the SID in order to expidite things for yourself and other traffic in the area. SIDs rarely give the most direct routing, as they are designed by default to be separated from other departure and arrival flows, military areas etc. If these restrictions are not relevant at the specific time you depart, a more direct routing may be possible. Or, maybe approach needs you off the SID because a following (faster moving) departure is coming up behind you.