AIM Text
Aircraft operating on an IFR flight plan, landing at an airport without an operating control tower will be advised to change to the airport advisory frequency when direct communications with ATC are no longer required. Towers and centers do not have nontower airport traffic and runway in use information. The instrument approach may not be aligned with the runway in use; therefore, if the information has not already been obtained, pilots should make an expeditious change to the airport advisory frequency when authorized.
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-1-5.
The short answer
This AIM section is FAA advisory guidance explaining that aircraft on an IFR flight plan landing at an airport without an operating control tower will be advised to change to the airport advisory frequency once direct ATC communication is no longer required. Towers and centers lack nontower traffic and runway-in-use information.
Research Notes
Common Questions
Why does ATC tell IFR aircraft to switch to the airport advisory frequency before landing at a nontower field?
Per this AIM guidance, towers and centers do not have nontower airport traffic or runway-in-use information, so once direct communication with ATC is no longer required the pilot is advised to change to the airport advisory frequency to obtain that local information.
Does the instrument approach always line up with the active runway at a nontower airport?
No. The AIM notes the instrument approach may not be aligned with the runway in use, which is why pilots should make an expeditious change to the airport advisory frequency when authorized to learn the runway in use.
When should a pilot switch to the advisory frequency on an IFR arrival to a nontower field?
When direct communications with ATC are no longer required and the change is authorized, pilots should make an expeditious change to the airport advisory frequency if the runway-in-use information has not already been obtained.