How do I contact a Flight Service Station (FSS) in the United States in order to receive weather information or open a flight plan after I've already departed.
For filing and opening VFR flight plans, pushing back your ETA because of a delay, and a few more obscure things which you can find in section 4-1-21 of the AIM you would contact Flight Service (addressed as "region Radio" - e.g. "New York Radio" or "Bridgeport Radio")
The universal flight service frequency is 122.2, and above 5000 feet you should be able to get someone on that frequency, but it's not guaranteed.
If yo can't raise someone on 122.2, or you're below 5000 feet, there are local Flight Service frequencies are listed on VORs as shown below. As a general rule if you can receive the VOR you can talk to Flight Service with the frequencies listed, and using these local frequencies helps avoid congestion on the universal frequency.
Initial calls to Flight Service should take the form:
<Facility Name> Radio, Cessna 12345, 5 northeast of Hampton VOR
(as above for Flight Watch, Who you're talking to ("New York Radio" in the example above), Who you are, and Where you are.)
There's a little catch with Flight Service frequencies: If there is an
R after the frequency (as shown for 122.1 here) it is a receive-only frequency. You will talk to flight service on that frequency, but you will listen to the VOR's audio channel to hear Flight Service's responses.
When you call Flight Service using one of these frequencies make sure your nav radio is tuned to the VOR and the voice channel volume is up, and let them know which VOR you're listening to as opposed to your location.
In this example if you were contacting Flight Service on 1221 the call would be
New York Radio, Cessna 12345, receiving on Hampton VOR.
For weather (giving PIREPs or asking for a weather report) you will also talk to Flight Service: There used to be a separate service (Flight Watch) and frequency for this, but weather-related tasks were consolidated into the general Flight Service frequency in late 2015.
Radio coverage and contact procedures are identical to what was mentioned above for dealing with flight plan stuff, but you should make it clear you're looking for weather updates (add "requesting enroute weather") or that you want to give a PIREP (add "With a pilot report") so the Flight Service Specialist on the other end of the radio knows what you're looking for.
As weather is generally a localized phenomenon (and weather updates or PIREPs can take some time to relay) it is probably be a good idea to use the local flight service frequencies for weather-relatedrather than the universal 122.2 frequency to avoid congestion there, though I don't believe the FAA or Flight Service has issued any official guidance on this since Flight Watch was consolidated with the rest of Flight Service.