When listening to ATC broadcasting (at least for ULLI), phrase
"Information <Charlie|India|whiskey|etc.> on board" may be heard. Charlie, India, Whiskey, etc. refer to "C", "I", "W" but what do these codes mean? What kind of information is this?
This is a simple versioning system used in ATIS (Automatic Terminal Information Service). ATIS messages contain information about the wheather at the airport, runways in use etc. This information is transmitted on a dedicated radio frequency and repeated in a loop. The ATIS messages are updated frequently as the wheather or operations on the airport change. Every time a new ATIS message is generated, the letter is changed.
Example of an ATIS message:
Now when for example the temperature or pressure at the airport changes, the ATIS information is updated:
The flight crew will listen to the ATIS often before they are in contact with the Air Traffic Control of the airport. They note the information in the ATIS, together with the information version letter.
On first contact with the airport's Air Traffic Control, they will then state what version of the ATIS they have listened to.
The Air Traffic Controller can then verify this is the latest version of the ATIS, or if there is newer information available that should be passed on to the flight crew.
In the US, ATIS info is typically updated hourly - unless something unusual/unpredicted happens that is relevant to approaching or departing aircraft. In the US, ATIS broadcasts include:
The letters refer to ATIS information, which stands for Automatic Terminal Information Service. ATIS messages are broadcast on a loop on a separate frequency from approach and tower frequencies and inform pilots of weather conditions, active runways, runway conditions, and other important data. Each time the information changes a new recording is made and the version letter increments.
Pilots listen to this information before contacting the approach/tower frequency so that the controllers don't spend their entire day reciting it over and over again.