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 weather 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 weather or operations at the airport change. Every time a new ATIS message is generated, the letter is changed.
Example of an ATIS message:
This is Schiphol arrival information Kilo, main landing runway 18Right. Transition level 50. Two zero zero degrees, one one knots. Visibility 10 kilometres. Few 1300 feet, scattered 1800 feet, broken 2000 feet. Temperature 15, dewpoint 13. QNH 995 hectopascal. No significant change. Contact Approach and Arrival callsign only. End of information Kilo.
Now when for example the temperature or pressure at the airport changes, the ATIS information is updated:
This is Schiphol arrival information Lima, main landing runway 18Right. Transition level 50. Two zero zero degrees, one two knots. Visibility 10 kilometres. Few 1300 feet, scattered 1800 feet, broken 2000 feet. Temperature 16, dewpoint 13. QNH 994 hectopascal. No significant change. Contact Approach and Arrival callsign only. End of information Lima.
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 state what version of the ATIS they have listened to.
Schiphol approach, this is KLM1234 with information Lima.
The Air Traffic Controller can then determine whether this is the latest version of the ATIS or if there is newer information available that should be passed on to the flight crew.
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.
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:
- Airport Name.
- Information letter (Alpha, Bravo, Charlie...).
- Time Hour/Minute in Zulu (GMT) when the observation was made
- Winds - direction, speed, and sometimes gusts, variability.
- Sky Condition - cloud cover (clear, few, scattered, broken, overcast) and altitude of the layer(s).
- Barometric Pressure - which is used to set the aircraft's onboard altimeter to an accurate reference for the airport it's approaching/departing).
- Other remarks, such as runway/taxiway closures, lighting or navigational aids down, etc.
In the USA, this is defined in the FAA's Air Traffic Organization Policy, Section 2-9-1. According to that source, ATC must:
(a) Identify each ATIS message by a phonetic letter code word at both the beginning and the end of the message. Automated systems will have the phonetic letter code automatically appended. Exceptions may be made where omissions are required because of special programs or equipment.
1. Each alphabet letter phonetic word must be used sequentially, except as authorized in subpara a2, beginning with “Alpha,” ending with “Zulu,” and repeated without regard to the beginning of a new day. Identify the first resumed broadcast message with “Alpha” or the first assigned alphabet letter word in the event of a broadcast interruption of more than 12 hours.