In order to conduct a flight that requires a flight plan, this flight plan needs to be made available to a list of aeronautical stations. The network over which flight plans and associated messages are transferred is called AFTN (Aeronautical Fixed Telecommunication Network). Every aeronautical station is identified by a unique address which follows a naming convention.
Each country defines the rules for flight plan distribution and published them as section ENR 1.11 in the AIP. Unfortunately, there is no standard on how flight plans are addressed and also no defined format for publishing the rules and every country uses different wording. In addition, the information in ENR 1.11 can be amended through the usual means, down to individual NOTAMs providing additional addressing instructions.
When filing a flight plan, a list of addresses has to be passed together with the flight plan. For pure IFR flights within the Eurocontrol area, this is simple as there are only two addresses:
Eurocontrol FP1 in Haren/Brussels, Belgium
Eurocontrol FP2 in Brétigny/Paris, France
Eurocontrol takes care of distributing the flight plans to the affected sectors.
When a flight plan is not entirely within the Eurocontrol area or contains a VFR segment (such as Z and Y flight plans), Eurocontrol will not do the addressing and the submitting party has to provide a list of addresses and is responsible for completeness and correctness. If an address is missing, there can be problems in flight with ATC units not knowing about the flight, not approving FIR crossing, etc.
To ensure correct flight plan addressing, the software needs to have knowledge of all addressing rules as set out by the countries in their AIP and the relevant amendments. This means that the information needs to be extracted and reformulated in a standardized and machine readable format. Such a format did not exist and was therefore created by the autorouter team with the goal of becoming the standard source of information which is used and maintained by all interested parties.
FPAL – Flight Plan Addressing Language
A descriptive computer language called FPAL was created. It allows expressing addressing rules which are parsed by a computer program and then matched against a flight plan. The result of a rule can be one or more AFTN addresses the flight plan has to be sent to.
FPAL is designed to be easy to use and efficient while at the same time supporting all possible complications of individual addressing rules defined by the countries. Addressing can depend on many things, among them:
- airspace/FIR penetrated
- flight rules (VFR / IFR)
- departure and destination aerodromes
- day of the week / time of day
- aircraft mass
For airspace definitions, FPAL relies on Eurocontrol’s ADR (Airspace Data Repository). Airspaces are referred to by the identifier found in ADR. Therefore, ADR is a requirement to evaluate the rules but it is available from Eurocontrol under a free of charge license.
In addition to the FPAL definition, we also offer access to the current addressing rules for which we are the initial contributor and maintainer.
As a service to the aviation community, our reference implementation contains a flight plan checker that returns the AFTN addresses for a given flight plan. This service is available free of charge as a RESTful service via HTTPS.
Both the language definition and the addressing data is offered under the Creative Commons Attribution-ShareAlike 4.0 International license. This license is very liberal and allows you to whatever you wish with the information. There is only one requirement: if you make changes to the information, you have to make it publicly available under the same license. This ensures that everybody plays in the same team and together the stakeholders work on the common goal of maintaining an up to date and correct flight plan addressing database.