What are the mandatory elements of an ICAO flightplan?
About the special characters at the beginning or in the body of the form: They are:

Formatting characters inserted when sending the FPL with a teletype interface on the AFTN network. e.g. &lt; is a carriage return, = is a line feed

Routing indication, e.g. FF is the priority level for a message of type FPL.


These characters are not part of the flight plan data of course.

Parser assumptions

All fields from 7 to 18 are mandatory and included in a Filled Flight Plan message (FPL). Being mandatory doesn't prevent some field to be filled with some dummy value, e.g: Field 18 may contain &quot;0&quot; to indicate no information is provided.

Field 19 can have survival items and remarks left blank. Field 19 is not part of the FPL message, but of the SPL message. The latter is transmitted to other ATS units by the ATS unit having registered the flight plan only in specific situations, e.g. when the contact has been lost with the aircraft (ALERFA).


ATS consistency
While they are mandatory, some fields are used only in specific conditions, in particular the ones related to navigation capabilities.

Example: Not all approaches or routes are PBN required. Not mentioning some navigation capabilities may not be a problem if the capability is not required by an ATS unit to provide the desired routing, but if you use a RNAV approach, the flight plan will be rejected.

For more information on the combinations that are consistent, see the interactive Eurocontrol Flight Plan Guide for IFPS (Integrated Initial Flight Plan Processing System). Example:


When PBN is specified in the item 18 of a flight plan, it indicates the Performance Based Navigation (PBN) levels and/or the Required
Navigation Performance (RNP) levels that can be met. It can only be
present if the aircraft is PBN approved which shall be notified by the
letter R in the equipment of that flight plan.



**Filling instructions**
The content of the ICAO flight plan is detailed in appendix 2 of ICAO Doc 4444 (PANS-ATM) and a filled sample is provided (page 361), but filling instructions are also available separately, e.g.:

ICAO Flightplan Form Basics
Flight Plan Filling at SKYbrary (Eurocontrol)

Note that FAA and ICAO flight plan forms have been different in the past, and while FAA is pushing for the adoption of the ICAO form, it seems the FAA form is still used for US domestic flights.