The NATS CEO indicated this morning that a piece of the system (which has to be the FPPS) failed because it didn’t recognize a message, which was almost certainly an FPL.
People are questioning how a “bad” FPL came to be accepted into the FPPS. It is important to recognize that an FPL has syntax (format) and semantics (meaning).
If the syntax is correct, it is a valid FPL. By far the most complex element of an FPL is the route. The other elements are just parameters that are checked for validity. For example, if aircraft type is stated as “C172” the FPPS checks this against a list of valid aircraft types in its database.
The route syntax is checked to make sure the expression follows the rules of how the route elements should be constructed. Whilst there are many different types of route element and many rules to follow, this checking is relatively straightforward. If a “bad” FPL in terms of format is recognized it will be rejected at this stage. If not, the FPL will be passed to route conversion, where the semantics are extracted.
This is why I can’t see how the statement “it didn’t recognize a message” could lead to a processing failure. If it didn’t recognize a message, it would be rejected at this stage – business as usual. I can only assume the message was recognized as valid and passed-on.
The FPPS must now work out what the actual route is within its airspace (the semantic meaning), and this is the really difficult bit. There is an infinite number of possibilities. For example, route fixes can be expressed as lat/long coordinates which could be literally anywhere. The programme works out what it needs to do, in terms of outputting information to controllers and adjacent centres, and my guess is that this is the source of the problem that caused the FPPS to crash.
The programme came across an unusual route it had not encountered before (and had not been programmed to expect), didn’t know what to do, and a graceful recovery was not available. In other words, encountered a bug and did something unpredictable.
Just my guess.
Last edited by CBSITCB; 30th Aug 2023 at 11:30.
Reason: Typo.