There are many issues, such as the criteria, differences between manufacturers, and the coding. It is interesting to see that there is quite a bit discussion on how the coding and FMC actually perform.
There are the differences between armed and engaged, and the flow matrix/sequencing for engagement. The code and criteria have not kept up with the more complicated or the advanced procedures.
As examples, while some systems let you arm on the ground, others do not, and some engage on wheels up and some at 500 feet.
Technically, with an approach, there is the initial segment, one intermediate segment, and a final approach segment. With RNP, there are frequently more than 3 segments to an approach procedure.
In the beginning, some systems would disco if you had more than one intermediate segment, or had a turn after the FAF. The first RNP procedures into the airports in China were done with Airbus, as the Boeing Smiths box would not accept a RWY over 10,000 feet.
Currently working on the RNP transitions to GBAS final, one can see just how difficult the flow management is with all of the if/then and arm/engage scenarios.
As a procedure designer, this is a very interesting thread to review, in fact, many of the threads have been used to show what the operators are actually experiencing and/or how they got there.
The certification process of aircraft avionics has kept the FMS stuck in the mid 80's, with 'new' versions simply being addendum to the coding with add ons, rather than re-writes. As you can imagine, this leaves many paths, and many potential scenarios that are very difficult to desktop/sim or even flight verify other than by thousands of different operators, hence the value of these types of conversations.
Cheers
Last edited by underfire; 5th Jun 2016 at 23:15.
Reason: oh jeeez, am I 51 now?!?!?!