Mandatory removal of unapproved avionics - NavWorx box

Peter wrote:

It looks like Navworx didn’t quite get it certified. Probably, it would seem, the FAA got really upset by something they did…

No, the problem is that the box is getting its quality indicators wrong after a software update. The GPS source can be uncertified, but if so the transmission has to say it’s based on an uncertified position (SIL=0). Instead, it is sending SIL=3, implying that the source is a certified GPS source. I’m surprised it can’t be addressed in software though.

The GPS position source needs to comply with one of the TSO’s that are acceptable to the FAA or it must meet the requirements of the TSO. The FAA is claiming that the data provided by NavWorx does not establish that the non certified GPS position source meets the requirements of an acceptable TSO. The box was certified with a SIL of 0, which means it can’t be used for meeting the 2020 mandate which requires a SIL of 3. NavWorx submitted a change to their box claiming that updated software was a setting the SIL to 3 was a minor change, the FAA did not agree and claim it is a major change. NavWorx went ahead without approval and changed it anyway. The FAA has issued a notice of a proposed AD.

One of the reasons why NavWorx appears to have gone ahead anyway was because of a change in how the ground stations provide services that occurred the beginning of this year. The FAA had been giving client status to these and other units with a SIL of 0, which meant that the ground station provided TISB services to these aircraft which is a ground station service that broadcasts the locations of mode A/C aircraft in the vicinity of the client to fill in the traffic picture. They also provide ADSR services to clients where the ground station rebroadcast equipped ADS-B out aircraft on the other frequency. So this is a UAT unit with only a UAT receiver, so it can’t receive air to air any aircraft on the 1090ES frequency. ADSR rebroadcasts the 1090ES on the UAT frequency. That way the UAT equipped aircraft can be able to see via air to air any UAT equipped aircraft and via ground station TISB (Mode A/C) and ADSR (Rebroadcast of 1090ES) to see a complete traffic picture in their vicinity.

The problem is that TSO C195B RTCA DO-317B certified systems are not permitted to display traffic that has a SIL of 0. So the Navworx box is invisible to my certified system, a GDL88/GNS530W. To solve this, the FAA made a change to the ground station software to treat a SIL=0 as not being a valid client and can’t get the TISB and ADSR services. The ground station will treat the SIL=0 as if it is not ADS-B Out equipped and will generate a TISB for such an aircraft, making it visible to certified systems. This change protected certified systems but removed services from SIL=0 systems. Customers of NavWorx must have complained and NavWorx went ahead with the change to set the SIL to 3. If the ground station change never took place, the NavWorx users would have been fat, dumb and happy, at least until 2020 when they would discover that they did not meet the mandate and would be excluded from the relevant airspace, as a SIL of 3 is called out in the regulation.

NCYankee wrote:

NavWorx submitted a change to their box claiming that updated software was a setting the SIL to 3 was a minor change, the FAA did not agree and claim it is a major change.

I would agree. You can’t just retroactively raise the SIL level of software. You have to essentially redo everything from scratch. (Unless, for some unfathomable reason, your initial development did follow SIL 3 standards with all the necessary processes and documentation but you just didn’t bother writing up the safety case.)