Posts about: "Digital Flight Data Acquisition Unit" [Posts: 9 Pages: 1]

But as previously posted a recorder is only as good as the systems that provide the data to it. If those systems, or some of those systems, are not powered the data is simply not available to be recorded. You need the DFDAU (or equivalent) to be powered and you need the systems that feed data to the DFDAU (or equivalent) to be powered and operational.

Edit to add - RIPS will likely maintain CVR function.

Correct, on the 787 it's the forward EARF that has the RIPS, and it will continue to record inputs from the CAM.

I'm pretty sure that within the 787 data is passed over the serial data protocols to a DFDAU - Digital Flight Data Acquisition Unit where it is stored as a form of database. Quite possibly XML

Any device subscribing to the AU will be getting the same information. This would include the DFDRs and whatever is used for ADS-B

Happy to be corrected

I am not aware of any requirement for a DFDAU (or equivalent) to store any data. I say "or equivalent" because in B717 the DFDAU is not an LRU. It is a functional partition of the VIA.

It's not clear to me that 787 EAFR even requires an external DFDAU. The GE EAFR does not -

"Provides Flight Data Acquisition function of ARINC 664 p7 data parameters – No need for a Digital Flight Data Acquisition Unit (DFDAU)."

ref https://www.geaerospace.com/sites/de...rder-3254F.pdf

In this case, there is still an AU - it's just integrated into the EAFR.

An unresolved difference in TLA between the channels is quite unlikely - the fault detection algorithm is quite good (sine squared plus cosine squared) - but it can happen. In the old days, we'd default to the higher TLA, but since the feds became preoccupied with Uncontrollable High Thrust, we tend to select the lower value. Worst case would be to default to idle. The fuel switch discrete doesn't really get used except for engine start - if it falsely indicates shutdown (on one or both channels), the FADEC won't do anything if the engine is already running. All this will set maintenance faults - and associated EICAS Status messages (L/R ENGINE CONTROL or ENGINE C1). I doubt that would be recorded on the DFDR - it would go to the QAR but that's unlikely to survive a crash. It would also be logged in the FADEC NVM - but again no guarantee that would survive either (although when the Lauda 767 crashed due to the thrust reverser deployment, the DFDR was destroyed but the FADEC NVMs both survived - much of what we know about that crash came from the FADEC NVM.)

Again, not familiar with the specifics of the 787, but on the 747-400/-8, one pole of the fuel switch feeds EICAS - which uses it in various message logic - and sends it out to any other aircraft systems that use it. There is "Digital Flight Data Acquisition Unit) DFDAU (pronounced Daff Du) that takes all the various system digital signals, sorts them and provides them to the DFDR and QAR. The 787 has something similar to the DFDAU but I don't recall what it's called.

It was stated earlier that 2 of the poles in the switch are wired to separate Remote Data Concentrators. The RDCs feed data into the Common Data Network (essentially an Ethernet LAN).

The EAFRs simply sniff the required data from the CDN, so there's no DFDAU as such.

For some parameters, including the fuel cutoff switches, the EAFRs also record a "Source Index." For the fuel cutoff switches, there are 4 sources numbered 1 through 4 as well as a valid/invalid flag. I don't know what the sources are, but perhaps they're the 2 RDCs plus the 2 channels of the EEC/FADEC. It appears that source 1 (the default) is not the EEC.

787 has RDC's - Remote Data Concentrators. Doing the same function. Two of the switch pole-sets go each to one of 2 different RDC's, that feed the EAFR's / QAR, and the common core network so that any system that wants to know, can. The wiring is positive voltage from the RDC's to the switch and to GND through the common pin. So the RDC's would be able to detect anomalies such as both contacts open, or both contacts closed. The EAFR will see two independent channels per switch.
The other 2 contact sets feed latching relays (again driven each coil independently by dropping to GND at the switch) that then drive spar valves and reset signals to the FADEC Channels. (and more).

So - for the benefit of those that hang on to the 'possibility' of electrical gremlins and 'ghost' switch signals.

Each switch has 4 mechanically separated 'channels' - 2 of which do electromechanical things to the engines through separate paths, the other 2 feed through independent paths the FDR and the rest of the computer systems. The results of the electromechanical actions also feed back to the FDR.

Thus the readout from the EAFR will PROVE that the switches MUST have been PHYSICALLY MOVED.

And - I'm nervous to challenge you tdracer - there's quite the authority gradient and I know I'm at the bottom end, but I can't let this fly...
Its 'Deefer Doo'. Fight me.

This has been discussed to death previously. I suggest looking for tdracer's excellent insights into FADEC design and implementation. From the beginning we've been discussing various very rare types of circumstances. Based on the preliminary report we can now lay most of those to rest.

Which actually brings me to this one because I would like to ask for a bit of clarification: By "fuel switch discrete" are you referring to the Fuel Control Switches discussed in the preliminary report?
I would assume from your statement, that if a mismatch in the NC/NO signal on the switch was detected the FADEC would not direct the Fuel Cutoff Valves to close (as far as the types you are familiar with are concerned), is that correct?

The report states:
So this would then mostly preclude the possibility of one or both switches being faulty electrically.

The report then states:
There is a 10 second gap between cutting fuel and re-enabling it and a 4 second gap between switches during re-enabling. Is there a mechanical reason why these switches would be slower to operate in either direction? There are obviously reasons such as startle factor and stress that might negatively affect the speedy operation of switches by anyone, but I am nonetheless curious if this might not be a pointer to some sort of mechanical issue after all, such as asymmetric wear or FOD.