SVARIN:
This means Normal law is preferred over any inferior laws. This is the very core of this flight controls design.
Quote:
If PRIM 2, in your view, is in control and able to deliver NORMAL LAW this means that PRIM 1 was not able to compute NORMAL LAW protections
Quite right, except PRIM1 is not faulted before 02:13. Which PRIM becomes master if PRIM1 is ALT2 and PRIM2 is Nz ?
Quote:
Both NORMAL LAW as ALTERNATE LAW are Nz laws, to keep it ‘simple’ you mean PRIM 1 is NOT able to compute NORMAL while PRIM 2 is capable.
Then I agree with you PRIM 2 will be in control.
And actually this happens once in a while in normal flight, our crew reports in such a case: 'Alternate Law message was transient'.
F/CTL ALTERNATE LAW did appear on ECAM and after a while (when system reverted to NORMAL again) the message disappeared from ECAM.
Technically speaking ‘ALTERNATE LAW was triggered at the start of the monitoring window and at the end the monitoring window the system reverted to NORMAL again’
A correlated failure, which triggered the ALTERNATE LAW may be present in CMC, maintenance action however is not required when crew reported ‘transient’.
When the system later on triggers an ALTERNATE LAW again the F/CTL ALTERNATE LAW warning will be set again with the correlated fault message at that time.
Back to AF447,
If PRIM 2 reverted to NORMAL t
he NAV ADR DISAGREE would set ALTERNATE LAW a 2nd time (at the start of the monitoring window) and that did never occur because system was already in ALT 2 and PRIM 2 NOT in NORMAL!
What we know is that F/CTL ALTERNATE LAW was triggered after (PITOT ICING) and latched to ALT 2 (due missing speed information) at the time of F/CTL RUD TRV LIM FAULT and confirmed by frozen position of Rudder Travel Limiter.
This mode is latched – for the remainder of the flight - to prevent cycling between the modes.
From the BEA report:
Like the FMGECs, the PRIMs consolidate the parameters that they use by
means of monitoring mechanisms.
Concerning the airspeed, it is the voted value that is used.
In normal operation, this is the median value.
When one of the three speeds deviates too much from the other two, it is automatically rejected by the PRIMs and the polled value then becomes the average of the two remaining values.
But if the difference between these two remaining values becomes too great the PRIMs reject them and the control law switches to alternate 2.
Furthermore, another monitoring procedure is applied to the value of the voted
airspeed and triggers switching to alternate 2 law when it falls by more than 30 kt in one second.
In my view, at that time PRIM 1 was in control, PRIM 2 and 3 couldn’t compute NORMAL LAW either.
Due to the PITOT problem they had all the same ADR information.
With all due respect to your extreme technical knowledge, may I introduce this disagreeing parameter ?
WRG:ADIRU1 BUS ADR1-2 TO FCPC2
This means PRIM2 does not have the same ADR set available to it than the other two PRIMs have.
Agree, but doesn’t necessarily mean this set gives a more reliable outcome to the median speed calculation. On the contrary 3 ADRs is more reliable, as long as you can speak of reliable in this case, than 2 sets of signals.
I can’t find it in the documents but it could be that the logics take the amount of available ADR signals in account.
Then for this Maintenance message:
The problem area:
As preface on my argumentation keep in mind, EFCS is the complete Electronic Flight Control System and consist of FCPC1, 2 & 3, FCSC1 & 2 and FCDC 1 & 2.
When a failure is detected, PCPCs and FCSCs take operational reconfigurations and sends failure information to FCDCs, FCDCs analyzes the failure and after confirmation generates a maintenance message. The message source will be EFCSx(FCDCx) as will be the identifier EFCSy(FCDCy).
If both FCDCs are available, FCDC1 sends external and internal (ATA 27 ex S/F) failures while FCDC2 sends all failures as external to avoid double storage within CMC.
ADIRU failures are considered as external failures, however because only FCPC2 reported this failure it is rated (ATA27) internal.
Furthermore FCPC 1, 2 & 3 are interchangeable units (as long as they have the same P/N and OBRM*) only their position in the racks determines if they act like FCPC 1,2 or 3.
*OBRM = On Board Replaceable Module
There are extensive BITE* tests available in the system, the logic however is not detailed.
(Wrap Around, SSM, parity and refresh rate tests may be expected within BITE)
*BITE = Build In Test Equipment
A is not B,
In A. a wiring failure is suspected only, if BITE analysis has revealed its fault. AMM states: “As soon as there is more than one component (more than one part) in the message, A/C wiring can be suspected but it is not indicated in the messages except if it is
clearly identified as being the origin of the fault.”
The failure messages in EFCS are always declared as HARD, even when it was INTERMITTENT (t<=2.5 s) or HARD due to several INTERMITTENTS during flight.
Based on this it should be considered as wiring fault i.s.o software flaw(OBRM) but can't conclude if it was of any influence (HARD) or not (INTERMITTENT) in law reconfiguration.