Originally Posted by HazelNuts39
The text you copied from the Air Caraibes memo appears on page 5 in the context of the protection named "CAS MONITORING" which in the BEA reports is the initiating event at 2:10:05 of the whole 2:10 sequence and results in the PROBE PITOT fault message after closing the one-minute correlation window.
Right. There is two independant functions for monitoring the speed for essential systems which should be the first to detect probe errors (beside plenty of other systems self monitoring their own recieved imputs, just in case).
a) ADR monitoring at Auto Flight System (AFS) level (-> FMGC). This should also be the first one to react as the threshold is set at 0.45 seconds for a variation of 20 KT.
b) ADR monitoring at Electronic Flight Control System (EFCS) level (-> FCPC/PRIM) which is "CAS Monitoring" you refered to. Here, the threshold is a variation of 30 KT during 1.0 second. It is also responsible for changing immediately the Flight LAW to ALT2 when a variation is detected. Hence, it takes 1 second to change the flight law, but it's at first temporary. After this point, the last valid value (before change) is retained and a comparison is made with the current value 10 seconds later: if the difference is > 50 KT, ALTERNATE 2 is confirmed, whithout the possibility to revert to NORMAL until post flight reset.
Originally Posted by HazelNuts39
Page 7 of the ACA memo shows pictures of the PFD speed scale in Normal and Alternate law in the context of an ADR DISAGREE message with Vsw in the Alternate Law 2 case.
This illustration is taken from the FCOM and is generic for ALTERNATE if airspeed is not unreliable. In case of UAS, What you will have instead is the red flag "SPD LIM" (see report previous page) without Vmax, VSL nor VSW and... It will last for the remainder of the flight like ALT2! (read me well: even if the airspeed come back to normal after a while, this is lost as it is displayed only when ALTERNATE low/high speed protections could work, which would not be the case after an UAS. Even if coherent, speed will always be considered spurious by the system). That is also the reason why some long haul flights turned back and landed after an UAS event which was not transient (lasted more than 10 seconds).
Originally Posted by HazelNuts39
While it may be unimportant whether or not a calculated Vsw is indicated on the speed scale when IAS has dropped to 60 kt or less, the intriguing question for me is whether it would be shown in the intermediate period when one or more of the indicated speeds had become valid again. EDIT:: Why should Vsw not be shown along the speed scale showing a 'valid' speed, first on PFD1, later on ISIS, and at an unknown time on PFD2?
Quite simply, it is an EFCS function that compute all the characteristic speeds from data comming from the FE part of FMGC and
ALL ADRs. This function become inoperative as soon as more than 1 ADR is rejected by the FCPC. After this point, it will consider both FMGC (1 & 2) as inop regardless if the data will be later coherent again. Hence, the display of probe data (speed) on each PFD or ISIS is directly available for the crew but it is unrelated with the function computing the characteristic speeds which is lost. Those data displayed are not what the system could use as it will need two "valid" sources, at any time, for safe operation.