TCAS/REAC W/S DET inoperative in case of RA 1+2 FAULT on A320
 

can anyone explain why TCAS and REACTIVE WINDSHEAR DETECTION fails in case of RA 1+2 FAULT on A320 ?

Thanks in advance

Because the TCAS commands and sensitivity levels are different depending on how high you are off the ground, and below 2350' it uses one RADALT. Above that it's above MSL.

Similarly the PWS mode is active below 2300' radalt..

Thanks.

PWS is not inop in dual RA. It is reactive w/s detection. why that would be inop?

Because Reactive Windshear warnings are only available below a given height agl. And the RA is used to measure that.

so does PWS warnings. but PWS is NOT inop during Dual RA. any thoughts?

The windshear detection function is provided by the Flight Augmentation Computer (FAC) in takeoff
and approach phase in the following conditions:
‐ At takeoff, 3 s after liftoff, up to 1 300 ft RA
‐ At landing, from 1 300 ft RA to 50 ft RA
‐ With at least CONF 1 selected.

No RA, no party.

PWS is a function of the Weather Radar, not connected to RAs.

The reactive wind shear, on the other hand:

It almost certainly is inop, but if the ECAM had to show the true list of inoperative systems following dual RA failure you'd still be reading them a week later.

It is not mentioned as an INOP system neither on the ECAM nor in the FCOM, hence it is very unlikely that it will be inop. As far as I remember the PWS takes position information from the EGPWS/T2CAS

The PWS even has an amber memo on the E/WD before take-off. Fursty's argument accepted in general, hiding that particular one from the INOP system does not sound realistic.

Checking the FCOM, my assertion about PWS not being connected to RA (in line with sonic's reasoning) is not true.
Possible solution? In the absence of valid RA signal, the PWS defaults to ON.

Yep very true, but this is for newer MSNs / non retrofitted ones, for older ones :

The weather radars have a Predictive Windshear System (PWS) that operates when:

‐ The PWS switch is in the AUTO position (Even if the weather radar is OFF), and
‐ The aircraft is below 2 300 ft AGL, and
‐ The ATC is switched to the ON, or AUTO, or XPDR, or XPNDR, position (depending on the ATC
panel), and
‐ Either engine is running.

radio height is not mentioned, nor it is mentioned in any other part of the FCOM whereas everything related to the PWS is mentioned as "AGL". My assumption is that the system takes its vertical information from the terrain database, not the RAs. On the other hand I had once engineering resetting both RA C/Bs after a weather radar malfunction during taxi-out, and that fixed it. Did not have enough time back then for a thorough explanation and later forgot about it... so it could indeed have something to do with the PWS too, but then why not list it as inop ?

older FCOM arrangement DSC-34-60-20-00006420.0005001 / 16 MAR 15 [MSN 271x to 471x]
The aircraft radio height is between 0 ft and 2 300 ft, and
(no mention of XPDR)

DSC-34-SURV-30-20-00020942.0004001 / 17 MAR 17 [oldest MSN 247x, the rest up to 691x]
the aircraft radio height is below 2 300 ft, and
(no mention of the XPDR)

DSC-34-SURV-30-20-00020942.0003001 / 17 MAR 17
WXR/PWS sw is in position 1 or 2 and the aircraft radio-height is below 1 800 ft, and
(no mention of the XPDR)

DSC-34-SURV-30-20-00020942.0001001 / 17 MAR 17 [newest MSN 319x, the rest down to 073x]
‐ The aircraft is below 2 300 ft AGL, and
‐ The ATC is switched to the ON, or AUTO

Confusing enough to me.
seem to remember that on the previous type PWS was only available on aircraft that came with Egpws from the factory, not the retrofitted ones around the time of the mandate. But could be imagining things.