Airbus 320 A/THR question.
 

Hello all,

I ve tried looking into the airbus Fcom and other tech sources for this question but I simply can't find the answer so was wondering if anyone could help.

In the cruise the auto thrust fails. When AP1 was active. To regain the auto thrust you select AP2 which I understand uses the other auto thrust channel so you now have AP2 and auto thrust active. My question is if on clearence for an ILS and I select AP1 as per the norm SOP will AP1 re engage and I m able to get Cat 3 dual or would either AP1 not engage or AP1 engage but because I ve lost an A/THR channel I am only able to get Cat 3 single?

In my company QRH for a cat 3 dual approach it states A/THR required 1. Does this mean 1 channel of auto thrust or simply auto thrust working.

Any help much appreciated.

Without getting out the manuals and from experience I would say:

Cat3 single only, don't engage both APs for approach otherwise you would be Cat2 (no Athr)

Also sim testing thing: if on LVO approach both AP engaged and Athr fail, just press AP1 to disengage it - back to Cat3a

From memory I think that the selection of AP1 on ILS approach when AP2 is already engaged makes Ap1 the master and if Ch1 of the athr has failed the landing capability becomes Cat 2. You have to deselect AP1 and re-engage Athr and you get Cat 3 single.
It appears that tom has edited his post with additional info whilst I was writing mine.

Dear tubby linton:

From our responses, any one would think we work at the same airline, with similar sim scenarios ;)

You may think that but I couldn't possibly comment!

Mooneyboy
The answer is in auto flight chapter master FMGC logic. When both APs are engaged FMGC1 becomes the master and controls ATHR, Since FMGC1 ATHR channel is not available you loose ATHR and landing CAT degrades. It is good enough for CAT2 but for CAT3 you will have to use only AP2 which can use ATHR second channel.

AP1 becomes the master when both are engaged. I often realise this because while flying on AP 2 and I engage AP 1 whilst in the turn to intercept the ILS, I get a momentary inappropriate yaw as AP1 takes over from AP2.

Thankyou for all your replies. It would be sometimes nice to have a sim to try these things out. It does make sense now that if AP one is re engaged you ll lose A/THR and a/c now cat 2 so better leaving AP2 to leave cat 3 single. I was focusing on the auto thrust chapter in auto flight section so that's why I missed the FMGC logic section.

So this should mean if AP2 is engaged in the cruise and for no obvious reason status page then shows cat 3 single it could be the autothrust channel failed for AP1.

ATHR chanel one can only fail if AP one is engaged and not when AP2 is engaged.

Mooneyboy doing your tech quiz by any chance? Both auto pilots on means FMGC 1 is master. A/THR by channel one which is failed so cat 3 single.