AP/FD off during TCAS RA - A320

[Oakglade]

Hi there, apologies if this has been posted before.

Can someone explain the reasoning behind AP/FD 'Off' when responding to a TCAS RA on A320 family? AP "Off" I can understand, but FD "Off" is something I'm having difficulty with. FCOM appears devoid of explanation however any references would also be appreciated.

Thanks in advance.

[Cough]

Forces A/THR into SPEED mode (prevents hi/low speed issues) and also we are very focused on following F/D's which may be leading you in the wrong direction.

[Cough]

From FCTM SI-60

• The PF disconnects the AP, and smoothly and firmly follows the Vertical Speed Indicator (VSI) green sector within 5 s, and requests that both FDs be disconnected.
Note: Both FDs must be disconnected once APs are disconnected:
- To ensure autothrust speed mode
- To avoid possible confusion between FD bar orders and, TCAS aural and VSI orders

[Oakglade]

Cough,

Many thanks for the reply, very informative and appreciate the FCTM quote. I did a recheck of the FCOM (DSC 22) and found some additional helpful info about the Autothrust response.

Regards.

[Fly3]

Imagine you were in a climb with auto thrust and autopilot engaged and FD's on. You then get a TCAS RA to descend. You disconnect the autopilot and pitch down but leave the FD's on. Thrust would remain at climb power and an overspeed could occur very rapidly. As already mentioned, if both autopilot and FD's are disconnected then SPD is always the thrust mode and you get some protection from both under and overspeed.

[A4]

In addition to Fly3's descriptor picture this.....

FO is PF (AP2) and you're descending in OP DES (i.e. Thrust is idle with speed controlled by Elevator). You now get a TCAS CLIMB command. The FO disconnects AP2 (FMGC1 now becomes "Master") and only switches his FD(2)off.

You now have a situation where the FO is following a TCAS climb by pitching up to keep the v/s in the green, but the A/THR will remain commanding IDLE thrust due to FD1 being on. So the aircraft is trying to climb with IDLE thrust and will loose speed rapidly.....ALPHA FLOOR will eventually kick in.

So the GOLDEN RULE is only ONE person (PNF) switches the FD's on or off and when they do it's always BOTH FD's to ensure A/THR goes into SPEED mode.

[Oakglade]

Ok, some good examples there and helpful to visual the scenarios and outcomes. I've got a far better understanding of the system now.

Thanks A4 and Fly3, appreciate the additional info.

[CONF iture]

So the GOLDEN RULE is only ONE person (PNF) switches the FD's on or off and when they do it's always BOTH FD's to ensure A/THR goes into SPEED mode.
Now for the 380 they apparently decided to go for a single Push Button to deselect both FDs at the same time.

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