FLIGHT DIECTOR push buttons A320

[bflyer]

Hi everybody

Why is it that BOTH flight director push buttons on the A320's EFIS panel must either be both ON or both OFF?
Why is it that the aircraft should never be flown with only one flight director selected ON?
thanks for the anticipated informations

[NigelOnDraft]

Suggest you research the Bangalore A320 accident... I think this was the basis of it

[Shot Nancy]

Suggest you research the Bangalore A320 accident... I think this was the basis of it

True.
There were a few accidents where Airbus reacted my modifying various flight modes or MCP/FMA indications.
When the A320 was first certified only turning both FDs OFF activated the A/THR SPEED mode. Turning one FD OFF removed the FD from that PFD but the active A/THR mode was as determined by the active FD. If you didn't follow the FD commands you could, and some obviously did, get into trouble. Now turning a FD OFF does not remove FD commands and appropriatre modes remain displayed. Turning both OFF forces A/THR to SPEED, neither FD is displayed and A/THR will target the selected or managed speed and therefore provide some form of protection.

[Dan Winterland]

Which is why the FDs are switched off when responding to a TCAS RA. You need the speed mode for protection when flying manually.

[ChristiaanJ]

Eastern 401 (1972, Everglades, Florida) ring a bell with anybody here?

I remember tearing into the mode selection logic on Concorde within months after the accident, as soon as we had some of the preliminary info.

[Dream Land]

What does Eastern 401 have to do with FD PB's on the Airbus?

[SIDSTAR]

Switching both FDs OFF, puts the autothrust into speed mode as Dan says and it also automatically changes to SELECTED speed (your current speed) and brings up the blue HDG target on your PFD.