A320 protection speeds

[ggofpac]

Hi all. Can someone shed some light on the protection speeds on A320 plz ( in normal law)?
I'm still confused even after reading the fcom.
IN Normal Law, A/P on , Alt Crz mode ( or Alt it doesnt matter) , if the Thrust levers are pull back to idle.....
1) Speed decrease below VLS nothing happens = Correct?
2) Next speed is V alpha prot?= correct?
3) A/P auto disconnect before it reaches V alpha prot? = Correct?
4) Assuming no sidestick input from pilots, a/c descend to maintain V alpha prot speed? or?
5)Will it go below V Alpha prot ? in which case alpha floor activated ( assuming no sidestick input- a/c left on its own )


Many thanks. ( so much to read.......)

[rudderrudderrat]

Hi ggofpac,

1) Correct
2) Correct
3) In the sim - the A/P will disconnect a little way into V Alpha tiger's tail and aircraft will nose down a bit to fly stick free at V alpha prot.
4) Correct
5) From what I've seen - if you now pitch up, so that the speed trend shows that you'll hit V Alpha Max in "X" seconds - alpha floor is activated with TOGA Lock.

[Port Strobe]

For some expansion on your points, in the example you give then pulling the thrust levers back to idle will disconnect autothrust obviously, so Vls won't have any great practical implication in that phase of flight for by definition it's merely the lowest selectable speed with autothrust on. Speed then further decays down the amber band towards V alpha prot, where the sidestick now commands changes in alpha. Given the autopilot is still in ALT mode then it'll keep pulling back until alpha reaches alpha prot plus one degree, at which point the autopilot will disconnect. If the stick is left alone, as far as I understand it, the "aft stick input" made by the autopilot since speed decayed below V alpha prot will now be released and IIRC auto trimming will also have ceased then so the aircraft will nose over to maintain V alpha prot which is the trimmed state. To further reduce speed then aft stick must be introduced. As long as autothrust is serviceable then it'll intervene in alpha floor mode (not entirely certain what it's trigger is, think it depends on pitch rate amongst other things) and you'll get into TOGA lock. However if you disconnect autothrust at the onset of alpha floor then you should be allowed to fly merrily down to V alpha max. Any further aft stick input will be ignored essentially and the aircraft descends at V alpha max. Willing to stand corrected of course on any of that but that's how I understand it.

[ggofpac]

Thanks so much :)

[killer.a]

Ok, lets see if I understand this with a slight change in scenario.

1) With auto thrust active and you pull the stick back, once the speed reaches VLS, auto thrust will cause the speed to maintain VLS even with the full aft stick pressure? Is that correct?

2) If auto thrust is not armed/active, then holding the stick aft will cause it to eventually reach VaMax. Somewhere between VaProt & VaMax, Alpha Floor will kick in to help maintain VaMax even though it was not armed/active?


Thanks

[MD83FO]

thats correct killer.

now.. what will happen if the A/T is inop and you select VS plus 6000
with AP on?

[Wingswinger]

FWIW,

A Floor is triggered when the aircraft exceeds a pre-determined Angle of Attack. It has nothing to do with speed per se. The name gives it away.

I've had a quick look in the FCOM for the A thresholds for activation of A-floor but I can no longer find them so I think the information must have been removed at a recent revision.

The A-floor threshold depends on variant (318/319/320/321) and configuration (clean, F1/F2/F3/FFull).

IIRC, the threshold for a 320 is 9.5 Alpha in clean wing, 13 in F1, 14 in F2/3 and 15 in FFull or something like that.

It is possible to provoke A floor when the aircraft is still above VLS/AProt simply by snatching the sidestick back and so instaneously exceeding the A floor threshold. On releasing the sidestick the A FLOOR FMA will change to TOGA LK.

There is a very convincing demonstration of the efficacy of Normal Law protections and A FLOOR which can be done in the simulator by setting TL to idle, pulling hard back on the sidestick and simultaneously rolling to maximum achievable bank angle before letting the sidestick go. The simulator (aircraft) will gently spiral upwards at AProt, 30 deg AoB with ATHR in TOGA LK - safely away from terra firma.

HTH,
WS