View Full Version : A320, speedbrakes and Vls
9th Jun 2012, 13:09
When using speedbrake on the -320, the Vls is going up. Therfore, I've been warned to be careful with the speedbrake, especially when heavy/slow...
Now, can anyone tell me what will actually happen if the pilot allows the Vls to reach and exceed the current speed? Will the autothrust add thrust to maintain Vls? Or maybe in Open Des will the AP/FD command pitch down? Or will the FBW/ATHR just allow to fly below the speedbrake Vls?
At between vls-2 to vls-19 a mode reversion will occur. FD bars removed, if Autothrust engaged thrust is added to recover the speed target. This is from the FCOM. If AThr is not engaged you'll hit alpha prot and/or alpha floor with all the usual results.
10th Jun 2012, 06:44
I am aware of the FCOM passage you posted, but it pertains only when FD bars are not followed during descent and the pilot allows the speed to drop from speed target, all the way to Vls...
This, however, is not what I am looking for. My question is:
What is the system behaviour if Vls suddenly becomes higher than speed target, due to speedbrake extension?
10th Jun 2012, 08:31
Ok, if the Autothrust is active it will attempt to increase speed so as to maintain Vls, if the speedbrake is also deployed the SPEEDBRAKE display on the EWD will flash amber as it senses a power against speedbrake condition. (this indication will not occur on early FMS A320's).
If the Autothrust is off, then the speedbrake will automatically retract when the AoA reachs AFLOOR, (as it will eventually do with in your scenario) and all the protections will try to fly you back to VLS.
I think the scenario you refer to is when selected (or managed) speed is slow, say green dot, but you are a bit high, so use the speedbrake which raises the vls so that it is just under your selected speed, you then turn and the aerodynamic increase in stall speed sends your amber vls tape over the top of your selected speed. Autoth will try to increase your speed and the amber speedbrake caution will come on.
In real life this scenario is quite easily reached if you ask the aircraft to do too many things at once, for instance, asking it to accelerate, climb and turn at the same time when at high weights and slow speed after take off, small buses don;t have vast surpluses of energy to cope and the FMGC will prioritse what you ask of it to its own logic. It will sort itself eventually. Hope this helps.
10th Jun 2012, 08:58
If A/THR is in SPEED mode, then I guess it will add thrust to maintain Vls...
BUT I am especially interested in the following scenario, which is not uncommon in line operations:
You are descending at, say, 220KT in OP DES. (A/THR is in IDLE mode, speed controlled by pitch). You realize you're too high and you extend speedbrakes without paying attention to PFD. Your Vls increases to, say, 230KT - 10KT ABOVE target speed. Now, what will the AP/ATHR do?
b) pitch down to maintain Vls
c) increase thrust while maintaining pitch and revert to SPEED VS mode
d) none of the above
10th Jun 2012, 09:16
I'll have to check later, but I believe that so long as no protection criteria are breached the a/c will gently pitch down to increase airspeed and maintain the top of the VLs tape.
11th Jun 2012, 04:39
You are absolutely right.
the Aircraft pitches down to maintain the Vls
The FMA will not change and it will remain THR IDLE OPEN DES
P.S.:- Even if you make your current speed go into the Alpha max sector (red tape)[Radid Extension of spiloers]
the A/THR will not command A.Floor :8
But Fcom says
α is greater than α floor (9.5◦ in configuration 0; 15◦ in configuration 1, 2; 14◦ in configuration 3 ; 13◦ in configuration FULL)
Any one knows why didn't the Aircraft went into A.FLOOR
11th Jun 2012, 08:40
Thanks for confirming that, got called from Standby and didn't get a chance:sad:
12th Jun 2012, 14:13
Thanks for replies, gents! :ok: