A320 Guru's...

[AKAAB]

(Or is it Gurri?)

Please help me find a reference for this bit of trivia. If selected speed is greater than VMO/MMO, the max you will actually get is VMO (-)minus 6kts or so. (AP on, ATHR on)

I can't find it in black and white. Any idea where a reference to this apparent buffer can be found?

This one could win me a free round!

Cheers,

AKAAB
:D

[argyle]

The selected speed/Mach target may be set beyond VLS or VMAX, but when autothurst is active, the guidance limits the speed to to VLS or VMAX. The high speed protection symbols, two green horizontal lines is set at VMO+6.

FCOM 1 ref 1.22.30 will give more gen. Also 1.27.20.

Hope this will help.:)

[AKAAB]

Thanks. I have that exact reference, but it does not explain what really happens.

Let's say you select speed well into the overspeed regime - 380 knots, for example. The FCOM reference says you will get VMO/MMO. With ATHR on the most you will actually achieve is approximately 6 knots below redline - not exactly VMO/MMO.

That's the question...where is there any reference to this apparent buffer below VMO/MMO?

Is there a fine point I'm missing on the subject of VMAX vs VMO/MMO?

If it worked as stated in the FCOM, then we should be able to see a speed equal to, but not greater than VMO/MMO.

Cheers,

AKAAB

[idg]

AKAAB
Don't remember a limit set at Vmo -6 knts. As has been pointed out autothrust should limit the speed to Vmo. FCOM Bulletin 45 points out the tolerances for PFD protection speed indications. Although it doesn't quote a tolerence for Vmo from memory it too is +/- 5 knts. From what you say you have tried selecting speed above the limit to see what happens. Could it be that the tolerance was at the maximum? Don't forget that the FAC generates the speeds based on AoA and that a difference of just 0.3 degrees can generate a 3 tonne error in calculations. :)

[LanFranc]

AKAAB,
I can tell you from experience that in selected speed the A320 will cruise at Vmo. Never heard reference to the -6 kts you mentioned.
Rgds

[AKAAB]

Well, my friends,

I tested the question at hand last night. We were in hurry-up mode so I selected Vmo and the speed stabilized 5-6 knots below. Then tried selecting above Mmo to see what happens... no change.

We've been discussing it around the office and everyone agrees that this is what the aircraft does, but no one can find a reference as to why.

Try it next time you're out and let me know if your aircraft does different.

Cheers,

AKAAB


:confused:

[CaptSnails]

AKAAB it's been some time since I flew A320 and in fact I don't have the FCOM's anymore I fly the A340-300 now and looking up in the FCOM's I found the following in FCOM 1 under Autoflight and Flight controls.

1.22.30 p11

SELECTED SPEED/MACH TARGET

To use a selected speed/Mach target the pilot uses the knob on the FCU to set the target speed, which is then displayed in the FCU window. It is also displayed in blue on the PFD speed scale.
Note:
The selected speed/Mach target may be set beyond VLS or VMAX, but when autothrust is active, the guidance limits the speed to VLS or VMAX.

Additionally in FCOM1 under autoflight in section 1.22.30p62


SPEED/MACH mode

In SPEED/MACH mode, the A/THR adjusts the trust in order to acquire and hold a speed or Mach target.
The Speed or Mach target may be:
- selected on the FCU by the pilot
- managed by the FMGC
When in Speed or Mach mode, the A/THR does not allow speed excursions beyond the following limits regardless of the target speed or Mach number:

- For selected speed target, the limits are VLS and VMAX (VMO-MMO, VFE-VLE, whichever applies)

- For a managed speed target, the limits are maneuvering speed (Green Dot, S, F whichever applies) and maximum speed (320/.84-VFE-VLE whichever applies)

Under FCOM 1 Flight Controls (Normal Law) 1.27.20p5


HIGH SPEED PROTECTION

The aircraft automatically recovers following a high speed upset. Depending on the flight conditions (high acceleration, low pitch attitude), the High Speed Protection is activated at/or above VMO/MMO.
When it is activated, the pitch trim wheel is frozen. Positive spiral static stability is introduced at 0deg bank angle (instead of 33 deg in normal law), so that with the sidestick released the aircraft always returns to a bank angle of 0 deg. The bank angle limit is reduced from 67 deg to 45 deg.
As the speed increases above VMO/MMO, the sidestick nose down authority is progressively reduced, and a permanent nose up order is applied to aid recovery to normal flight conditions.
The high speed protection is deactivated when the aircraft speed decreases below VMO/MMO, where the usual normal control laws are recovered. The autopilot disconnects when high speed protection goes active.


In normal law the system will keep your speed to VMO/MMO for a select speed input. In other words in cruise if you were to select MMO +0.2 lets say the A/THR would accelerate to a max speed of MMO not above not below. The same is valid for the low speed regime. If you were to select VLS -10 (with flaps extended) the A/THR would decelerate to VLS and no lower.
Hope this helps.
Regards Snails

[Pegasus77]

My guess is you're talking about global speed protection. Maybe you can find somehting under that title.

P77