Airbus overspeed

[Bkdoss]

Recently when flying with a new captain on the bus there was a discussion about flap exceedance and Automatic Flap Retraction system. My take on the function was that the flaps retract automatically at 210 and then the aircraft accelerates to the managed speed 250kts during which time if we don't retract flaps then we get a Overspeed warning.

However the captain was quick to point out a line in the FCOM which reads

"If the managed speed/Mach target is set above VMAX (VFE , VMO , MMO ), the FMGS automatically limits the speed to VMAX"

Going by this line the aircraft should maintain 230 kts and we'd never get an OverSpeed warning. I sense something is amiss, who among us is correct?

[safelife]

You're correct and he is wrong.
The Airbus with happily fly into the flap overspeed.

[KayPam]

I worked on this topic recently so I'm absolutely sure of the answer :

Let's say you've got a managed speed target at 250kt (typically it's "speed limit")
The FMGS does not automatically limit the speed target to VMAX (which I initially thought true)
The FMGS (actually the FG subpart) will limit the speed to VMAX.

This means you will get a speed target at 250kt (displayed on the PFD) but the FG will typically pull up (open climb mode, for instance) to limit the speed to VFE.

However, what could happen is a wind gradient making your speed increase above VFE. Then the flaps loads relief system will trigger and retract to the flaps to a * conf.

This answer was given by an airbus test pilot from the "design" test pilots team, he's working in close relation to the engineers.

However I'm only certain for the 330.
The 320 might work differently.
IIRC, it's in FCOM DSC22-30-40.

Actually I don't think the 320 has an FLRS (not that I know of at least) so we must be talking about the 330.
Could we all clarify which aircraft we're talking about ?

[giggitygiggity]

Assume it's the A320 given the auto-retract and limiting speeds given.

[RUMBEAR]

The FCOM describes the A/THR when in SPEED mode will limit speed to VMAX even when the target speed is above this value. It doesn't say the same for THRUST mode ( I.e At Take off) however I tried this scenario in the simulator and at Flap 1 initially the auto retract occurred at 210 kts, then pitch up to maintain 230 kts and after lever off thrust reduced and 230 kts was maintained despite of a managed speed target of 250 kts.

I wouldn't try this inflight however as manoeuvres / turbulence etc would probably create a overspeed condition!

[handsome goafer]

Plus, the aircraft can accelerate faster than the flaps can retract

[vilas]

BKD
Airbus protections come from Flt Ctls, FMGS, ATHR. Unless one goes through these he cannot find correct answers.
1. While taking off with heavier weight S speed becomes close to VFE. Auto Retraction System retracts the flaps to increase the VFE to 230kts to help avoid VFE exceedance.
2. In OP CLB mode during TO/GA thrust even in TOGA with LVRCLB flashing FMGS will increase ROC to restrict speed at VFE. But if ALT* happens speed exceedance will happen unless thrust is brought to CLB detent.
3. With AP on the FMGS speed protection is available only with ATHR in Speed mode. Below FCOM:
When in SPEED/MACH mode, the A/THR does not allow speed excursions beyond the following limits, regardless of the target speed or Mach number: ‐ For a 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 (340/0.80-VFE -VLE, whichever applies).
4. With AP off only FD on in open mode and ATHR in THR if FDs not followed correctly and speed is allowed to increase then at VFE+4 FDs are removed ATHR will revert to speed and the original speed target will appear in selected and recover.
5. There is no flap load relief in A320.

[safelife]

And the solution is simple: pull speed.

[pineteam]

Originally Posted by vilas

3. With AP on the FMGS speed protection is available only with ATHR in Speed mode..

Hi Vilas,

Thank you again for your inputs. Those protections confused me for a long time. So could you or someone else confirm that if:
1: I'm flying in "alt" mode with the A/THR in CLB thrust or TOGA with AP ON or with AP OFF but following the FD the aircraft will accelerate over VMAX without any reversion mode or protection?
2: same conditions but now we pitch fully down with CLB/TOGA thrust in alt mode, still no protection?

Thanks.

[RUMBEAR]

Pineteam,

The A/Thr interacts with the vertical flight guidance modes. If the THR Lever is in the CLB detent the A/THR mode changes to speed/Mach mode at ALT *. So it's not possible to have ALT mode and THR CLB simultaneously.

TOGA thrust is a fixed thrust setting ( I.e. A/THR is armed but not active), so no thrust reduction will occur at VFE and over speed will occur.

I'm sure Vilas can explain this much better than me though!

[vilas]

pineteam
Hi

1: I'm flying in "alt" mode with the A/THR in CLB thrust or TOGA with AP ON or with AP OFF but following the FD the aircraft will accelerate over VMAX without any reversion mode or protection?

As RB pointed out if you were in THRCLB at ALT* it will change to SPEED and protection is available. But if you were in TOGA then unless you bring THR levers to CLB speed mode cannot engage and over speed will occur.

2: same conditions but now we pitch fully down with CLB/TOGA thrust in alt mode, still no protection?

The protection I mentioned is FMGS speed protection only available in OPCLB/OPDES with AP off. In this case ATHR is in THRCLB thrust aircraft will continue to accelerate till FLT CTL high speed protection activates.

[pineteam]

Hi Rumbear,

Yes, my bad; Thank you for your answer and for correcting me.


Hello Vilas,

Thanks for your reply. Once again, really appreciated!