A320 stall audio warning at high altitude

[Greenfly07]

I was teaching stalling recently in an A320 sim. We were at FL 350, clean, normal CG, GW about 58 tonnes, ALT Law (via FACs off). In level 1g flight we had to drop about 15 kts into the Vsw red/black strip before we got the ‘Stall, stall’ audio. During this time we were flying normally, around 7 degrees pitch. I had expected the audio to trigger as we entered Vsw, as it does at low level, certainly not as late as -15kts. I would have put it down to a sim problem, but then just about the same thing happened a couple of days’ later in a different sim.
I cannot find any relevant reference in the Airbus FCOMs, FCTM, or other documentation. There are merely rather vague allusions to the audio warning being triggered at an 'appropriate' time before the stall.
Am I missing something important about high alt stall warning? I'd be most grateful for any guidance.

[BBH]

I believe that your problem is due to FAC's of
as they are used to calculate Stalling speed you don't have any more this computation so only AOA devices trigger the stall warning without anticipation

[Des Dimona]

I think you will find that the "stall stall" is in fact occurring at the correct AoA.

My understanding is that the aural warning is generated by the FWS based on real AoA data, whereas the Vsw is based on Mach data.

At low levels the difference between the 2 is negligble, but at high levels the increased Mach causes a difference between the 2, which is enough to show the variation you describe.

So, in summary, I believe the "stall, stall" aural warning is indeed occurring at the correct angle of attack and providing you with the design margins.

Anyone else care to comment ?

[CONF iture]

Greenfly07,

I think you are correct in your appreciation, and something is wrong in the simulators you used.
You will find some excellent reading here.

[cwatters]

Interesting read. Section "8. Stall Warning and stall" makes it clear how little safe margin there is if you simply push the THR levers to TOGA and try to maintain altitude...

In approach configuration, if the pilot reacts immediately to the SW, the aircraft reaches AoA stall -2°.

In approach configuration, if the pilot reacts with a delay of 2 seconds to the SW, the aircraft stalls.

[CONF iture]

Originally Posted by cwatters

Interesting read. Section "8. Stall Warning and stall" makes it clear how little safe margin there is if you simply push the THR levers to TOGA and try to maintain altitude...

Especially as an error in the anemometric information (... AF447) will affect the Stall Warning.

Originally Posted by Section 6 "Protections against the stall in ALTERNATE and
DIRECT LAW on FBW and conventional aircraft"

The value of the AoA SW depends on the Mach number. At high Mach number, the AoA SW is set at a value such that the warning occurs just before encountering the pitch up effect and the buffeting.

If the anemometric information used to set the AoA SW is erroneous, the SW will not sound at the proper AoA.

[Greenfly07]

V stall warning at high altitude on the Airbus

Many thanks to you all for your inputs.
I have now had the opportunity to further check the high altitude approach to stall indications on different A320 simulators and have used 2 x ADRs off (rather than FACs). The result is the same: we go around 15 knots into Vsw before the audio "Stall, stall" alert. The audio seems to coincide with reaching the actual stall AoA, so I think the answer to my original question is that given by Des Dimona: the Vsw black/red band is Mach-based, while the audio is AoA based. The red/black band is therefore not as accurate a reflection of the imminence of the staff as is the audio.
Thanks to everyone who contributed thoughtful and interesting answers.

[john_tullamarine]

No specific knowledge for this Type.

However, given that high level stalls for sims were not the training driver until we saw the sorts of problems such as AF experienced, perhaps it is as simple as a sim programming artefact.

Suggest in the first instance, the sims be snagged and it gets referred back through the techs to the sim and Type OEMs for resolution.

[Goldenrivett]

Airbus are now trialling a new standard of Flight Warning Computer (FWC)

" STALL WARNING enhancement: Stall warning will work when:
Undetected erroneous computation of pitot
Pitot out of the airflow
Pitot obstructed by ice or any foreign material at any speed (function now possible below 60 kts)"

[tdehuya]

about the vsw

I dont understand on the sim,why there still have vsw and red/black
indication on the PFD after both FACs turned to off

[sierra_mike]

Originally Posted by BBH

I believe that your problem is due to FAC's of
as they are used to calculate Stalling speed you don't have any more this computation so only AOA devices trigger the stall warning without anticipation

​​​​​

Originally Posted by tdehuya

I dont understand on the sim,why there still have vsw and red/black
indication on the PFD after both FACs turned to off

from FCOM-DSC-22 40 30-Flight Envelope Function-General

"As long as one Flight Augmentation Computer (FAC) is valid, it governs the flight envelope function, the rudder position display, and the rudder trim indication regardless of what the flight crew does with the FAC pushbutton."

...different story when you pull both CBs

[inducedrag]

The subject Airbus Modification SB 31-3211 "INDICATING/RECORDING SYSTEMS - EIS - DEFINE RED STALL MESSAGE ON PFD"