Quote from Owen Glyndwr (my underscoring):
In this particular case the autopilot dropped out because of a strong temperature shear (not the windshear you suggest).

Whoops! :{ Thanks for the reminder that the sudden increase in Mach to beyond Mmo was due to the sudden fall in temperature, not a headwind shear.

Quote:
...the logic designed to protect against stall at low Mach numbers had unexpected side effects when applied at high Mach numbers.

It's the logic at low Mach numbers that I'm questioning. As I wrote:
on the face of it, it seems bizarre for the FBW to "grab" a transient, higher-than-normal AoA even if it results in more than 1G with a neutral sidestick (assuming wings level).

Here's the best scenario I can think of at the moment. I'm hand-flying at low altitude in the holding stack and there's a severe, momentary up-gust, causing a momentary (rapid) rise in AoA towards alpha-prot.

Phase-advanced alpha-protection puts the FBW into AoA Protection Law. Meanwhile, the AoA has returned (fallen) to what it was before the gust. However, FBW uses up-elevator to increase the AoA to alpha-prot. The a/c climbs suddenly until I push the stick forward more than half-travel, OR use less-than-half forward stick for more than a second.

Although I'm unlikely to allow the a/c to climb much (unless I'm having a bad day at the office, or get distracted), maintaining AoA Law seems an unnecessary presumption by the FBW.

So my question remains: why does it not revert to Normal Law once the AoA falls to a safe value?

Hang on a minute! Suppose it is an A320 at say 60 tonnes - holding at 200 kts?
That will be about 5.5 deg AoA with stall at 12 deg. The alphaprotect logic is a combination of alpha and rate of change of alpha, but take just alpha for the moment. You are going to need a gust of 38 fps to get to the stall and from published A320 flight statistics you will need to fly half a million flight hours at 200 kts before you hit that sort of gust. OK one needs to back off a bit to account for the phase advance contribution, but it will still be a helluva lot of hours :=

So in theory you may be right about the possibility but it would be an extremely rare case I think.

As for why it doesn't revert to normal law, surely it cannot, because once alphaprotect has taken charge the AOA is held at the alphaprotect value?

Re: 330, 340 near miss not AF447
 

Another question for OG,

I'm still struggling with the contents of the AAIB bulletin concerning the 330 & 340 near miss....

....specifically the 340 Captain's statement that he observed an 'alpha lock' message and then the board's conclusion that it was probably an 'alpha floor' annunciation when under the circumstances, the lowest IAS & IMN stated as 205 & 0.67, it seems highly likely that neither of these messages would or even could be displayed.

Why would the board glom onto 'alpha floor' as a probable when it's inhibited above 0.53 IMN?

What am I missing here? Has someone else covered this earlier?

@OK465
The straight answer is that I don't know :O Partly because I'm not sure what "alpha lock" entails and partly because the recorded behaviour of the engines is consistent with an 'alphafloor' occurrence regardless of what the book says said about its 0.53M threshold.
The AIB report text is:
Its all a bit confusing, as the thrust increase came at about the same time, or even a little after, the A/T was disconnected, but the report makes no mention of any throttle increase by the PF, only the thrust reduction. Also the phraseology "if airspeed is likely to reduce to a value approaching alpha max" leaves one wondering! It does imply though that alpha floor switching is also a function of rate of change (of speed?)

Ducking the issue somewhat, I imagine that the AIB will have taken advice from Airbus on what might have been and I would think (well hope anyway) that AI would understand their own system :ok:

Edit: OK I now understand what alpha lock is, (prohibiting slat retraction in certain circumstances) but so far as I can see it would in no way be relevant to the cruise condition where this incident occurred.

Thanks OG,

Just an observation....

....if done as a demo or ops check in a level D FFS, using idle thrust just to expedite a level entry from say a weight dependent ~200 KCAS at FL350 into AOA protection, hands-off it will drift on down at alpha prot no thrust increase, if the SS is cycled full aft aggressively to achieve a short term transient alpha max there is still no thrust increase....

Like clockwork at around Fl280 (0.53 IMN) thrust comes up and alpha floor is then annunciated.

But it is only a sim, and real air can do some strange things. :)

I agree it is unlikely it was an SFCC alpha lock indication, I believe that is annunciated by the slat/flap indicator display anyway (don't recall for sure).

It's just the word probably that is intriguing and the fact that the Captain would have had to take positive action to exit alpha floor and one would think observe the alpha floor annunciation extinguish. Of course he was 60 years old and I noticed myself that at that point my powers of observation were waning somewhat also. :}

Quotes from Owain Glyndwr:

"Hang on a minute! Suppose it is an A320 at say 60 tonnes - holding at 200 kts? That will be about 5.5 deg AoA with stall at 12 deg."

Yes, Owain, that's very much the speed I would expect for holding an A320 (clean) at 60T - "Green Dot" speed would be 205, which is well above VLS, and the FMGC sometimes suggests a slightly lower figure than Green Dot for holding. A level-flight pitch of about +5 or so is typical, suggesting an AoA of that value. You quote an AoA of "stall at 12 deg". Bearing in mind that AoA Protection Law is based on alpha-prot, is that figure alpha-prot, or alpha-max? (I presume the former.)

"The alphaprotect logic is a combination of alpha and rate of change of alpha, but take just alpha for the moment. You are going to need a gust of 38 fps to get to the stall..."

Agree that a sudden up-gust of 38 fps (2280 fpm) must be extremely rare, but you admit that 38 fps doesn't take phase-advance into account.

"As for why it doesn't revert to normal law, surely it cannot, because once alphaprotect has taken charge the AOA is held at the alphaprotect value?"

...Precisely the logic I'm calling into question! :ok:

No, that is stall for a completely clean airframe (slats and flaps retracted) at low Mach No.

That is a very intriguing question. I would love to know the answer to that.

Probably to prevent osicillations between NORMAL and AoA Law, so it needs a positive ND SS input to revert to NORMAL Law.

A33Zab
Now that sounds a very sensible answer :ok:

And lest we forget, when talking about the airprox incident (as opposed to AF447) - I think it would be wise to remember - before getting tied up in suggesting problems with the systems - that all the crew had to do, both to arrest the climb and exit the AoA/Alpha Prot mode, was to *push forward on the stick*. Hardly rocket science and, by my estimation, fairly intuitive piloting** - no?

Of course, the same held true in many ways for AF447. I continue to believe that the Stall Warning issue is to some extent a red herring in the grand scheme of things, because the thing was working fine for more than enough time to have been picked up by the crew before the aircraft and its sensors went too far into the unknown.

Also, I'm still seeing references to "automation-dependent crew", which as I said before doesn't square with the sailplane qualifications of the AF447 PF - there is no automation (or for that matter TOGA power) on a glider!

** - In the sense that whether you're flying anything from a Cessna to Concorde, if the aircraft isn't pointing where you want it to go - point it where you want it to go!

@Dozy
Which is what they did of course!

@Chris Scott
Reason for that is that with the pretty well universal right handed system of axes used to describe airplane motions a clockwise (nose up, tail down) rotation is positive. That means that 'down' elevator is positive. Although the RHS axes don't have to be applied inside the aircraft it makes sense to have a positive (forward) movement of the stick correspond to a positive (downwards) elevator movement.

Agreed, although it is definitely concerning that it didn't occur to them until they'd already busted their flight level.

A340 AIRPROX Alpha-Floor
 

Quote from Owain Glyndwr:
"...the recorded behaviour of the engines is consistent with an 'alphafloor' occurrence regardless of what the book says said about its 0.53M threshold... ...the thrust increase came at about the same time, or even a little after, the A/T was disconnected, but the report makes no mention of any throttle increase by the PF..."

I can't shed any light on the alleged (by OK465) maximum of M0.53 for Alpha-Floor initiation. Just in case there's any misunderstanding, however, my recollection is that Alpha-Floor itself engages A/THR if the latter is currently OFF, regardless of thrust-lever position.

@Chris Scott:

If I recall correctly the Alpha Floor functions with A/THR OFF, but it will not function if A/THR is *disabled*, if not in Normal Law, or below 100ft RA. I'm not sure about the Mach limit** - OK465, would you have a reference handy?

One thing that struck me when I read the airprox report was that the A340 Captain said he noted "ALPHA LOCK" on his PFD, which the AAIB took as a misreading of "A. FLOOR". The thought occurred to me that it might possibly have been a misread of ALPHA PROT (from the transient AoA anomaly) and THR. LOCK (from the AP disconnect). Just a thought...

** - I should note however, that in my experience OK465 is usually correct! :ok:

Well that would clear up one mystery!

Here we go again
 

Salute!

Fer chrissakes, we are back to the basic design and such of the magenta line jets and the associated autopilot modes and their connection with the FBW systems or even the old systems.

I simply cannot understand the rationale of all the sub-modes of the FLCS ( flight control system) and the autopilot (AP) connections and on and on. Good grief!!!

I do understand that the heavies appreciate some of the AP modes and such to reduce workload and so forth. Funny for me, as I was the only guy in the jet and didn't have a PNF or anyone but Otto to help.

I must insist upon iterating my view of the Viper's FBW contol laws and some of the logic behind the design. This is compared to the 'bus design and more importantly, the implementation. I feel that most of the old school farts here would have liked what I flew with 35 yerars ago.

I cannot understand an implementation that switches modes and laws and such based upon "noise level" variations in temperature or mach changes or whatever. This is especially true for the heavies, that have basic inertial flight paths like most objects zooming thru the air, and lots more inertia than I ever had. And then there's the connect with nav systems and AP functions and on and on and on.

It all came down to AoA for we Viper pilots. Our FLCS trusted the AoA vanes all the way unless deemed FUBAR for a few seconds. I once went vertical and was looking over my shoulder when the "slow speed horn" sounded. "Gums, you are below 120 knots and gear is up!" OK, let go of the sidestick and let HAL do what it can do. So came back down in a tailslide and all the FLCS warning lights came on, Master Caution light came on, and so forth. Jet pitched down and I didn't touch a thing and forced myself not to pull back or forward. A few seconds later I was flying again and reset all the stuff and the fight was still on.

I realize that the heavy pilots here don't have those situations, but they sure as hell have some situations that require basic airmanship and a complete understanding of all the capabilities of their plane and maybe even all the modes and sub-modes of all the the FLCS and the AP modes and so forth. I empathize with them.

'nuf philosophy from this old FBW pioneer.

@gums:

You must know by now that I have the deepest respect for you. That said, you're not really saying anything you haven't said before. We covered the rationale behind the law changes repeatedly long ago - if you don't like it, that's all good, but it doesn't make the implementation bad.

Well yes, because it was a fighter and the expected operating parameter ranges of those AoA vanes was significantly different. The current flight control law has absolutely no effect on the validity of data from the AoA vanes on the A330, so I'm at somewhat of a loss to understand the point you're making.

But OWAIN, if you can still set the gains for the flight control system, do you need to drop out of Normal Law in the first place?
I don't believe that the accuracy would be sufficient to manage an auto throttle system, but it should be possible to calculate gain settings that would permit Normal Law or perhaps a variant of Alternate 1 Law. Any improvement in stall warning calculation is just icing on the cake.

For us non Airbus pilots that don't really care if the autopilot or autothrottle even works because we don't rely on them at all why is automation so important to Airbus pilots and how can it be blamed for any crash? We like to let it relieve our work load but if it quits we just hand fly. If the IAS goes away on all three air speed indicators we fly attitude and power using a QRH. We don't have to know any laws. We are either in automatic or manual. One is a little more work but no big deal. I don't get it.