AF 447 Thread No. 11
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?
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?
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@OK465
The straight answer is that I don't know 
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
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.
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?
What am I missing here? Has someone else covered this earlier?
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:
Alpha floor is an autothrottle function which applies full thrust, irrespective of the position of the thrust levers, if the airspeed is likely to reduce to a value approaching alpha max
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
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.
Last edited by Owain Glyndwr; 1st Nov 2013 at 15:17.
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.
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!
"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!
Last edited by Chris Scott; 1st Nov 2013 at 16:08.
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Bearing in mind that AoA Protection Law is based on alpha-prot, is that figure alpha-prot, or alpha-max? (I presume the former.)
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Originally Posted by Chris Scott
So my question remains: why does it not revert to Normal Law once the AoA falls to a safe value?
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So my question remains: why does it not revert to Normal Law once the AoA
falls to a safe value?
falls to a safe value?
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A33Zab
Now that sounds a very sensible answer
Probably to prevent osicillations between NORMAL and AoA Law, so it needs a positive ND SS input to revert to NORMAL Law.
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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!
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!
Last edited by DozyWannabe; 1st Nov 2013 at 18:49.
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@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.
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?
@Chris Scott
BTW, in the AAIB's DFDR trace, the sidestick parameter seems to give negative values for "up" inputs, i.e., stick back.
Last edited by Owain Glyndwr; 1st Nov 2013 at 18:55.
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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.
"...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.
Last edited by Chris Scott; 1st Nov 2013 at 19:11.
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@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!
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!
Last edited by DozyWannabe; 1st Nov 2013 at 19:23.
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.
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.
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@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.
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.
Our FLCS trusted the AoA vanes all the way unless deemed FUBAR for a few seconds.
Last edited by DozyWannabe; 2nd Nov 2013 at 02:33.
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Originally Posted by OWAIN
I think you are missing a point however. If the warning were to be latched until safe conditions were restored (as suggested by BOAC and myself) the airspeed indications would necessarily be restored to normal and the proper stall warning threshold for the Mach number computed before the warning would be cancelled. You will remember that when the AoA was reduced temporarily the airspeed came back on line and the stall warning with it. That is why I think your next suggestion -
Quote: Machinbird
If, for example, the aircraft could compare g to measured AOA, it should then have been possible to estimate IAS with sufficient accuracy to set flight control gains as well as estimating Mach number based on OAT and altitude.
Is an unnecessary complication
Quote: Machinbird
If, for example, the aircraft could compare g to measured AOA, it should then have been possible to estimate IAS with sufficient accuracy to set flight control gains as well as estimating Mach number based on OAT and altitude.
Is an unnecessary complication
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.
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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.
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bubbers44
That is the advantage of flying archaic aircraft. its like somebody staying in remote regions of the world saying why do you need internate we don't have it and we don'tcare. Why do you need calculators mug up multiplication tables. You will care about automation if you were flying long range if you were in RVSM. If there was any virtue in 737 design Boeing wouldn't have changed to fly by wire.
That is the advantage of flying archaic aircraft. its like somebody staying in remote regions of the world saying why do you need internate we don't have it and we don'tcare. Why do you need calculators mug up multiplication tables. You will care about automation if you were flying long range if you were in RVSM. If there was any virtue in 737 design Boeing wouldn't have changed to fly by wire.
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I guess us flying these Boeings around the sky knowing we don't need the automation isn't important to you but it is to us. We don't go into a full stall at FL350 like two Airbus pilots did out of RIO on AF447 just because the pitot tubes froze over but you are right we don't rely on automation much and you do.
Good luck with your automatic airplanes. Hope automation doesn't fail like AF447. It could get nasty.
Good luck with your automatic airplanes. Hope automation doesn't fail like AF447. It could get nasty.