AF 447 Thread no. 4
AoA Protection
@a33z
Very good description of the "intent" of the AoA protection. It's what "protection" should be.
My question has to do with at least one reversion mode in which it appears the system establishes an AoA other than max and more than what is required for level flight. In other words, the gee bias is reduced as far as trimming to the normal pitch/roll-corrected one gee.
This action would appear to move the THS trim posiiton, with or without pilot holding other than neutral stick. So I question, "why not maintain the basic gee command and only use AoA only as a limit?"
Secondly, there are some very good reasons to use less than the AoA required for level flight - like recovering from a stall!
BTW, ol' Gums did not call the system "ridiculous", that was another contributor. I still feel the reversion laws could be streamlined and some work could be done with respect to "standby gains" when air data is unreliable.
Very good description of the "intent" of the AoA protection. It's what "protection" should be.
My question has to do with at least one reversion mode in which it appears the system establishes an AoA other than max and more than what is required for level flight. In other words, the gee bias is reduced as far as trimming to the normal pitch/roll-corrected one gee.
This action would appear to move the THS trim posiiton, with or without pilot holding other than neutral stick. So I question, "why not maintain the basic gee command and only use AoA only as a limit?"
Secondly, there are some very good reasons to use less than the AoA required for level flight - like recovering from a stall!
BTW, ol' Gums did not call the system "ridiculous", that was another contributor. I still feel the reversion laws could be streamlined and some work could be done with respect to "standby gains" when air data is unreliable.
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@A33Zab re Probe heating
A33Zab re #551
Described monitoring is task of the installed Probe Heat Computer (3x)
Can you supply a reference to the functionality of the PHCs; and the warning messages they can generate?
My searches have had very limited success so far. However they do not point to any PHC functionality other than ensuring the pitots have voltage applied when appropriate and are drawing reasonable current (i.e. the element is not burning/burned out?).
So far I've not seen any indication that the PHC does anything other than control the normal operation of pitot heating, and monitor for faults.
My suggestion was to infer the presence of high-altitude icing conditions from the pitot's electrical behaviour; which in turn reflects the thermal load on the pitot. I'm delighted if somebody is already doing this, but have seen no indication of it in any of the reports of high-altitude pitot icing I've read.
Described monitoring is task of the installed Probe Heat Computer (3x)
Can you supply a reference to the functionality of the PHCs; and the warning messages they can generate?
My searches have had very limited success so far. However they do not point to any PHC functionality other than ensuring the pitots have voltage applied when appropriate and are drawing reasonable current (i.e. the element is not burning/burned out?).
So far I've not seen any indication that the PHC does anything other than control the normal operation of pitot heating, and monitor for faults.
My suggestion was to infer the presence of high-altitude icing conditions from the pitot's electrical behaviour; which in turn reflects the thermal load on the pitot. I'm delighted if somebody is already doing this, but have seen no indication of it in any of the reports of high-altitude pitot icing I've read.
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Originally Posted by A33Zab
It's nowhere mentioned in the manuals.
So what is changed? Environmental issues, seeking the limits due operational demand, lack of training or taking more risk because sophisticated weather radar is installed nowadays?
- inadequate maintenance ?
- lack of maintenance ?
- degradation due to aging ?
- altered manufacturing process ?
- ... ?
The reported problems on the GOODRICH are very few.
HN39;
Unless I have misunderstood your comments I'm not sure where or why my views weren't clear on pitch and power. I think we agree completely on this, so permit me to clarify, and if I have misunderstood your thoughts, please help me out, 
Specifically on your point regarding 'the better strategy', contrary to trying to control either speed or altitude within tight limits and short time frames, letting airspeed and altitude "float" in response to turbulence sufficiently strong to displace the aircraft has been an SOP and a trained response for a long time.
From the FCOM, the Turbulence Penetration SOP, in part, reads:
With all due respect to you both, I cannot fully agree with your views. In my view, disregarding ECAM messages has to do with setting priorities and the different roles that PF and PNF have at times of crisis.
I believe the prime motivation for "pitch and power" does not lie there but in flight dynamics. A string of incidents and accidents in the early years of jet transport operations led to the insight that chasing airspeed and altitude in turbulence very quickly leads to combinations of pitch and power (e.g. high pitch, low power) at which the airplane is very vulnerable to the next gust, frequently leading to loss of control. It was quickly realized that the better strategy is to concentrate on maintaining a safe combination of pitch and power, while allowing airspeed and altitude to float (within limits, of course).
I believe the prime motivation for "pitch and power" does not lie there but in flight dynamics. A string of incidents and accidents in the early years of jet transport operations led to the insight that chasing airspeed and altitude in turbulence very quickly leads to combinations of pitch and power (e.g. high pitch, low power) at which the airplane is very vulnerable to the next gust, frequently leading to loss of control. It was quickly realized that the better strategy is to concentrate on maintaining a safe combination of pitch and power, while allowing airspeed and altitude to float (within limits, of course).
Specifically on your point regarding 'the better strategy', contrary to trying to control either speed or altitude within tight limits and short time frames, letting airspeed and altitude "float" in response to turbulence sufficiently strong to displace the aircraft has been an SOP and a trained response for a long time.
From the FCOM, the Turbulence Penetration SOP, in part, reads:
When encountering turbulence, maintain wings level and smoothly control pitch attitude. Use the ADI as the primary instrument. In extreme vertical wind currents, large attitude changes may occur. Do not use sudden or large control inputs.
. . . .
THRUST AND AIRSPEED
Set the thrust to give the recommended speed . . . .
THRUST AND AIRSPEED
(Refer to QRH). This thrust setting aims to obtain, in stabilized conditions, the speed for turbulence penetration.
Change thrust only in case of an extreme variation in airspeed, and do not chase your Mach or airspeed.
A transient increase is preferable to a loss of speed that decreases buffet margins and is difficult to recover.
I haven't said anything about 'chasing airspeed and altitude in turbulence' and in fact indirectly stated the opposite by saying a number of times to "do nothing", meaning leave the pitch and power settings where they were before the UAS event and don't touch them, especially if one enters turbulence, and waiting to see what is needed to further stabilize the aircraft. I was implying but didn't think it needed stating, that this action does let the airplane "ride", ignoring minor variations in speed and altitude.
As I responded to Machinbird, "doing nothing", means leaving pitch and power alone until the QRH can be brought out and the PNF can read and supply the UAS cruise numbers has been my expressed view from the beginning. In fact, except for heavy turbulence and sustained changes in wind speed or direction, the variations in airspeed and/or altitude in moderate to light turbulence generally are small and one would certainly not make pitch/power changes to 'chase' those kinds of variations.
Small changes are going to occur anyway, but as long as such alterations center around a 'nodal point' (ie, even their effects on airspeed and altitude out so as to remain a neutral influence), then one has a stable airplane. Would you agree with this or have I truly missed something?
My main point in the post in response to Ian_W's comments was, the first order of business is to stabilize the aircraft.
This means, no drills or checklists or ECAM actions are begun until the aircraft is clearly under control. That means that the ECAM is not as high a priority as stabilizing the aircraft.
It is my view that at no time was the UAS event an "emergency" which required an instant response in the same way an engine fire, failure, depressurization, loss of energy would. That said, it absolutely did require careful, disciplined crew coordination between PF and PNF but the correct sequence of responses remains control, then drills/checklists.
Maintaining a pitch attitude and a QRH power setting would stabilize the airplane, and would then provide lots of time for the crew to then execute the necessary drills, should they still even be required.
In heavy to severe turbulence, one does one's best, difficult though that may be, but does not change the priorities or the SOPs.
If the pitch is displaced upwards dramatically for whatever reasons, one provides sufficient sidestick input to get the nose down quickly, to the QRH-supplied pitch attitude. Regarding pitch-ups caused by spurious or rogue computer behaviours, in all EFCS (FCPC) - directed pitch-ups (for speed or AoA), the only one in which ND sidestick input is inhibited until the aircraft is below VMO/MMO is the High Speed Law when in Normal Law. The Alpha Protection Law does not apply in Alternate Law, (notwithstanding the scenario proposed in which one PRIM stays momentarily in Normal Law and the others in Alternate Law which I do remain curious about but doubtful), and the High Speed Protection Law in Alternate Law which does permit pilot ND intervention.
HN39, I do understand that the pitch/power matter is flight dynamics and that ECAM actions are important and I hope this clarification helps you understand my post better.
Last edited by PJ2; 29th Jun 2011 at 23:35.
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InfrquentFlyer 789, "Clearly if above 30kft, we shouldn't be needing a GPWS escape, so maybe disable it. Then what happens if the plane has dodgy altitude info ? Ah, but what's the probability of needing a GPWS escape and at the same time the altitude being screwed ? "
Um, er, ah, please explain how a modern plane could experience dodgy altitude readings over water in the middle of the Atlantic. And do the extent of small thousands of feet even over any place on the face of the Earth. (Small 10s of feet if good DEM data is used along with GPS.)
Now, if you need barometric altitude for aerodynamics purposes I can understand it. However, for the simple task of noticing you are falling very rapidly GPS should be more than adequate for the job as should the ring laser gyros and accelerometers on the plane.
For a couple minutes GPS/Inertial data should be perfectly a adequante substitute for air speed once translated.
Um, er, ah, please explain how a modern plane could experience dodgy altitude readings over water in the middle of the Atlantic. And do the extent of small thousands of feet even over any place on the face of the Earth. (Small 10s of feet if good DEM data is used along with GPS.)
Now, if you need barometric altitude for aerodynamics purposes I can understand it. However, for the simple task of noticing you are falling very rapidly GPS should be more than adequate for the job as should the ring laser gyros and accelerometers on the plane.
For a couple minutes GPS/Inertial data should be perfectly a adequante substitute for air speed once translated.
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jcjeant, "BTW .. all Airbus accidents so far are due to pilots errors."
And it seems nobody important enough is asking the obvious next question, "Why?"
And it seems nobody important enough is asking the obvious next question, "Why?"
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Peter H
There is a bit more info in the design specification of Airbus pitot tubes systems in the ATSB Guam report AO-2009-065 (involving the preferred Goodrich probe).
but again it just mentions monitoring current against thresholds. To justify the term Probe Heat Computer I trust it measures true AC (115V 400Hz?) rms power using V*I*cos(phi), but that used to be done in simple hardware devices. There should be a measurable change in heater resistance with heater coil temperature, but guess not simple to compensate for 'wind chill effect' of different humidity air streams - its also possible external ice build up creates a thermal insulating layer. If you just want to detect icing conditions use an ice sensor- melting point close to 0 deg C despite air mass at -40C at cruise altitude?
There is a bit more info in the design specification of Airbus pitot tubes systems in the ATSB Guam report AO-2009-065 (involving the preferred Goodrich probe).
but again it just mentions monitoring current against thresholds. To justify the term Probe Heat Computer I trust it measures true AC (115V 400Hz?) rms power using V*I*cos(phi), but that used to be done in simple hardware devices. There should be a measurable change in heater resistance with heater coil temperature, but guess not simple to compensate for 'wind chill effect' of different humidity air streams - its also possible external ice build up creates a thermal insulating layer. If you just want to detect icing conditions use an ice sensor- melting point close to 0 deg C despite air mass at -40C at cruise altitude?
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galaxy flyer, 
if it is so hard to fly AirBus fly by wire because pilots learn to fly as God and the Wright Brothers intended then how on Earth do those who manage to transition to F18s, F16s, and other fine fly by wire aircraft able to manage the feat and do it so well they can out fly any other aircraft in the world?
I realize these young men are minor gods. But that should not stop "mere morals" who might find themselves at the stick of a modern FBW aircraft from doing a better job than we see with AF447. And, as I harp on excessively, "WHY do we see this, repeatedly?" Are the FBW rules wrong at some point? Is the training wrong? (That is a VERY real probability considering that AB and AF decided to change the stall training for all pilots.) Yatta and more yatta.
I would like to learn why the pilots screwed up flying the plane when it was handed to them in pitch darkness with no speed indications and zero visibility. They committed some blunders, TWO OF THEM committed or permitted some blunders (without screaming at the top of their lungs or physically assaulting each other). Why did this happen? And blaming the FBW and side stick is not going to cut it with me. It was bad decisions. Where did the FBW do something the pilots did not tell it to do? Why did they persist until at least some here (I'm not sure) suspect the pilots saw light glinting off the ocean and BOTH made ND inputs.
edit: Oops - not F4, yes F18 (and many others.)
{o.o}
if it is so hard to fly AirBus fly by wire because pilots learn to fly as God and the Wright Brothers intended then how on Earth do those who manage to transition to F18s, F16s, and other fine fly by wire aircraft able to manage the feat and do it so well they can out fly any other aircraft in the world?I realize these young men are minor gods. But that should not stop "mere morals" who might find themselves at the stick of a modern FBW aircraft from doing a better job than we see with AF447. And, as I harp on excessively, "WHY do we see this, repeatedly?" Are the FBW rules wrong at some point? Is the training wrong? (That is a VERY real probability considering that AB and AF decided to change the stall training for all pilots.) Yatta and more yatta.
I would like to learn why the pilots screwed up flying the plane when it was handed to them in pitch darkness with no speed indications and zero visibility. They committed some blunders, TWO OF THEM committed or permitted some blunders (without screaming at the top of their lungs or physically assaulting each other). Why did this happen? And blaming the FBW and side stick is not going to cut it with me. It was bad decisions. Where did the FBW do something the pilots did not tell it to do? Why did they persist until at least some here (I'm not sure) suspect the pilots saw light glinting off the ocean and BOTH made ND inputs.
edit: Oops - not F4, yes F18 (and many others.)
{o.o}
Last edited by JD-EE; 30th Jun 2011 at 20:16.
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(mumble) 1g (mumble) (mumble)
Grity, Gums, et al.
I presume you are using "1g" as a shorthand for 1g normal to the surface of the Earth, right?
If not I can see a pilot getting into trouble with an indicated 1g acceleration vector that points off somewhere other than straight down in the direction of the Earth's gravity. Worst case, of course, would be 1g straight down regardless of the heading of the aircraft.
My fool head sees 1g with no direction vector and simply cringes.
I presume you are using "1g" as a shorthand for 1g normal to the surface of the Earth, right?
If not I can see a pilot getting into trouble with an indicated 1g acceleration vector that points off somewhere other than straight down in the direction of the Earth's gravity. Worst case, of course, would be 1g straight down regardless of the heading of the aircraft.
My fool head sees 1g with no direction vector and simply cringes.
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RR_NDB, for all your asking for reasons for the pitot redundancy you surely don't think that the failure was in the pitots rather than the obvious common mode failure of the air itself, do you?
If this icing can affect a Thales probe it can also affect any other probe if conditions are "just right." So simply changing probes is more like sweeping the problem under the rug than solving it.
You're on about just one of the contributing factors that led up to the AP/AT disconnect. Yes, any one of those factors changed could have changed the outcome here. It won't change the outcome in some other cases.
Some of the things I can see for improvement (with my personal bias to "communicate") include:
(The AirBus is a remarkably safe airplane already, possibly with 10s of billions (American) of air miles flown with this accident representing around 4000 miles of that or a one in 7.5 million accident rate based on air miles. Even if nothing substantive is done will it materially change the aircraft's safety record over its remaining life in the air?)
(Wikipedia says 789 planes. I figured 750 flying 18 hours a day 300 days a year at a piddly 400 MPH for 20 years - 32 billion miles flown. Heh, that's a bit more than the number of milliseconds in one year.)
If this icing can affect a Thales probe it can also affect any other probe if conditions are "just right." So simply changing probes is more like sweeping the problem under the rug than solving it.
You're on about just one of the contributing factors that led up to the AP/AT disconnect. Yes, any one of those factors changed could have changed the outcome here. It won't change the outcome in some other cases.
Some of the things I can see for improvement (with my personal bias to "communicate") include:
- communicate not shrug when DAKAR does not answer,
- deviate around storms, know your radar (radar training),
- pseudo-airspeed (GPS derived for as long as altitude and ground speed remain "sane"),
- a variety of probes (sadly not possible as only two are qualified),
- modified pilot training (What do I do when a stall warning appears? What do I do on AP/AT disconnect?),
- reduce cockpit information overload in crisis (obviously conflicting warnings MUST be solved),
- and more.
(The AirBus is a remarkably safe airplane already, possibly with 10s of billions (American) of air miles flown with this accident representing around 4000 miles of that or a one in 7.5 million accident rate based on air miles. Even if nothing substantive is done will it materially change the aircraft's safety record over its remaining life in the air?)
(Wikipedia says 789 planes. I figured 750 flying 18 hours a day 300 days a year at a piddly 400 MPH for 20 years - 32 billion miles flown. Heh, that's a bit more than the number of milliseconds in one year.)
Normal gee
@JD
We are talking about the aircraft reference frame for one gee. That's down from your brain thru your butt if you are not at Creech AFB flying a Predator drone 10,000 miles away, heh heh.
It's the same as when driving a car or flying a Cessna or riding on a roller coaster.
What makes this concept for FBW attractive is that the rate and acceleration sensors so essential to a FBW system do not depend upon earth-referenced gee or air data. Using body rates is also much closer to what we old fossils experienced in every plane we flew. You still have to use your attitude indicator and such when IMC, but somehow the old "seat of the pants" expression holds true.
The Airbus does not "trim" for one gee all the time, but corrects for attitude ( best I can glean from the manuals). So it trims for less than one gee when in a climb/dive. Also adds a bit of gee if in a bank. So in a level 30deg bank it trims for 1.15 gee +/- as the neutral position for the stick. The "other FBW jet" I flew didn't/doesn't do this. The gee command was always thru the body axis ( the jet and the human), plus we could trim the thing for something besides one gee.
Using a gee command system doesn't work real well in a video game or flying a Predator. You can't "feel" the gee.. So a body rate/attitude blend seems better for those applications. Have to check, but I think the Shuttle does that until the system switches over to air data at low altitude, prolly below 100,000 feet or so.
Questions?
We are talking about the aircraft reference frame for one gee. That's down from your brain thru your butt if you are not at Creech AFB flying a Predator drone 10,000 miles away, heh heh.
It's the same as when driving a car or flying a Cessna or riding on a roller coaster.
What makes this concept for FBW attractive is that the rate and acceleration sensors so essential to a FBW system do not depend upon earth-referenced gee or air data. Using body rates is also much closer to what we old fossils experienced in every plane we flew. You still have to use your attitude indicator and such when IMC, but somehow the old "seat of the pants" expression holds true.
The Airbus does not "trim" for one gee all the time, but corrects for attitude ( best I can glean from the manuals). So it trims for less than one gee when in a climb/dive. Also adds a bit of gee if in a bank. So in a level 30deg bank it trims for 1.15 gee +/- as the neutral position for the stick. The "other FBW jet" I flew didn't/doesn't do this. The gee command was always thru the body axis ( the jet and the human), plus we could trim the thing for something besides one gee.
Using a gee command system doesn't work real well in a video game or flying a Predator. You can't "feel" the gee.. So a body rate/attitude blend seems better for those applications. Have to check, but I think the Shuttle does that until the system switches over to air data at low altitude, prolly below 100,000 feet or so.
Questions?
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Originally Posted by gums
We are talking about the aircraft reference frame for one gee. That's down from your brain thru your butt if you are not at Creech AFB flying a Predator drone 10,000 miles away, heh heh.
(And I'm only mildly surprised the Predator pilots don't have motion base chairs to simulate what the plane feels. One industry I do work for dotes on that sort of stuff as means of separating rubes from dollars. It's convention is in Orlando just before Thanksgiving.)
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SV Interesting thought exercise - what would have happened if pilot didn't recover Normal control?
HN Interesting indeed. ..... At best, the airplane ends up in 1g flight at a speed corresponding to alpha-prot, descending or climbing depending on the thrust available (set).
HN Interesting indeed. ..... At best, the airplane ends up in 1g flight at a speed corresponding to alpha-prot, descending or climbing depending on the thrust available (set).
but I think, first we have to draw a raw AoA line with: AoA = pitch-FPA and look how good the Mr.alpha-protector did his work
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PJ2;
Thanks for replying. Yes, we agree completely on what you wrote in your #548 and #559, except for the phrase I quoted and which I may have misunderstood. The only thing I commented on, as a 'flight mechanics' guy, was the notion that "the reason for 'set power and pitch' is to 'disregard all those messages' ". And yes, the SOP and trained response has been along for a long time, but only after a series of mishaps, as I remember it. Somewhat similar to the insight that you have to reduce AoA to unstall an airplane.
Thanks for replying. Yes, we agree completely on what you wrote in your #548 and #559, except for the phrase I quoted and which I may have misunderstood. The only thing I commented on, as a 'flight mechanics' guy, was the notion that "the reason for 'set power and pitch' is to 'disregard all those messages' ". And yes, the SOP and trained response has been along for a long time, but only after a series of mishaps, as I remember it. Somewhat similar to the insight that you have to reduce AoA to unstall an airplane.
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Originally Posted by A33Zab
I realize these probes failed (iced up) more on A. than other A/C recently, while there only few suppliers (also GOODRICH had problems on these A/C).
HN39;
Yes, agree. Hopefully the regulators, the sim builders and the airlines will continue the encouraging trend towards such awareness and changes in training. From a recent flight crew training manual revision, "High Speed Protection may also result in activation of the angle of attack protection. In all events, check the AP engagement status, and re-engage it when appropriate. It may have tripped and the associated aural warning may have been superseded by the overspeed aural warning.". I recall our original discussion on AoA in the second thread and how much I've learnt since then from discussions like the present one. The great difference between the stall AoA at low altitude and cruise altitudes was one of the few things that Davies did not discuss, (I suspect because he would never have expected that line pilots would be anywhere near situations requiring that knowledge) and was never discussed as a flight dynamics matter in any training. Many thanks, HN39.
Somewhat similar to the insight that you have to reduce AoA to unstall an airplane.
Per Ardua ad Astraeus
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PJ - can you explain the logic of the angle of attack protection? I can understand why the system might want to LIMIT AoA to alpha-prot (desirable), but why design it to pitch up to it from level flight (bizarre)