AF 447 Search to resume
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PJ2
Thank you for your reply. I modified my previous posting a bit with additional information regarding the BackUp Speed Scale option, and a bit more info on an A-310 accident on takeoff which occurred in Africa following bogus stall warning activation.
The reference to airspeed derived AOA validation was only a comment on a possible engineering approach which should not be done. AOA and airspeed are each primary data, and they may conflict for various reasons. This is just the engineer in me coming out and I apologize if it causes confusion.
The AOA indication can also be an effective forewarning of a potential bogus stall warning after liftoff. If it is pegged at high AOA while going down the runway, something is wrong with the sensor, and you will know that before rotation, probably even before V1. Airspeed and AOA go together to tell you important data about your aircraft's performance and enable you to cross check each indication. When something doesn't agree between them it can be a serious problem and you need to have an idea which to believe in. If all you have on your panel is airspeed, you are open for an AOA sucker punch (IMHO), particularly on a high wing loading aircraft.
Machinbird
Thank you for your reply. I modified my previous posting a bit with additional information regarding the BackUp Speed Scale option, and a bit more info on an A-310 accident on takeoff which occurred in Africa following bogus stall warning activation.
The reference to airspeed derived AOA validation was only a comment on a possible engineering approach which should not be done. AOA and airspeed are each primary data, and they may conflict for various reasons. This is just the engineer in me coming out and I apologize if it causes confusion.
The AOA indication can also be an effective forewarning of a potential bogus stall warning after liftoff. If it is pegged at high AOA while going down the runway, something is wrong with the sensor, and you will know that before rotation, probably even before V1. Airspeed and AOA go together to tell you important data about your aircraft's performance and enable you to cross check each indication. When something doesn't agree between them it can be a serious problem and you need to have an idea which to believe in. If all you have on your panel is airspeed, you are open for an AOA sucker punch (IMHO), particularly on a high wing loading aircraft.
Machinbird
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AoA from FPV
Originally posted by Machinbird in #540:
Once the ADIRU outputs are disqualified, the velocity vector information they provide is removed and the aircraft systems have to make do with what is left (which isn't much).
Once the ADIRU outputs are disqualified, the velocity vector information they provide is removed and the aircraft systems have to make do with what is left (which isn't much).
Originally posted by PJ2 in #541:
The question and argument here I think is, what does AoA offer that CAS does not.
The question and argument here I think is, what does AoA offer that CAS does not.
regards,
HN39
PJ2,
Thanks for posting that excellent graphic of the Airbus PFD in FPA mode. This originated with the Orca (A310) in 1983/4.
I think it may be helpful to some other readers, unfamiliar with Airbus glass cockpits, to clarify what we are looking at.
It is clear in the graphic that Alpha is +15 (but see my penultimate paragraph below). But if the drift increases, and particularly when varying amounts of bank are present, it's not so easy for the eye to interpret quickly.
The second problem is that, unfortunately, we are not looking at a primary indication of AoA, even though that could be made available from the appropriate AoA sensor. The "bird" is dependent entirely on IRU data (from the appropriate ADIRU), as Machinbird pointed out. So, of course, is the pitch indication on the same PFD.
HN39,
As far as I know, the FPA ("bird") derives its data from the IRU, not the AD. I would be astonished if it used VS from the AD, because the latter (presumably coupled with TAS) could only give a trajectory relative to the air – not the ground. When you are using the FPA to fly a steady glide-slope on a non-precision approach (standard procedure in British Airways), the FPA has to be relative to the ground.
Everyone,
My response to HN39 has belatedly reminded me that AoA CANNOT accurately be derived from a comparison of Pitch and FPA (flight-path angle): it is only strictly true in a stationary air mass.
Coming from a purely civil (airline) background, I've nothing useful to contribute on the concept of flying AoA as an alternative (or complimentary) to airspeed. We were just never taught it. To illustrate the point, both the 1960s VC10 and BAC 1-11 (and, presumably, Trident) had AoA sensors for their sophisticated stall-protection systems. But the only time we saw an AoA gauge in the cockpit was when the engineers fitted it temporarily for a flight test, after which it was removed.
Thanks for posting that excellent graphic of the Airbus PFD in FPA mode. This originated with the Orca (A310) in 1983/4.
I think it may be helpful to some other readers, unfamiliar with Airbus glass cockpits, to clarify what we are looking at.
It is clear in the graphic that Alpha is +15 (but see my penultimate paragraph below). But if the drift increases, and particularly when varying amounts of bank are present, it's not so easy for the eye to interpret quickly.
The second problem is that, unfortunately, we are not looking at a primary indication of AoA, even though that could be made available from the appropriate AoA sensor. The "bird" is dependent entirely on IRU data (from the appropriate ADIRU), as Machinbird pointed out. So, of course, is the pitch indication on the same PFD.
HN39,
As far as I know, the FPA ("bird") derives its data from the IRU, not the AD. I would be astonished if it used VS from the AD, because the latter (presumably coupled with TAS) could only give a trajectory relative to the air – not the ground. When you are using the FPA to fly a steady glide-slope on a non-precision approach (standard procedure in British Airways), the FPA has to be relative to the ground.
Everyone,
My response to HN39 has belatedly reminded me that AoA CANNOT accurately be derived from a comparison of Pitch and FPA (flight-path angle): it is only strictly true in a stationary air mass.
Coming from a purely civil (airline) background, I've nothing useful to contribute on the concept of flying AoA as an alternative (or complimentary) to airspeed. We were just never taught it. To illustrate the point, both the 1960s VC10 and BAC 1-11 (and, presumably, Trident) had AoA sensors for their sophisticated stall-protection systems. But the only time we saw an AoA gauge in the cockpit was when the engineers fitted it temporarily for a flight test, after which it was removed.
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Certainly a direct AoA reading would be of benefit. The only experience I have of such is from the Harrier GR3 where you were taught to 'maintain 8 units' during transition to and from slow speed (unspecified 'units' - didn't matter!). Providing one stuck to this rule the actual business of 'transferring' lift backwards and forwards to thrust was simple. Driving the AoA reading from an alpha vane (suitably heated
) would take away many of the problems caused by the 'sophistication' of modern aircraft systems - let's keep computers and IRS out of it - and if crews were given 'ball park' figures to set to maintain safe flight well away from the stall I can only see a positive safety addition.
) would take away many of the problems caused by the 'sophistication' of modern aircraft systems - let's keep computers and IRS out of it - and if crews were given 'ball park' figures to set to maintain safe flight well away from the stall I can only see a positive safety addition.
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AoA from FPV
Originally posted by Chris Scott in #544:
As far as I know, the FPA ("bird") derives its data from the IRU, not the AD.
As far as I know, the FPA ("bird") derives its data from the IRU, not the AD.
From BEA’s no.2; 1.6.11.2 Autopilot, flight director and autothrust:
The FPV is elaborated in the IR part of the ADIRU which, for this purpose,
uses inertial parameters and also an anemometric parameter: the barometric
vertical speed. It is thus necessary for the IR to have at least one valid ADR
at its disposal. From the perspective of the IR, an ADR is valid if the three
parameters, altitude, barometric vertical speed and true airspeed are valid
(SSM status is NO) If the three ADRs are considered invalid by the IR it is no
longer possible to calculate the FPV and the red FPV flag appears on the PFD.
The FPV is elaborated in the IR part of the ADIRU which, for this purpose,
uses inertial parameters and also an anemometric parameter: the barometric
vertical speed. It is thus necessary for the IR to have at least one valid ADR
at its disposal. From the perspective of the IR, an ADR is valid if the three
parameters, altitude, barometric vertical speed and true airspeed are valid
(SSM status is NO) If the three ADRs are considered invalid by the IR it is no
longer possible to calculate the FPV and the red FPV flag appears on the PFD.
HN39
Last edited by HazelNuts39; 22nd Mar 2010 at 13:17. Reason: paragraphing
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Originally Posted by Chris Scott
But the only time we saw an AoA gauge in the cockpit was when the engineers fitted it temporarily for a flight test, after which it was removed.
I'll have to ask a pilot again, in what circumstances and to what extent it was used and useful.. I honestly no longer remember.
CJ
Thanks, HN39, I'm suitably "astonished"!
Will assume the BEA is right on that point, at least. So the horizontal data (TRK & GS) come from the IRU part of the ADIRU, and the vertical speed is simply the VS supplied by the AD system part. I had overlooked the fact that pressure-derived VS indications moved on a bit during my career, in terms of lag (lack of), as well as accuracy and reliability.
I should not have inferred that you were suggesting the horizontal data was also from the air-data (AD) system – my apologies.
But I agree with you that it seems odd that the displayed FPA/FPV ("bird") does not, apparently, derive its vertical datum inertially, from the IRU.
The rest of my comments still stand to be corrected...
Chris
Will assume the BEA is right on that point, at least. So the horizontal data (TRK & GS) come from the IRU part of the ADIRU, and the vertical speed is simply the VS supplied by the AD system part. I had overlooked the fact that pressure-derived VS indications moved on a bit during my career, in terms of lag (lack of), as well as accuracy and reliability.
I should not have inferred that you were suggesting the horizontal data was also from the air-data (AD) system – my apologies.
But I agree with you that it seems odd that the displayed FPA/FPV ("bird") does not, apparently, derive its vertical datum inertially, from the IRU.
The rest of my comments still stand to be corrected...
Chris
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AoA and FPV
Chris Scott:
Thanks for your reply. I agree with the rest of your comments, except that you might wish to rethink slightly:
Barometric VS differs from inertial (tapeline or geometric) VS when temperature differs from ISA. Another reason why the use of barometric seems odd.
PJ2:
I owe you an apology for setting you off on the wrong foot. Although I am used to defining FPA relative to the airflow, the mention of 'drift' should have alerted me to the fact that FPV on the PFD is relative to the ground.
regards,
HN39
Thanks for your reply. I agree with the rest of your comments, except that you might wish to rethink slightly:
VS from the AD, (...) could only give a trajectory relative to the air – not the ground
PJ2:
I owe you an apology for setting you off on the wrong foot. Although I am used to defining FPA relative to the airflow, the mention of 'drift' should have alerted me to the fact that FPV on the PFD is relative to the ground.
regards,
HN39
Last edited by HazelNuts39; 22nd Mar 2010 at 18:18. Reason: 'except ...' added
Good evening HN39,
The sentence of mine you quote has effectively been rubbished by me, by virtue of my apology above. [I was mistakenly under the impression that you were arguing that FPV was based on TAS, which would have made the FPA relative to the airmass, not the earth.] Am tempted to amend my post, but that seems dishonest, and I doubt it will be read in isolation.
You raise another interesting point about the alleged dependence of the FPA/FPV ("bird") on VS data from the AD. The pressure lapse rate in the atmosphere does indeed vary with temperature deviation from ISA. I suppose the air-data system may correct for that, but that's me guessing again...
The cockpit altimeters do not correct for temperature deviation, presumably to keep all aircraft "singing from the same hymn sheet" for ATC separation purposes, and also because the AD system cannot know the temperature deviation of the entire atmosphere below the aircraft.
The sentence of mine you quote has effectively been rubbished by me, by virtue of my apology above. [I was mistakenly under the impression that you were arguing that FPV was based on TAS, which would have made the FPA relative to the airmass, not the earth.] Am tempted to amend my post, but that seems dishonest, and I doubt it will be read in isolation.
You raise another interesting point about the alleged dependence of the FPA/FPV ("bird") on VS data from the AD. The pressure lapse rate in the atmosphere does indeed vary with temperature deviation from ISA. I suppose the air-data system may correct for that, but that's me guessing again...
The cockpit altimeters do not correct for temperature deviation, presumably to keep all aircraft "singing from the same hymn sheet" for ATC separation purposes, and also because the AD system cannot know the temperature deviation of the entire atmosphere below the aircraft.
Last edited by Chris Scott; 22nd Mar 2010 at 22:53. Reason: Syntax improvement.
HazelNuts39, Chris Scott;
Exactly right, HN39.
So, with reference to Chris Scott's remark, where are we?
We have two cases: One in which the crew sets the FPA and the aircraft follows it under autoflight/FMGC guidance, and one in which the airplane is descending "normally" under autoflight/FMGC guidance either in the "OpDes/FltLvl Chg" modes which are the Airbus and Boeing settings respectively, or a form of VNAV which is typically altitude or airspeed constrained (not following an FPA in other words) and referenced to the destination airport taking into consideration the entire flight planned path entered in the FMGC.
Just so everyone is aware, the use of FPA for descent from cruise is almost never used in routine operations. Some here who don't fly but who understand what's going on here may know why right away but if not its discussed below.
Discussion points numbered for reference only:
1. First, when an FPA is selected by the crew on the autoflight control panel, (I believe for all types and not just the A330), it is defined by and referenced to the inertial platform, (essentially, the aircraft) and is not referenced to the airmass or the aircraft groundspeed. Once we set, say "3.0" in the FCU (Flight Control Unit), the FPA is a physical descent path.
2. Now I understand that airmass characteristics such as temperature and humidity do affect barometrically-referenced data. It is my impression that this data would indeed be corrected for "local" deviations from standard, (understand the comment that the ADIRU's/ADC's cannot know what airmass characteristics are below the aircraft).
3. In other words, while I understand that a fine-grained aerodynamics analysis of the notions set out in para.1 would disagree, neither a pilot nor the autoflight system nor the instrumentation on the PFD/Standbys can expect as an outcome, "3-decimal accuracy" in flight, even though the guidance equipment will likely be sending such fine-grained data to the autoflight system; - it's just not possible with the airmass variations and aircraft inertia involved. But the "approximation" is, (we have to assume, because its all certified and we do it all the time), good for all practical work both enroute and during the approach/landing on, for example, the non-precision FPA approach described above.
4. So back to FPA and AoA, let's assume the crew has selected a 3.0 FPA descent: As the wind component changes and as temperature (and humidity) changes during the descent minor adjustments are made to pitch to maintain the 3.0deg FPA - that means that the IVSI, (Inertial Vertical Speed) changes and of course the AoA may change slightly as well. In other words, the autoflight system will increase/decrease descent and in the latter case even add power to maintain the FPA set by the crew. You can now readily see why we don't use FPA for a routine descent. Both pitch and power are all over the place trying to maintain the theoretical path in space. In busy air, it's a mess and not very smooth for the passengers.
5. In the case where the FPA is set by the crew and the autoflight system flies it, the FPV symbol will be close to the setting because that's what the autoflight system is designed to do. When under VNAV or OpDes/FLCHG guidance, the FPV symbol will still indicate the actual FPA, whatever it is in the VNAV-guided descent.
6. The key here is, even as the descent is taking place the FPV symbol on the PFD indicates actual FPA for the aircraft in real time and so may be (guessing here guys) reliable as a practical working AoA.
7. I'm not sure if this got us anywhere in terms of deciding whether the algebraic difference between displayed pitch and displayed FPA by the FPV symbol is accurate in all cases, (the non-still-air case as described by Chris) but it may provide some grist for further thought. PJ2
Originally Posted by HN39
Although I am used to defining FPA relative to the airflow, the mention of 'drift' should have alerted me to the fact that FPV on the PFD is relative to the ground.
Originally Posted by Chris Scott
My response to HN39 has belatedly reminded me that AoA CANNOT accurately be derived from a comparison of Pitch and FPA (flight-path angle): it is only strictly true in a stationary air mass.
So, with reference to Chris Scott's remark, where are we?
We have two cases: One in which the crew sets the FPA and the aircraft follows it under autoflight/FMGC guidance, and one in which the airplane is descending "normally" under autoflight/FMGC guidance either in the "OpDes/FltLvl Chg" modes which are the Airbus and Boeing settings respectively, or a form of VNAV which is typically altitude or airspeed constrained (not following an FPA in other words) and referenced to the destination airport taking into consideration the entire flight planned path entered in the FMGC.
Just so everyone is aware, the use of FPA for descent from cruise is almost never used in routine operations. Some here who don't fly but who understand what's going on here may know why right away but if not its discussed below.
Discussion points numbered for reference only:
1. First, when an FPA is selected by the crew on the autoflight control panel, (I believe for all types and not just the A330), it is defined by and referenced to the inertial platform, (essentially, the aircraft) and is not referenced to the airmass or the aircraft groundspeed. Once we set, say "3.0" in the FCU (Flight Control Unit), the FPA is a physical descent path.
2. Now I understand that airmass characteristics such as temperature and humidity do affect barometrically-referenced data. It is my impression that this data would indeed be corrected for "local" deviations from standard, (understand the comment that the ADIRU's/ADC's cannot know what airmass characteristics are below the aircraft).
3. In other words, while I understand that a fine-grained aerodynamics analysis of the notions set out in para.1 would disagree, neither a pilot nor the autoflight system nor the instrumentation on the PFD/Standbys can expect as an outcome, "3-decimal accuracy" in flight, even though the guidance equipment will likely be sending such fine-grained data to the autoflight system; - it's just not possible with the airmass variations and aircraft inertia involved. But the "approximation" is, (we have to assume, because its all certified and we do it all the time), good for all practical work both enroute and during the approach/landing on, for example, the non-precision FPA approach described above.
4. So back to FPA and AoA, let's assume the crew has selected a 3.0 FPA descent: As the wind component changes and as temperature (and humidity) changes during the descent minor adjustments are made to pitch to maintain the 3.0deg FPA - that means that the IVSI, (Inertial Vertical Speed) changes and of course the AoA may change slightly as well. In other words, the autoflight system will increase/decrease descent and in the latter case even add power to maintain the FPA set by the crew. You can now readily see why we don't use FPA for a routine descent. Both pitch and power are all over the place trying to maintain the theoretical path in space. In busy air, it's a mess and not very smooth for the passengers.
5. In the case where the FPA is set by the crew and the autoflight system flies it, the FPV symbol will be close to the setting because that's what the autoflight system is designed to do. When under VNAV or OpDes/FLCHG guidance, the FPV symbol will still indicate the actual FPA, whatever it is in the VNAV-guided descent.
6. The key here is, even as the descent is taking place the FPV symbol on the PFD indicates actual FPA for the aircraft in real time and so may be (guessing here guys) reliable as a practical working AoA.
7. I'm not sure if this got us anywhere in terms of deciding whether the algebraic difference between displayed pitch and displayed FPA by the FPV symbol is accurate in all cases, (the non-still-air case as described by Chris) but it may provide some grist for further thought. PJ2
Hi PJ2,
Re your item 4:
Wish we understood this "IVSI" better. Is it inertial, or barometric, or a fiendish combination?
Not really relevant, but – in response to your comment – one or two pilots on my (A320) fleet used to use the FPA (AP engaged) for routine descents. They achieved continuous idle thrust by asking the A/THR to try and achieve an IAS of, say, 250, when the selected FPA was good for, say, 300 at idle thrust.
Re items 6 and 7, the following is my best shot.
In still air, including the absence of thermals or downdrafts, the pitch angle minus the FPA represents the AoA (provided, I suggest, due allowance is made for what used to be called the "riggers angle of incidence" of the wing mounting; which is a constant).
In air which is moving vertically, there would be a correction to be made. For example, imagine a jet in the cruise, with pitch +3 and FPA 0. Alpha is +3. Now it enters an area of continuous updraft, and pitch has to be reduced to +2 to maintain FPA 0. But Alpha must have been maintained at +3. If Alpha had reduced to +2, the aeroplane would accelerate downwards due to loss of lift.
The case where the air mass is only moving horizontally ("wind"), also leads to an error. This may be best illustrated by looking at an extreme scenario. Imagine a microlight equipped with similar flight instrumentation to an Airbus (!). It is flying straight and level at a TAS of 50 kts into a 50-knot headwind (GS 0), pitch +3, FPA 0, Alpha +3. Because the aeroplane is stationary in relation to the earth, an FPA of zero would only be possible if the pilot maintains the VS precisely at zero. The tiniest rate of descent would presumably result in FPA -90 (vertical descent), suggesting an Alpha of +93. Similarly, a tiny rate of climb would give FPA +90 (vertical climb), suggesting Alpha -87. But, in fact, Alpha remains at +3.
Not having tried to work out any figures, I don't know how significant these two types of errors would be to a jet pilot, experiencing spurious airspeed indications at high altitude and high TAS, trying to use AoA to recover from an upset. Perhaps not very significant?
In the AF447 case, I presume the predominant upper wind in the area of the accident would have been fairly light; but the thermals and downbursts potentially severe.
Re your item 4:
Wish we understood this "IVSI" better. Is it inertial, or barometric, or a fiendish combination?
Not really relevant, but – in response to your comment – one or two pilots on my (A320) fleet used to use the FPA (AP engaged) for routine descents. They achieved continuous idle thrust by asking the A/THR to try and achieve an IAS of, say, 250, when the selected FPA was good for, say, 300 at idle thrust.
Re items 6 and 7, the following is my best shot.
In still air, including the absence of thermals or downdrafts, the pitch angle minus the FPA represents the AoA (provided, I suggest, due allowance is made for what used to be called the "riggers angle of incidence" of the wing mounting; which is a constant).
In air which is moving vertically, there would be a correction to be made. For example, imagine a jet in the cruise, with pitch +3 and FPA 0. Alpha is +3. Now it enters an area of continuous updraft, and pitch has to be reduced to +2 to maintain FPA 0. But Alpha must have been maintained at +3. If Alpha had reduced to +2, the aeroplane would accelerate downwards due to loss of lift.
The case where the air mass is only moving horizontally ("wind"), also leads to an error. This may be best illustrated by looking at an extreme scenario. Imagine a microlight equipped with similar flight instrumentation to an Airbus (!). It is flying straight and level at a TAS of 50 kts into a 50-knot headwind (GS 0), pitch +3, FPA 0, Alpha +3. Because the aeroplane is stationary in relation to the earth, an FPA of zero would only be possible if the pilot maintains the VS precisely at zero. The tiniest rate of descent would presumably result in FPA -90 (vertical descent), suggesting an Alpha of +93. Similarly, a tiny rate of climb would give FPA +90 (vertical climb), suggesting Alpha -87. But, in fact, Alpha remains at +3.
Not having tried to work out any figures, I don't know how significant these two types of errors would be to a jet pilot, experiencing spurious airspeed indications at high altitude and high TAS, trying to use AoA to recover from an upset. Perhaps not very significant?
In the AF447 case, I presume the predominant upper wind in the area of the accident would have been fairly light; but the thermals and downbursts potentially severe.
Last edited by Chris Scott; 23rd Mar 2010 at 13:24. Reason: Glider replaced by microlight.
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FDR,
thanks for your post here but you remind me that I should not have left school that early .... For sure that luizmonteiro link is going straight to my favorites.
What I can say regarding AF447 and the relation between AoA and Indicated Airspeed is that the AoA will vary only marginally for a variation of speed between MMO and VLS.
I mention AoA but I could say Pitch as both match for a A330 in clean configuration for a level flight.
As put by HN39 earlier, maintaining green dot speed which is below usual cruising speeds, will produce a pitch attitude anywhere between 3.5 and 4 degrees. Green dot speed is not that far above VLS, so anything close to 5 degrees already, must be considered very seriously.
In case of erroneous airspeed indication, the crew will not necessarily notice such a minimal variation of pitch on their PFD. And I refrain mentioning turbulence ...
These unreliable airspeed indications at usual cruising flight levels can be a real trap for a crew, especially when the erroneous indicated airspeed is not recognised as such by the system, crew included.
Then you can also take account of the "protections" that may play around, but on the base of erroneous data …
thanks for your post here but you remind me that I should not have left school that early .... For sure that luizmonteiro link is going straight to my favorites.
What I can say regarding AF447 and the relation between AoA and Indicated Airspeed is that the AoA will vary only marginally for a variation of speed between MMO and VLS.
I mention AoA but I could say Pitch as both match for a A330 in clean configuration for a level flight.
As put by HN39 earlier, maintaining green dot speed which is below usual cruising speeds, will produce a pitch attitude anywhere between 3.5 and 4 degrees. Green dot speed is not that far above VLS, so anything close to 5 degrees already, must be considered very seriously.
In case of erroneous airspeed indication, the crew will not necessarily notice such a minimal variation of pitch on their PFD. And I refrain mentioning turbulence ...
These unreliable airspeed indications at usual cruising flight levels can be a real trap for a crew, especially when the erroneous indicated airspeed is not recognised as such by the system, crew included.
Then you can also take account of the "protections" that may play around, but on the base of erroneous data …
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I am again finding myself drawn into arcane mathematics and physics here about 'descent' and 'FPA'. Surely the whole point of having an independent AoA indication would be to enable a crew to ensure they are flying a safe AoA (away from any stall angles) in the event of corruption of the electronic wizardry? I do not see the need for such with fully functioning triple IRS and 'correct' IAS inputs.
Certainly trying to work out my AoA from PJ's picture, whilst wrestling with conflicting IAS inputs and possible IRU problems would have significantly challenged me! A simple vane driven display would do that with delightful ease. As with my example with the Harrier, IAS was IRRELEVANT - all that was needed was the magic 8 units from 0 kts to normal wing-borne flight. So easy!
Certainly trying to work out my AoA from PJ's picture, whilst wrestling with conflicting IAS inputs and possible IRU problems would have significantly challenged me! A simple vane driven display would do that with delightful ease. As with my example with the Harrier, IAS was IRRELEVANT - all that was needed was the magic 8 units from 0 kts to normal wing-borne flight. So easy!
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"Seabed Worker" and "Anne Candies"
The "Seabed Worker " berthed Recife at 0810m (1110z) 2010-03-22 and the "Annie Candies" ETA Recife is 0800m (1100z) 2010-03-24.
Will advise when "Anne Candies" berths.
mm43
Will advise when "Anne Candies" berths.
mm43
Chris Scott re your post 544
I also have only a civil background and therefore heve never flown AoA in normal operations. However, I used to fly as co-pilot or observer on many VC10 C of A air tests. As I remember, the aircraft was prepared by having a cabin window removed and replaced with a blanking plate on which a vane was fixed. This in turn was connected to an AoA indicator which was mounted in the centre on top of the glare shield. During stall tests to stick push it was always someone's sole responsibility to monitor the AoA indicator and shout if the push had not occured before 15 deg AoA. At which point both pilots pushed like hell!
May I congratulate the various contributors to this thread on some really excellent posts. Thank you!
I also have only a civil background and therefore heve never flown AoA in normal operations. However, I used to fly as co-pilot or observer on many VC10 C of A air tests. As I remember, the aircraft was prepared by having a cabin window removed and replaced with a blanking plate on which a vane was fixed. This in turn was connected to an AoA indicator which was mounted in the centre on top of the glare shield. During stall tests to stick push it was always someone's sole responsibility to monitor the AoA indicator and shout if the push had not occured before 15 deg AoA. At which point both pilots pushed like hell!
May I congratulate the various contributors to this thread on some really excellent posts. Thank you!
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A numerical example
RE: PJ2 #551; Chris Scott #552
Perhaps I can add to Chris Scott’s remarks by giving a numerical example:
Ambient conditions: close to sealevel, ISA, still air (initially)
Airplane: A330, 205t, cg as QF72, airspeed 200 kt CAS (= TAS), AoA = 5.5 degrees
Set thrust to achieve FPA = -3 degrees (down), pitch attitude 2.5 deg up, rate of descent 1061 fpm, maintaining 200 kt.
Next, throw in 20 kt headwind, leaving everything else unchanged, and FPA increases to –3.3 degrees.
To return to FPA –3, increase thrust to achieve rate of descent of 955 fpm, and pitch angle will increase to 2.8 deg up. AoA remains 5.5 throughout because airspeed remains 200 kt.
regards,
HN39
Perhaps I can add to Chris Scott’s remarks by giving a numerical example:
Ambient conditions: close to sealevel, ISA, still air (initially)
Airplane: A330, 205t, cg as QF72, airspeed 200 kt CAS (= TAS), AoA = 5.5 degrees
Set thrust to achieve FPA = -3 degrees (down), pitch attitude 2.5 deg up, rate of descent 1061 fpm, maintaining 200 kt.
Next, throw in 20 kt headwind, leaving everything else unchanged, and FPA increases to –3.3 degrees.
To return to FPA –3, increase thrust to achieve rate of descent of 955 fpm, and pitch angle will increase to 2.8 deg up. AoA remains 5.5 throughout because airspeed remains 200 kt.
regards,
HN39
Last edited by HazelNuts39; 23rd Mar 2010 at 11:12. Reason: Weight added
Chris, HazelNuts39;
and,
For me, these two examples make the matter quite clear; thank you both.
BOAC;
Absolutely agree with you. I think it would challenge anyone primarily because, though in now-understood cases where it may work, the FPV symbol was not intended as an AoA indicator. Throw in degrading systems as described and the task would be very difficult indeed.
I have asked myself in the face of many expressed dispositions towards AoA displays (almost exclusively from those who have flown military vice civilian equipment and thus have experience with direct AoA displays) why the industry has not moved in a similar direction given the other significant computing, system and display capabilities now available. In his post #540, Machinbird suggested examining accidents in which the presence of an AoA indication may have prevented the accident through better awareness of AoA. I sought a possible answer, albeit informally, in my response #541 citing six relatively recent fatal accidents which resulted from stalling the aircraft. I thought that an AoA indication may possibly have assisted the crew of only one accident, the USAirways B737 near Pittsburgh. Unless associated with sufficient and unmistakble warning, (we are into complexity and computers again), it appears as though the availability of an AoA indication would not have assisted, for example, the THY B737 crew. The Colgan case is similar in this regard as again, airspeed was permitted to bleed off and it is difficult to know if the either crew would have responded to another, different indication. It is clear to all that an AoA indication would not have prevented accidents in which the stall resulted from a contaminated/damaged wing.
It is interesting that, from the discussion anyway, only military aircraft appear to have AoA displays as standard. Would this point to the fact that military aircraft are operated much closer to aerodynamic extremes than airliners and that the benefits of having such information do not outweigh the cost? I can only speak from my own airline experience and the need for a direct AoA display was never apparent.
I am certainly not arguing against the display - the more information the better providing it is unequivocably clear to the crew and they are trained to use the information effectively; Given the many comments however, I am trying to understand why the display has never been installed in civilian airliners.
Whether such information would have assisted the crew of AF447 is of course speculation. However, given the large speed difference between a M0.80 cruise CAS and VLS (about 60kts and another 20+kts or so to the stall) it is difficult to comprehend a situation, even in most turbulence (unless direct entry into a cell), in which airspeed is lost that, as per QRH memory items, setting power, flying pitch in alternate law, (not difficult) would not in and of itself have stabilized the situation. I can't help but wonder what else went on that so severely destabilized the airplane.
PJ2
Originally Posted by HazelNuts39
Ambient conditions: close to sealevel, ISA, still air (initially)
Airplane: A330, 205t, cg as QF72, airspeed 200 kt CAS (= TAS), AoA = 5.5 degrees
Set thrust to achieve FPA = -3 degrees (down), pitch attitude 2.5 deg up, rate of descent 1061 fpm, maintaining 200 kt.
Next, throw in 20 kt headwind, leaving everything else unchanged, and FPA increases to –3.3 degrees.
To return to FPA –3, increase thrust to achieve rate of descent of 955 fpm, and pitch angle will increase to 2.8 deg up. AoA remains 5.5 throughout because airspeed remains 200 kt.
Airplane: A330, 205t, cg as QF72, airspeed 200 kt CAS (= TAS), AoA = 5.5 degrees
Set thrust to achieve FPA = -3 degrees (down), pitch attitude 2.5 deg up, rate of descent 1061 fpm, maintaining 200 kt.
Next, throw in 20 kt headwind, leaving everything else unchanged, and FPA increases to –3.3 degrees.
To return to FPA –3, increase thrust to achieve rate of descent of 955 fpm, and pitch angle will increase to 2.8 deg up. AoA remains 5.5 throughout because airspeed remains 200 kt.
Originally Posted by Chris Scott
It is flying straight and level at a TAS of 50 kts into a 50-knot headwind (GS 0), pitch +3, FPA 0, Alpha +3. Because the glider is stationary in relation to the earth, an FPA of zero would only be possible if the pilot maintains the VS precisely at zero. The tiniest rate of descent would presumably result in FPA -90 (vertical descent), suggesting an Alpha of +93. Similarly, a tiny rate of climb would give FPA +90 (vertical climb), suggesting Alpha -87. But, in fact, Alpha remains at +3.
BOAC;
Certainly trying to work out my AoA from PJ's picture, whilst wrestling with conflicting IAS inputs and possible IRU problems would have significantly challenged me!
I have asked myself in the face of many expressed dispositions towards AoA displays (almost exclusively from those who have flown military vice civilian equipment and thus have experience with direct AoA displays) why the industry has not moved in a similar direction given the other significant computing, system and display capabilities now available. In his post #540, Machinbird suggested examining accidents in which the presence of an AoA indication may have prevented the accident through better awareness of AoA. I sought a possible answer, albeit informally, in my response #541 citing six relatively recent fatal accidents which resulted from stalling the aircraft. I thought that an AoA indication may possibly have assisted the crew of only one accident, the USAirways B737 near Pittsburgh. Unless associated with sufficient and unmistakble warning, (we are into complexity and computers again), it appears as though the availability of an AoA indication would not have assisted, for example, the THY B737 crew. The Colgan case is similar in this regard as again, airspeed was permitted to bleed off and it is difficult to know if the either crew would have responded to another, different indication. It is clear to all that an AoA indication would not have prevented accidents in which the stall resulted from a contaminated/damaged wing.
It is interesting that, from the discussion anyway, only military aircraft appear to have AoA displays as standard. Would this point to the fact that military aircraft are operated much closer to aerodynamic extremes than airliners and that the benefits of having such information do not outweigh the cost? I can only speak from my own airline experience and the need for a direct AoA display was never apparent.
I am certainly not arguing against the display - the more information the better providing it is unequivocably clear to the crew and they are trained to use the information effectively; Given the many comments however, I am trying to understand why the display has never been installed in civilian airliners.
Whether such information would have assisted the crew of AF447 is of course speculation. However, given the large speed difference between a M0.80 cruise CAS and VLS (about 60kts and another 20+kts or so to the stall) it is difficult to comprehend a situation, even in most turbulence (unless direct entry into a cell), in which airspeed is lost that, as per QRH memory items, setting power, flying pitch in alternate law, (not difficult) would not in and of itself have stabilized the situation. I can't help but wonder what else went on that so severely destabilized the airplane.
PJ2
Last edited by PJ2; 23rd Mar 2010 at 13:46. Reason: Correct speeds in last para.
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All your detailed comments on how the FPV is working are certainly interesting, but as the ACARS messages might suggest, the AF447 crew has probably tried to get the help of that bird to stabilize the situation ... but the bird was just INOP !
The only available tool was a pitch and a thrust setting.
As PJ2 said, the bird is not usually used in routine descent or cruising phases. Its utilization is more dedicated to approach phases, a great help, almost magical to stabilize an non precision approach, a bit like having a flight director on its own.
To think that the bird was selected at FL350 makes me believe that the crew was looking for some kind of certitude. The FPV red flag was probably not what they were expecting ...
The only available tool was a pitch and a thrust setting.
As PJ2 said, the bird is not usually used in routine descent or cruising phases. Its utilization is more dedicated to approach phases, a great help, almost magical to stabilize an non precision approach, a bit like having a flight director on its own.
To think that the bird was selected at FL350 makes me believe that the crew was looking for some kind of certitude. The FPV red flag was probably not what they were expecting ...
CONF iture;
It would be unusual but I didn't get the sense from the ACARS messages that the crew was actually using the 'bird'. I believe the ACARS receives/records failure messages for transmission independently of crew selection of such system. Such messages are based upon system capability and availability and wouldnt' wait until the crew had selected the system before recording/sending such a message.
. . . as the ACARS messages might suggest, the AF447 crew has probably tried to get the help of that bird to stabilize the situation ... but the bird was just INOP !
. . .
To think that the bird was selected at FL350 makes me believe that the crew was looking for some kind of certitude. The FPV red flag was probably not what they were expecting ...
. . .
To think that the bird was selected at FL350 makes me believe that the crew was looking for some kind of certitude. The FPV red flag was probably not what they were expecting ...
