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AF447

Old 4th Aug 2009, 13:33
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fundemental question;
If I fly my (small) aircraft into an active Cu and the conditions in there cause my pitot tube to ice over resulting in my loss of control.....
Is it really the pitot tube failure that has caused the crash?
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Old 4th Aug 2009, 13:39
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DJ - all I have to go on as non-Airbus is 'Smart Cockpit' which says that 'Low Speed Stability' (which it says incorporates the audio stall warning) is ABSENT in ALT2 with 2 ADR disagree. I take it you are saying the audio is still available?
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Old 4th Aug 2009, 13:50
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Stall Warnings

RE: Hyperveloce (#4116)
Does anyone know how these alpha threshold are computed ?
No, you would have to ask Airbus Flight Test/Engineering. If you were in their shoes, this is what you could do:

You do a stall test (or several) in the particular configuration. Starting at a speed well above the stall, you gradually reduce airspeed until the airplane stalls. From the recorded data, you derive a curve of CL vs alpha (see for example page 5 in the Lutz presentation). On that curve you select an alpha-max. The CL at alpha-max defines the reference stall speed. If you want the minimum stall warning margin required by regulation, you calculate a speed equal to 103 percent of stall speed, and the corresponding CL. With that CL you go into the CL-alpha curve to find the stall warning threshold. Bear in mind that you may select a stall warning margin greater than the regulatory minimum.

regards,
HN39

Last edited by HazelNuts39; 4th Aug 2009 at 14:59. Reason: typo correction (2)
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Old 4th Aug 2009, 17:33
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AoA thresholds, Stall warnings & procedures

Originally Posted by HazelNuts39
RE: Hyperveloce (#4116)
No, you would have to ask Airbus Flight Test/Engineering.
Thank you HN39. Yes: CFD, wind tunnels, flight tests, and the return of experience will shape these AoA tables. But wouldn't it be of interest, for a pilot, to know these limits ? Aren't they the limits of his own authority as a pilot ? (before the automation overules him). Not knowing these limits could lead to a bad surprise from the automation: one day, nearing the limits of the flight envelope, these tables could activate a primary reflexes of the plane & litterally sort the pilot out of the control loop ? In fact, this already happened. Wouldn't it be usefull for a pilot who is handflying his plane (all the more in a critical phase, say at landing/take off) to know/be warned that the automation is about to take over the plane in a surprising move ?
Jeff
PS) Assuming a pilot decides to apply the stall procedure (in reaction to several stall warnings) and lowers the nose (and possibly increases the thrust if there is a margin left). Two flights have initiated a descent and departed from their initial route to avoid a possible traffic but they recovered their airspeeds rapidly during the descent and stabilized their altitude. How would the crew monitor the consequences/results of their actions (stall procedure) in the absence of airspeeds ? If these latter do not recover their normal values as rapidly as in the two previous cases, when would a crew abord his descent ?
Is it only necessary to get a couple of ADRs in agreement to recover the airspeeds and the normal display on the PFDs or is it required that the 3 ADRs are simultaneously consistent ?
Where is the GPS ground speed & altitude displayed in the cockpit ?
PPS) About the crew reaction to stall warnings: wouldn't it make a difference that the crew be {CoPi 32 yrs; CoPi 37 yrs} or {CPT 58 yrs; CoPi 32 yrs} if stall alarms sounded like in several cases of Pitot event ? If yes, is it probable that a crew rotation had occured before 02:10Z ?
..............CPT.... CoPi37....CoPi32.....................................
ATPL ......1990... 2001...... 2000* (*) only the theoretical part
Flight Hrs 10988* 6547...... 2936 (All types) (*) CaravelleXII,A320,Boeing 737
Flight Hrs 1747.... 4479 .... 807.. (A330/A340)
South Am 16....... 39........ 5..... (flights toward south America)

Last edited by Hyperveloce; 4th Aug 2009 at 21:32.
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Old 4th Aug 2009, 19:11
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DJ77:
More seriously, stalling an airliner in normal operations involves many risks that a sound captain cannot allow himself to take. eg: passenger or cabin crew injuries, inflight collision, engine flameout, aircraft damage, and more.
DJ, I think you misunderstood what I was getting at. My point is that with appropriate situational awareness, a flight crew should instantly be able to recognize whether a stall warning is real or spurious. Of course a stall is a serious matter, but if the cockpit crew is awake they shouldn't have to reflexively carry out some rote procedure without reflecting on it for a few seconds. From most of the posts here, I gather that some of our brethren here are spring-loaded to do things that might not be appropriate. I was asking people knowledgeable in AB operations whether a stall warning is something to panic over. From the responses, I gather not. There should be plenty of time to decide on the proper response.

Regards,

Ed
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Old 4th Aug 2009, 20:57
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RE BOAC (#4121)
Not being AB myself, I just try to understand the FCOM (Smart Cochpit and another source). I understand that "low speed protection" is a flight control function (ATA 27) while "audio stall warning" is generated by the Flight Warning Computer (see "smart cockpit", ATA 31, Indicating and Recording Systems 1-31-10 Page 4). I may be wrong but I think these two functions are completely independent.
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Old 4th Aug 2009, 21:35
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RE: HN39 (#4119)
With cruise AoA typically 2 degrees, wouldn't that leave 2.2 degrees?
I found the 3.5 deg cruise pitch attitude in the "Unreliable Airspeed Indication / ADR check proc." (FCOM 3.02.34 P21). I acknowledge it is for an IAS of 260 kt. Nominal IAS at M 0.80 / FL 350 is 273 kt so the pitch attitude at M 0.80 may be perhaps a half deg lower. Remember however that Air Caraïbe started and ended their 1 min and 26 sec airspeed rollback at exactly M 0.80. One can think that their airspeed did not vary much in between and still they had two stall warnings.

Regards,

DJ.
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Old 4th Aug 2009, 22:36
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Smilind_Ed (#4124),

I generally agree with your post, except on one point: you don't really have "plenty of time". Look, a stall warning should normally never happen aboard an airliner. If it happens, you must have missed something big or ... it is spurious. If you cannot determine which is true almost instantly, you have to take the safest course of action: believe the alarm. Usually, this cannot hurt and you buy time to analyse later. Otherwise, you would need to mobilise cognitive resources to analyse the available parameters and that takes time, very possibly too much time. We are only humans.

Regards,

DJ.

Last edited by DJ77; 4th Aug 2009 at 22:44. Reason: typo
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Old 5th Aug 2009, 01:03
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How Much Is "Plenty of Time"?

DJ, I guess we need to define "plenty of time". Five seconds, ten seconds? In a couple of seconds, a properly alert crew should be able to say to themselves, "Hey look, the airspeed just dropped and we got a stall warning, but the plane keeps on flying normally. Must be a failure of the airspeed indicators." As long as you're cruising smoothly do you really want to push over and add power, or whatever? I wouldn't think so but that's why I think an independent AoA indicator would be appropriate. Lately there has been a rash of erratic airspeed indications reported. It turns out that there have been incidents going back some time. Apparently the airlines don't want to pay for it but until the erratic airspeed problem is solved, if I were flying one of these, I'd like to have an AoA gauge. That way I wouldn't unnecessarily disturb the passengers.
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Old 5th Aug 2009, 02:20
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Re the stall discussion; ‘all warnings must be respected’, is generally true. Great care is taken to ensure that technology has a very low false warning rate for critical warnings – hazardous situations, but in some circumstances the crew are also alerted to an impending hazardous situation, e.g. TAWS - Amber alerts, then Red warnings. Crews, in normal circumstances are not required to, and should not attempt to identify ‘false’ warnings.

In conventional aircraft, many stall ‘warning’ systems have two levels.
An alert level is given before the aircraft stalls, e.g. 1.05 -1.1 Vs, where usually stick-shake provides additional awareness. Somewhat misleadingly, the majority of alerting signals are called “stall warning” (vice alerting), but the recovery action only requires avoidance of the approaching stall.
The actual stall (Vs) is defined by an identification signal (stall ident); often via a stick push which cues the full recovery action.

In aircraft such as the A330, there may be no requirement for one or both parts of a stall identification system, particularly where control/AOA limiting protections are used.
However, when flying in a reversionary mode – the loss of the protections, aspects of a conventional stall ‘alert’ are used.
AFAIK the setting of this alert provides the crew with information that they are approaching a stall and that ‘avoidance’ action is required (an alert). This is not necessarily stall recovery, just the alleviation of the impending situation.

I doubt that many crews either have or even those with this knowledge (as applicable to the A330) would be able to recall and apply it in such difficult circumstances – no display of primary information, unreliable airspeed, convective weather.
In the absence of specific training, any crew might well revert to old habits – basic training, a full-stall recovery.
In this, there may be similarities with the Colgan Air accident.

It appears that the specific combination of circumstances in this accident is very, very rare. Remove any one of the main contributions and the outcome could have been significantly different – note previous incidents.
The design assumptions about the use of alternative stall awareness, procedures for unreliable airspeed, and even the continued airworthiness status after reports of pitot problems were probably justifiable before this event; - after, now with hindsight, a change is required.
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Old 5th Aug 2009, 02:41
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my question safetypee would then be, if a/s (indications) are lost, and the STALL alert appears, does the pilot know if it is appropriate to 'recover', or merely to 'avoid'. I somehow have a hard time envisioning the small stick vibrating, and then articulating 'forward' at Stall. Wouldn't you think an AI would be critical at such a time as well, rather than 'optional' ??
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Old 5th Aug 2009, 02:52
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Re “does the pilot know if it is appropriate to 'recover', or merely to 'avoid'”.
The emphasis of my text was that the indication would only require avoidance - perhaps a subsequent nose drop or roll asymmetry would indicate a stall.
I do not know what specific procedures are recommended for an alert level, but I would suspect a small nose down pitch adjustment and application of power.
AFAIK the ‘alert’ is audio only.
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Old 5th Aug 2009, 09:06
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Hyperveloce:
Is it only necessary to get a couple of ADRs in agreement to recover the airspeeds and the normal display on the PFDs or is it required that the 3 ADRs are simultaneously consistent ?
The FCOM don't say airspeed indications, either exact or erroneous, are removed from the PFDs and ISIS (otherwise, you could not sort them out in case of ADR DISAGREE).
How would the crew monitor the consequences/results of their actions (stall procedure) in the absence of airspeeds ?
There are other cues, like pitch/thrust or pitch/vertical speed relationships.
As was already mentionned by others, stall warning procedure is different from stall recovery. the former was very well described by HazelNuts39 in post #4100.

DJ.
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Old 5th Aug 2009, 09:23
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Imagination

Has there been a single confirmed false stall warning in all these unreliable airspeed incidents?
regards,
HN39
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Old 5th Aug 2009, 11:31
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Pitot freezing event & false stall alarms: imagination ?

-during the two Air Caraïbe, the crew got two series of stall warnings a few (~30s and ~40s for the 2nd incident) seconds after the deluge of fault reports & the loss of flight assistances and protections (severe turbulence penetration proc.: N1 fixed between 81 and 82%, Mach 0.80).
-during a flight between Paris and Antananarivo (flight AF 908), FL370, A/THR OFF, Mach 0.80, after a similar sequence of faults, the crew immediately faced stall warnings although the A/THR was TO/GA locked (-50kts of speed trend), and decided to initiate a descent (and a departure from the initial route to avoid traffic during the descent).
-flight AF 422 (FGLZT) Paris/Roissy-Bogota, entering a cirrus veil at FL370, severe turbulence, A/P manually disengaged, deployment of the airbrakes to avoid MMO, Mach 0.78 selected, airbrakes retracted and A/P reengaged. Then again, new brutal acceleration, A/P automatically OFF, many red echoes on the weather radar, ice is beginning to accrete on the windshield. Altitude is maintained +/- 200 ft. Many thunderbolts around, key phrase to the PNC in the cabin. Suddenly the airspeed rolls back in the red area, recovers its value, then again the two airspeed indications are lost on the PFD and stall alarms are sounded. The PF was about to implement the stall procedure when the airspeeds recovered.
Over 6 recent known cases (incl. the two cases investigated by the NTSB), 4 produced stall warnings a few seconds (40s max for the AC case) after the classic sequence of faults had occurred, one produced a late overspeed warning (Northwest flight ?). If these weren't false alarms, this would be a very strange combination of coincidences, how to explain that without obvious alterations of the flight parameters (or with a thrust increase to TO/GA), the planes initially in stabilized cruise flight were approaching a stall in a blink of an eye, in all these cases ?
Jeff
PS) when you read the account of these flights, you don't really get the feeling that the crew had "plenty of time" (say more than 15-20 s) to think about the stall alarms, and you don't get the feeling that these were "smooth" quiet flights when the sequence of problems occurred. In the case of the AF 422, the turbulences had increased just before the event (maintaining altitude +/- 200 ft) and most of the flights had disengaged their A/THR (turbulence penetration), AF 447 excepted. There was a large number of sudden & confusing events concentrated within 1 or 2 minutes.

Last edited by Hyperveloce; 5th Aug 2009 at 12:32.
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Old 5th Aug 2009, 13:01
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Suspected false stall warnings

RE: Hyperveloce (#4134)

Jeff,
The Air Caraibe incidents have been properly analysed and the stall warnings have been found to be valid. I strongly suspect that the other events have been similarly analysed with the same result. The only result of Air Caraibe's meeting with Airbus 'engineers' in Toulouse has been that Airbus "would consider a possible amendment of the checklist" (my own words). Has the checklist been amended?
regards,
HN39
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Old 5th Aug 2009, 13:40
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Originally Posted by DJ77
Hyperveloce:
The FCOM don't say airspeed indications, either exact or erroneous, are removed from the PFDs and ISIS (otherwise, you could not sort them out in case of ADR DISAGREE).
My wording was not precise enought, sorry for that. The Air Caraïbe safety report explains how the PFD are reconfigurated in the event of an ADR disagree: "SPD LIM", RED FLAG (like in the AF 447 ACARS btw) and many of the following indicators are lost: VLS, S, F, the green dot, Vtrend, Vmax, VFEnext, Vsw (resulting from the CAS monitoring process which excluded the 3 ADR and triggered the ALTN2 and PROT LOST). Though we know that the Vtrend is still displayed (-50 kts during the Paris-Antananarivo flight), and I wonder if it is opportunate in the case ADR disagreement event.
But in the case of the AF 422, when it is said "réapparition de la vitesse puis conditions VMC", what does exactly mean "réapparition de la vitesse" ? Back to the normal PFD speed display with all the previously lost speeds indicators ?
There are other cues, like pitch/thrust or pitch/vertical speed relationships.
As was already mentionned by others, stall warning procedure is different from stall recovery. the former was very well described by HazelNuts39 in post #4100.
DJ.
Yes, I was assuming that the nose had been lowered a bit (and maybe the thrust had been increased) with the idea to avoid stalling conditions, not to recover from these (and in my thinking [*], these would tend to be spurious). Is the pich&thrust procedure only applicable to maintain a constant altitude (zero vertical speed) or is it easily implementable during a descent ? Once the plane is in descent, given moderated turbulence and probably no outside clues, how much time would you need to monitor the time evolution of {pich; thrust; altitude evolution or vertical speed} to get a sufficient confidence about your flight point ? Because it seems that 10s, 30s or 50s could lead to different outcomes if my rudimentary computations bear a sufficient degree of representativity.
Jeff[*] would there be also the opposite case (overspeed alarms, nose up/climbing and critical loss of speed in high altitude, nose up stall) ?

Last edited by Hyperveloce; 5th Aug 2009 at 13:51.
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Old 5th Aug 2009, 14:10
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Originally Posted by HazelNuts39
RE: Hyperveloce (#4134)

Jeff,
The Air Caraibe incidents have been properly analysed and the stall warnings have been found to be valid. I strongly suspect that the other events have been similarly analysed with the same result. The only result of Air Caraibe's meeting with Airbus 'engineers' in Toulouse has been that Airbus "would consider a possible amendment of the checklist" (my own words). Has the checklist been amended?
regards,
HN39
I don't know if the check-list has been amended (Airbus said it was working on it at the end of 2008) and if it was, if it could explain the differences shown by the BEA report between Airbus and AF SOPs.
I agree with you HN39, that AC report shows that the AoA exceeded the 4.2° threshold when the stall alarm sounded. But this 4.2° threshold is probably different under a normal law and normal airspeeds ? (since it is derived from these two parameters, and from the aerodynamical config.). Hence to me, this event would not be independant from the event "Pitot freezing" and to prove my point or disprove it, the only piece of information needed would be to get the normal AoA threshold (under a normal law) and compare it to 4.2°. I don't know if the AoA exceeding 4.2° was a definitive sign of a "valid"/real stall (probably not, since the plane's AOA reached 4.48° without entering a stall or having shown precursors of a stall), or if it was an AoA threshold with a margin to take in account the degraded situation (ALTN2).
Besides, there are two stall warnings, the one triggered by the alpha prot. (lost in ALTN2) and the one mentioned above (comparison to 4.2°). In this latter case, the stall warning comes with the cricket sounds: the AC crew heard the cricket when the stall warnings sounded, but not all the other crew: the AF 908 reported "stall warnings without the cricket sound".
Jeff

Last edited by Hyperveloce; 5th Aug 2009 at 14:24.
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Old 5th Aug 2009, 14:42
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RE: Hyperveloce (#4136)

Jeff,
Stall avoidance, properly executed, is very gentle. Few, if any, person in the cabin would even notice it in still air, and even less in turbulence.
Perhaps it needs to be explained that, unlike most other warnings, stall warning continues as long as AoA exceeds the warning threshold, and ends when AoA is reduced below that threshold. Thats where the avoidance ends and 'normal' flight is resumed.
regards,
HN39
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Old 5th Aug 2009, 15:01
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PRIM1/SEC1 voluntary reset

I sincerely hope that every pilot flying Airbuses have read this Air Caraïbe flight safety report.
Among the last ACARS messages are the PRIM1/SEC1 faults.
The Air Caraïbe and the Qantas flights tried to reset their master flight computer in the hope to regain the normal law for the rest of the flight (Qantas was advised by the ground maintenance/technical center).
Counting from the A/P and A/THR OFF event, the unreliable airspeeds events have never lasted more than 3mn. Taking an upper range duration of 3mn would give us ~02:13:30Z, which is the time when the PRIM1 and SEC1 fault appear, the plane was probably already loosing altitude rapidly (if we exclude the high dive scenario, with 20 000, 30 000 or 40 000 fpm vertical rates). We are sure that the airbrakes (if they were deployed before) were retracted at this moment. The other planes reset their PRIM only after they had stabilized the situation.
Jeff

Last edited by Hyperveloce; 5th Aug 2009 at 15:20.
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