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AF447

Old 2nd Aug 2009, 17:53
  #4081 (permalink)  
 
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If the AD calls for r/r of two pitots only, and AB believes all three failed on 447, you see the problem? Something doesn't fit.
Could the folks who wrote the AD be aware of a logistics problem, i.e. there are only so many Goodrich probes available so they're spreading them out over the entire fleet? Agreed that something doesn't fit, at least from where I sit.
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Old 2nd Aug 2009, 18:04
  #4082 (permalink)  
 
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With autos not selling, tire sales are off, Goodrich isn't that busy. I'm convinced it's money, and I'm not the only one. AF and AB are in a tough spot.
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Old 2nd Aug 2009, 18:30
  #4083 (permalink)  
 
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Goodrich doesn't make auto tires anymore. They sold the tire brand to someone else, maybe a Japanese company, and are strictly in the aerospace business now. I think they make landing gear, wheels, and brakes. Didn't realize they were in the pitot tube business too until this came up.
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Old 2nd Aug 2009, 18:42
  #4084 (permalink)  
 
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Pitot tube business

Smilin,
Didn't realize they were in the pitot tube business too until this came up.
I believe the 'pitot tube business' was previously called "Rosemount".

regards,
HN39
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Old 2nd Aug 2009, 19:01
  #4085 (permalink)  
 
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Stall Warning

RE: Hyperveloce (#4076)

The Conclusion of the the NTSB Airplane Incident Report:
The National Transportation Safety Board determines the probable cause(s) of this incident as follows:
The captain's improper procedures regarding stall avoidance and recovery.
Contributing to the incident were ...
Pilots MUST respect stall warning.

regards,
HN39
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Old 2nd Aug 2009, 19:13
  #4086 (permalink)  
 
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It is reasonable to assume that more is known than publicly available. There has been a lot of speculation concerning the probes. So I am assuming that by now a significant number of probes have been inspected, and problems were found. Possibly, corrosion has been mentioned; if so, it is only a matter of time before some pictures become available? Or, are the probes being replaced without a clue of what's going on?
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Old 2nd Aug 2009, 20:48
  #4087 (permalink)  
 
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Question

As a non-pilot I have a question. I know this is off-topic but it has been bugging me reading this forum.

What I do not understand is the sequence "IAS declines - stall warning sounds - stall procedure initiated". If the IAS declines during a well defined stable cruise with a specific engine thrust I would think that the stall warning and IAS would immediately be suspect. In fact a stall procedure at this stage might even be unsafe.

Last edited by ernst_mulder; 3rd Aug 2009 at 08:59. Reason: Bad spelling
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Old 2nd Aug 2009, 21:01
  #4088 (permalink)  
 
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Am I missing something regarding faulty airspeed on the 757/767? The pilot seeing erroneous airspeed just has to reach forward and switch to Alternate ADC.

GB
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Old 2nd Aug 2009, 21:11
  #4089 (permalink)  
 
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from FAA (2005)

TSO C16, Air-speed tubes (electrically heated), requires compliance
to the performance specifications of SAE Aerospace Standard AS393. SAE
AS393A includes a test to demonstrate deicing and anti-icing capability, but only temperature and airspeed are specified. Liquid water content is not specified but it influences heat requirements. Although functioning of pitot probes are evaluated in natural icing conditions during certification test programs, there is no requirement to flight test at the Appendix C or X icing limits because the low probability of finding those conditions imposes a burden. The airframe manufacturer is responsible for showing the pitot heat is adequate for the Appendix C and Appendix X icing conditions. If not obtained in flight test, analysis or icing tunnel test data should be submitted.
Although Appendices C and X of part 25 only consider the liquid
water content of icing conditions, recent cloud characterization research has indicated that approximately 40 percent of icing condition events consist of liquid water drops and ice crystals (mixed-phase icing conditions). The ice crystal environment may be more critical than liquid water for thermal systems since more energy is required to evaporate the ice crystals.
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Old 2nd Aug 2009, 22:17
  #4090 (permalink)  
 
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syseng68k, what use is a closed loop control of you do not know the sweet spot for applying the sensor and have a way of doing so without destroying the probe's calibration? I strongly suspect that presuming the entire probe is all at one temperature is a serious mistake.

Something changed in the last few years that is affecting the probes of at least one manufacturer rather adversely. It's not global warming, the time frame is wrong to declare that. We've been cooling this last decade. So let's look, instead, at things like probe maintenance, probe mounting, flight regimes used, and so forth.

That might be some worthwhile study even if it does not apply to the accident under discussion here. I've certainly seen no proof that this is what happened to AF447. "It's too simple," is the way I'd describe it as a cause. AF447 was more likely done in by a rather rare cascade of failures, perhaps improper radar settings (lack of training), perhaps asleep in the cockpit, perhaps distracted by preparations for the 4 hour crew change, perhaps it involved engine icing, perhaps any number of things mentioned here. (Maybe some of them can be surmised from noting the strange position at which the plane apparently hit the ocean with a very strong vertical velocity component. A really good "it went down here" position might sharply limit the flight (or spin) profiles that could have taken place. That would also require some good guesses at just how high a vertical velocity component was needed to cause such a breakup.)

Look to things that somewhat limits the probability that it's a daily event for other AF447 flights before this one. I note one, it was particularly dark with very little illumination from the Moon. That limits any visual horizon for orientation. That happens half or less of each month. Another is the somewhat exceptional storm. And, yes, icing on pitot probes seems to be an exceptional event. But is this enough for explaining why it's only happened once since A340s were flying that route at that time of day? (Does the flight more commonly have its 4 hour crew change happen at a different position in the flight? Could this have been caused by the captain being overdue in the cockpit leaving the F/Os preoccupied with "where is he?")

JD-EE
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Old 2nd Aug 2009, 23:43
  #4091 (permalink)  
PJ2
 
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BOAC;

Yes, the Aeroperu had the statics taped over with clear tape while the airplane was washed. The Birgenair 757 pitot loss cause was undetermined but there was a theory that mud-dauber wasps had built a nest in one of the pitot tubes. The current issue is disconcerting for sure. Because of the autoflight system depth of component and software integration the effects can be very distracting and initially difficult to assess while the airplane's warning systems settle down. Cascading failures can be very distracting, but that is not the primary issue here. The key is to do nothing and change nothing because the airplane was just fine moments before the event. The issue here is surprise and confusion not the loss of data and that is not a situation which will inevitably lead to a LoC.

GB, not sure who you're asking re "the pilot seeing erroneous airspeed" and re the point you're missing. I don't recall a lot about the 1990 incident and what we did but if it was my post your asking about, the point you're missing is obvious I think - the unreliable airspeed problem was essentially a non-event. We determined which indicator was correct using the Standby ASI and we flew the airplane to destination without further incident.

I don't recall any drills in the books which called for switching ADRs as, at the time, there was no such thing as an unreliable airspeed. We just used our heads and made it up as we went and it worked just like it should. The Birgenair and Aeroperu accidents occurred in 1996 I believe.

mulder:
I would think that the stall warning and IAS would immediately be suspect. In fact a stall procedure at this stage might even be unsafe
In my opinion, you are correct. "Respect the stall warning" must be understood as meaning, respect "legitimate" stall warnings. We expected (but did not get) an overspeed warning when the captain's ASI exceeded the barber pole, (Max operating speed). We would have ignored the overspeed warning in that case.
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Old 3rd Aug 2009, 11:48
  #4092 (permalink)  
 
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In US terminology, incidence is defined as the angle between the reference chord line of the wing and the longitudinal axis of the fuselage. As far as I know, this angle generally lies in the range 0 to 3 deg. With this definition, we have in level flight:
Pitch attitude = Angle of Attack - incidence.

The A330 "Unreliable Speed Indic. / ADR Check Proc." contains a table of Pitch / Thrust values. In this table, for parameters corresponding to AF447 known data at the time of the events, ie:

clean configuration,
cruise phase,
gross weight > 190 t,
FL250 – FL 370,

you can read:

Speed = 260 kt,
Pitch attitude = 3.5 deg,
N1 = 90%.

The pitch attitudes values in the table are rounded to the closest 0.5 deg.

The report on the ACA incident says (page 11) that the "STALL" audio alarm is triggered when the AoA exceeds a predetermined value that depends on

Flaps or Slats position,
Speed or Mach (???),
Active control law (normal, alternate or direct).

Then it states that two STALL audio alarms occured during the incident for AoAs of 4.48 deg and 4.31 deg respectively. Those values were obviously obtained from FDR data. A note adds that the STALL alarm is triggered at a threshold AoA of 4.2 deg. I believe that this value only applies in the configuration of the incident and was obtained from Airbus personnel.

It may look stange that AoA depends on airspeed but probe litterature generally speak of "local AoA" and "corrected AoA". An important design stage for AoA probe installation is to find a place on the fuselage where the correction is quasi linear relative to airspeed. This is done experimentally in wind tunnel. One would expect that if the associated Air Data Computer detects unreliable airspeed, it uses a default conservative correction factor. Alternatively, the warning computer could use increased margins, which would account for the threshold AoA dependancy upon the active control law.

Since the configurations of the ACA flight and AF 447 were very close (cruise, clean, about same weight, same level), It seems very likely that AF 447 experienced STALL alarms. Moreover, if the 4.2 deg AoA threshold value is true, the formula:
Pitch attitude = Angle of Attack - incidence
3.5 deg = 4.2 deg - incidence
shows that, at this weight and flight level, even taking into account rounding and using zero incidence, the a/c is very close (less than one deg) to the stall warning alarm threshold. A little pitch up move of the sidestick or a small updraft could trigger it.

Posted by HazelNuts39: Pilots MUST respect stall warning
That is fairly true. This alarm is in the category of "time critical warnings", like GPWS or windshear alarms, that require immediate crew awareness and corrective action. Crew Resource Management teaches that when such an alarm occurs, it's not the time to start wondering what caused it, but to act.
The PF of the ACA flight fortunately did not perform the stall recovery procedure because he was convinced the plane was not stalled. This conviction was certainly based, among other cues, on the fact that he had been controlling airspeed manually for some time when the airspeed rollback event occurred. In fact, FDR data show they exited the event at the same airspeed they entered it. They did respect the stall warning in an appropriate way since they voluntarily ignored it.

Contrary to ACA, the A/T was controlling airspeed aboard AF447, until it disconnected. The crew was not at the same level of awareness about the airspeed before the indicators went astray. If a STALL alarm sounded, which I think is very likely, they logically respected the alarm the standard way: nose down, full thrust then ... loss of control ?

The main cause of the accident could be ... failure of the crew to disregard a false alarm.

Last edited by DJ77; 3rd Aug 2009 at 15:23. Reason: Misspelled HN39 pseudo (apologies to him).
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Old 3rd Aug 2009, 12:11
  #4093 (permalink)  
 
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"legitimate" stall warning

RE: PJ2 (#4090)

"Respect the stall warning" must be understood as meaning, respect "legitimate" stall warnings.
I’m inclined to question the wisdom of advocating “respect only ‘legitimate’ stall warnings”. Firstly, I believe that the risk involved in the appropriate response to stall warning tends to be overstated, considering that in most of these incidents the stall warning occurs after the speed has been reduced to turbulence penetration speed, typically Mach 0.78 – 0.80. Secondly, I believe that we (several posters including myself) may have been wrong in assuming that these stall warnings are “false”.

To elaborate on the first point:
(a) from ASR on FGNIH AF908 CDG-TNR -
Suivi immédiatement de l'alarme STALL STALL STALL (sans l'alarme cricket associée) avec indication de TOGA LK. Comme j'avais toujours le speed trend à moins 50 Kts, j'ai piloté l'avion en manuel avec mise en descente et léger virage à droite pour sortir de l'AWY. L'avion répondant très mollement avec sur le PFD CDB plusieurs régressions de vitesse dans le bandeau rouge inférieur. (...) Descente jusqu'au FL 340. La vitesse avion étant redevenue correcte j'ai débrayé l'ATHR pour sortir du TOGA LK. Les indication de vitesse étant similaires coté CDB et OPL, ...
and (b) from NTSB Report on Icelandair B757 incident; Icelandair Stall Recovery Training; as per Boeing 757 Flight Crew Training Manual; see source document for Approach-to-Stall Recovery procedure -
A stall must not be confused with the stall warning that alerts the pilot to an approaching stall. Recovery from an approach to a stall is not the same as recovery from an actual stall. An approach to a stall is a controlled flight maneuver; a stall is an out-of-control, but recoverable condition.
To elaborate on the second point, I would like to re-examine information provided in the Air Caraibe memo:
On page 12 -
la check-list « UNRELIABLE SPEED INDICATION » ... , dans sa partie développée en 3.02.34 page 17, celle-ci stipule «RELY ON THE STALL WARNING THAT COULD BE TRIGGERED IN ALTERNATE OR DIRECT LAW. IT IS NOT AFFECTED BY UNRELIABLE SPEEDS, BECAUSE IT IS BASED ON ANGLE OF ATTACK ».
On page 11 -
l'alarme « STALL » se déclenche deux fois pour un « AOA » de 4.48° puis 4.31°. NB 5, seuil pour le déclenchement de l'alarme « STALL » : « AOA »=4.2°
On page 6 -
«HIGH ANGLE OF ATTACK PROTECTION»: (...) Faisant suite à un problème technique et dans le cadre d'une reconfiguration de la loi de pilotage en « ALTERNATE LAW 1 », il existe une nouvelle protection appelée « LOW SPEED STABILITY » laquelle s'active entre 5KT et 10KT au dessus du décrochage. La profondeur passe alors en « DIRECT LAW». Néanmoins, un ordre qui peut être surpassé par le pilote et visant une diminution de l'assiette est introduit pour maintenir une marge par rapport à la vitesse de décrochage. Une alarme sonore « STALL » s'active. Sur le « PFD » les vitesses « Va PROT » et « Va MAX » sont remplacés par Vsw. Pour terminer, il faut souligner que le givrage des sondes « PITOTS » et « TAT » entraîne au préalable l'apparition du message « F/CTL ADR DISAGREE » qui engendre à son tour le passage en « ALTERNATE LAW 2 » avec dans ce cas la perte de la protection « LOW SPEED STABILITY ».
I’m taking the text quoted from page 12 for what it says: stall warning in alternate or direct law is based on AoA; it is not affected by unreliable speeds.
Likewise for the facts quoted from page 11: two stall warnings occurred at AoA’s 4.48 and 4.31 degrees, for a threshold of 4.2 degrees. In other words, these are valid stall warnings as per system design.
The text on page 6 is not entirely clear to me. Does it mean that stall warning in Alternate Law 1 occurs at an AoA corresponding to ‘between 5 and 10 kt’ above stall speed? In that case the stall warning margin in Alternate Law 1 ‘as per system design’ would be about twice the regulatory minimum of 3 knots or 3 percent, which ever is greater. This could make sense in view of the lost alpha-prot protection, while the stall warning margin in Normal Law is presumably closer to the regulatory value. The last sentence (Pour terminer ...) is not understood. If it means that stall warning is lost, that would contradict the procedure which aims to retain stall warning by keeping one ADR ‘online’.

regards,
HN39


PS: After writing the above, I read DJ77's #4091, but decided to post my contribution anyway because I think it's still valid. DJ77's last paragraph raises the question: Would the occurrence of a stall warning normally trigger an ACARS message?
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Old 3rd Aug 2009, 13:40
  #4094 (permalink)  
 
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Hi there. Thank you D77, PJ2, all, for these reasonning.
-The Air Caraïbe report states that the stall warnings are triggered as soon as the AoA exceeds 4.2° (can't we find the tables for these AoA thresholds as a function of the aerodynamical configuration & the Mach ?), so indeed, a downdraft of ~10 kts would be sufficient to biase the AoA so as to induce a stall alarm ?
-In most of the known recent cases (about Pitot icing & corrupted airspeeds), stall warnings were sounded, in one occurence it was an overspeed warning. The freezing event durations seem to lie between 1mn and 4-5mn (3mn average).
-It appears that some pilots (were about to) react(ed) to the stall alarms
-The simulation trainings only implement true stall/overspeed alarms justified by conditions in most cases known and prepared beforehand.
-Facing a decision about a stall warning, wouldn't a pilot consider both the cost of disreguarding a justified stall warning and the cost of taking in account an undue stall warning ? (depends on where the PF thinks his plane is within the flight envelope)
-Which are the signals a pilot should consider to make his decision about stall warnings ? (ADR disagree context)
D77 suggests that the Air Caraïbe crew may have had a better, real time awareness of their airspeeds/thrust (A/THR manually OFF) than the AF 447 crew when they faced the stall warnings.
A cross-checking in the BUSS fashion (pich/altitude time evolution for a given pich,thrust)
A cross(checking in the OCTAVE fashion: airspeeds directly compared to GPS ground speed corrected by the OCTAVE
wind database (time required for that ? accuracy versus the aero margins ?)
.../...
on the contrary what would be the false friends to avoid when you have to decide about stall alarms ?
the speed trends if they are maintained/still displayed on the PFDs !
the part of the unreliable airspeeds procedure that tells to take stall alarms in account (and to disreguard the
ECAM warning "risks of undue stall warnings")
.../...
If it appears that the AF 447 may have lost control in high altitude cruise because it reacted badly to a stall alarm, it should be possible to assess the amount of time required to increase the airspeed from Mach 0.80 (~cruise speed) to Mach 0.85 (reaching MMO) then to Mach 90 (severe overspeed) under a few hypothesis ( thrust ; descent angle )
couples ?
Does anone understand why one of the planes which went through a Pitot freezing event (with the deluge of similar ECAM messages) found itself in TO/GA LK ? (the A/THR was manually disengaged before)
Jeff
PS) stall/ overspeed warnings are not transmitted by the ACARS

Last edited by Hyperveloce; 3rd Aug 2009 at 13:54.
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Old 3rd Aug 2009, 14:17
  #4095 (permalink)  
 
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On the A330 in manual thrust if speed drops below a certain point, ( 'alpha floor', close to the stall), auto thrust re-engages at TOGA power (thrust lock). The only thing that will stop it is if auto thrust is unserviceable.
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Old 3rd Aug 2009, 14:49
  #4096 (permalink)  
 
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Yes jshg, I have also read that the TO/GA LK could only result from the activation of the alpha prot (the AoA law was only available before the ADR disagree and the deluge of ECAM alerts, it is lost in ALTN2). The thrust index N1 will also be locked if the A/THR fails, then "THR LK" will flash on the FMA at the top of the PFD (the PF should not miss it if he is hand flying the plane ?) when the ECAM will display "AUTO FLT A/THR OFF". Will the FMA diplay THR LK or TOGA LK if the N1 was full thrust (TO/GA) when it was locked ?
If one of the planes (Pitot freezing) found itself in TO/GA LK after the ADR disagree, does it suggest that its alpha prot had activated just before ? If the A/THR was manually set to OFF, it shouldn't have been chasing airspeeds in this case (but wouldn't the alpha prot. also reduce the AoA or the pich ?)
As the AF 447 is the only case with an engaged A/THR when the problems occured, I would really like to know whether there is a mean to confirm that an A/THR in managed speed mode can chase declining airspeeds (thus reducing the margins left to the pilots). But whether it was an alpha prot. activation or an A/THR chasing declining airspeeds, the result would be similar: the automation reducing the margins of the pilots/crew before it threw the towel ? The next question would be to know whether these two possibilities can remain outside the situational awareness of the crew (taking in account the context).
In any case, assuming that the pich was lowered and the thrust was increased either by automation or the crew, I feel it would be really interesting to know if the crew had only 30 sec and 60 sec before the plane reached MMO and Mach 0.9 (for example assuming a 2° descent from FL350 and TOGA). To know how much time left the crew had to recognize/acknowledge the overspeed situation.
(actions: thrust on idle, airbrakes deployed, severe overspeed: lower the landing gear ?)
keeping in mind the possible PRIM1/SEC1 voluntary reset (between 02:13:15 and 02:13:30): if the airbrakes were deployed, they would have been retracted by this event whether it be automatic or manual.
Jeff

Last edited by Hyperveloce; 3rd Aug 2009 at 15:31.
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Old 3rd Aug 2009, 15:17
  #4097 (permalink)  
 
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HazelNuts39:
The text on page 6 is not entirely clear to me. Does it mean that stall warning in Alternate Law 1 occurs at an AoA corresponding to ‘between 5 and 10 kt’ above stall speed?
Yes, that's what it means.
The last sentence (Pour terminer ...) is not understood. If it means that stall warning is lost, that would contradict the procedure which aims to retain stall warning by keeping one ADR ‘online’.
My tentative translation:
"Finally, it is worth noticing that PITOT and TAT probes icing entails first the << F/CTL ADR DISAGREE>> message pop up, which then generates switching to <<ALTERNATE LAW 2>> and, in this case, loss of <<LOW SPEED STABILITY>> protection."

the <<LOW SPEED STABILITY>> protection is described in the first part of the paragraph. Since the system can no longer determine if there remains a valid airspeed, it seems logical to cancel the protection. That does not preclude stall warnings based on AoA.

btw, I doubt that TAT probe icing plays a role in this sequence.
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Old 3rd Aug 2009, 15:33
  #4098 (permalink)  
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Back to the AB experts please. Again apologies if I am going over old ground, but the reading of the thread remains a challenge.

I understand the ABI instruction is to 'respect the stall warning', but to me it looks as if there will not be one?

From my reading of the 330 FC section, it appears that in ALT 1 there will be an IAS driven stall warning, but in ALT 2 with 2 ADRs out there will not, and with 3 out there will be also no overspeed warning.

Obviously there cannot be a reliable 'overspeed' detection, but I cannot see why an AoA triggered stall warning is denied in a worst case scenario of no IAS to work on. Are AB pilots actually happy with this or should the warning be available? (E&OE of course) Surely this is the only 'long-stop' left to a crew with multiple IAS failures?
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Old 3rd Aug 2009, 15:46
  #4099 (permalink)  
 
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The Bowling Ball Again

I continue to get the feeling from this conversation that at least some pilots feel that a stall warning requires instantaneous action to prevent a major upset. When I mentioned this earlier, someone, I forget who, reassured me that flying a Bus in AF442's situation was NOT like trying to balance on a bowling ball. Is there not time to assess the situation and take deliberate response vs. instantaneous reaction? Would they not have had time to determine whether a stall warning was legitimate? What's the rush? If, unlike the Colgan and Turkish crews, the AF crew was not asleep, would not a thoughtful assessment of the situation be the prudent course?

Also, are angle of attack data not derived from a vane (or whatever) independent of the pitot/static, ADIRU, gyro, and inertial systems? Would not a glance at an independent AoA readout be the way to verify or validate a stall warning while in cruise. Is there such a readout?
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Old 3rd Aug 2009, 16:07
  #4100 (permalink)  
 
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I agree with SmilinEd. Just because one sees a warning is no reason to go into Panic mode. My sense of this crew tells me with their experience and skill, they were calm and professional. As A330 pilots, they were the best informed and current in type than any, "Stall? mmm...what gives?".

One can get carried away with 'nonchalance', however. That 1011 that plopped in the swamp had everyone fixated on a broken green lamp. That's taking 'detachment' a bit far.
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