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AF447

Old 3rd Aug 2009, 17:13
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Bowling Ball

RE: Smilin Ed (#4098) and Will Fraser (#4099)

Smilin,
The issue we're discussing has nothing to do with AB vs the rest of the world. A distinction needs to be made between stall avoidance and stall recovery. Stall warning warns that stall is approaching, it is NOT identification that the airplane is stalled. Stall warning begins when AoA exceeds a threshold below the stall, and continues until AoA is reduced below the threshold. Stall avoidance demands prompt response. No panic, just a gentle reduction of an abnormally nose-high attitude will stop the warning. The immediate effect of pitching down is to reduce AoA. The reduced lift causes the airplane to descend so that it will pick up some speed. Meanwhile the engines will have spooled up to provide the extra thrust that will recover the lost altitude. If you don't respond, the stall warning may cease while maintaining pitch and thrust, e.g. if it has been caused by an upward gust. If you allow the stall warning to persist, you don't know whether you are close to the warning threshold or close to stalling. In the latter case, a more agressive maneuver may be called for to avoid stalling. If you do stall, no need to panic either - the airplane has demonstrated safe stalling characteristics in many (hundreds?) stalls before type certification. You will loose a couple of thousand feet, will see some unusual attitudes and g-levels, but if you stay cool you will end up straight and level. You may have some explaining to do after you get home - like that Icelandair captain.

regards,
HN39

Last edited by HazelNuts39; 8th Aug 2009 at 17:43. Reason: expanded explanation
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Old 3rd Aug 2009, 17:38
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Well, if you get stall alarms at a flight level of 350 and if you see -50 kts of speed trend (trend over 10 seconds) on your PFD, isn't it a powerfull invitation to think quick, given your thin margin ? (probably <50 kts at FL350 ?). No need to panic of course, but you have to take this decision just after being rained down with multiple ECAM alerts, aural and visual alerts, and after having lost most of your flight protections. Maybe you had a little time to think about these multiple signals, to try to understand all these confusing signals displayed on the ECAM and the PFDs, then a new problem arises, you hear that your plane is stalling. No need to panic, but a clear & quick analyzis in a high workload & confusing situation ? The misleading signals: the stall warnings themselves, the airspeeds, the airspeed trends, the ECAM warning about "risks of undue stall warnings" and the SOPs urging you to take stall warnings into account (and to disreguard the ECAM warnings).
Sorry but I know how I feel when my PC goes haywire, produces strange & conflicting signals, and when my troubleshooting book or service is of no help. I how it can be frustrating, although I have all my time to comprehend between two coffees, in a flat world. I can't figure out how I would be, if I had only tens of seconds to solve a very confusing problem, with a plane to handfly in the same time with no visibility, in the night, buffeted in a tropical thunderstorm. I guess I would really be tempted to reboot the failed automation, but I am not a pilot.
All I know are past incidents, and past incidents show that some pilots are taking the stall alarms (and some other flight parameters) seriously. And some other incidents show attempts to reboot the PRIM/SEC inflight to regain a normal law. I don't know if you have the time to think, but in certain situations, you don't even have the time to send a MAYDAY.
Jeff
----------------
PS) Are there formulae or tables to compute the time needed to reach Mach 0.85 and Mach 0.90 starting from FL350, Mach 0.80, according to several descent angles (1░,2░,3░...) and different thrust levels (N1=0.85, 0.90, 0.95, Max) ? is it possible to compute the same thing but with a banking angle ?
----------------

Last edited by Hyperveloce; 3rd Aug 2009 at 19:17.
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Old 3rd Aug 2009, 18:02
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Super Fast Response Required?

HN39, my comment/questions are not unique to AB to any other manufacturer. It just happens that this is an AB accident. I still think the attitude on this thread, regardless of make of plane, seems to be that response to a stall warning has to be super fast. Does it? If the crew is awake and the plane seems to be flying OK, there's no need to panic and consequently take the wrong action. Just deliberate assessment of the situation followed by appropriate action.A glance at the AoA, should quiet any fears. IMHO, there should be an independent AoA readout available at all times.
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Old 3rd Aug 2009, 18:42
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They still fit an ADF though......

ELAC explained the reason why some time back. This is getting to be a really long thread........

Rgds.

24V

Actually, the instrument exists and has been an available option for FBW Airbus's for at least 16 years, and probably since the introduction of the A320 in 1989. If you look at a cockpit photo you can even see the "blank" in the upper left corner of the Capt.'s instrument panel where the AoA indicator is meant to be fitted. The problem is not the lack of the instrument, just an unwillingness of the airline's to pay for an instrument that they don't consider necessary.

ELAC
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Old 3rd Aug 2009, 18:47
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Regardless of the many opinions surrounding such a corporate position, ELAC has framed the discussion, a discussion that transcends this thread, and may or may not have something to do with the fate of AF447, Rocket junk and histrionic writing aside.
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Old 3rd Aug 2009, 20:16
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Smilin Ed, if the occupants of the flight deck were paying attention to their instruments rather than doing something silly like reading their Playboys or playing canasta they should have a good awareness of what the AP has the plane doing. So if there is a sudden stall warning for no known good reason why should they act by doing something which might be wrong. If they are in the correct regime and an input to the stall warning system went sour almost anything they do other than intentionally ignoding the warning would be the wrong thing.

You ARE doing something when you intentionally sit there and make no changes.

JD-EE
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Old 3rd Aug 2009, 20:33
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Jeff;
Are there formulae or tables to compute the time needed to reach Mach 0.85 and Mach 0.90 starting from FL350, Mach 0.80, according to several descent angles (1░,2░,3░...) and different thrust levels (N1=0.85, 0.90, 0.95, Max) ? is it possible to compute the same thing but with a banking angle ?
No. Such tables would serve no useful purpose or solve no problem which carries risk. No airline pilot has a right to expect that such tables would be used or useful, mainly because most seasoned pilots know pretty well what it would take.

To regain speed at high altitude it takes a very long time in level flight especially if the air is warm, (ISA + 15 and warmer). However, speed can increase very rapidly even with a 0-degree pitch or perhaps 1, maybe 2 degrees at most, depending upon power setting and what the air is doing around you. Most transports cruise at 2deg NU or so - some slightly higher, others a bit lower and it is of course, dependant upon IAS. A five degree nose-down pitch is a very serious pitch down and 10 degrees could be defined as a loss of control without speed/Mach limit.

It's very simple and explained very well in Davies. The L/D curve for a jet transport when Davies wrote back then and still today, is very shallow. The airplane will slide along the curve left and right, very easily where as for propeller-driven aircraft the curve is much more "U" shaped mainly due to the thrust and drag offered by the large disc(s) described by the props.

Last edited by PJ2; 3rd Aug 2009 at 21:11.
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Old 3rd Aug 2009, 20:35
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Guys - where is this 'stall warning' you keep talking about going to come from? Is it not determined they were in Alt 2 Law with at least two ADR's 'suspect'?
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Old 3rd Aug 2009, 21:21
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Super Fast Response Required?

RE: Smilin Ed (#4102)

I still think the attitude on this thread, regardless of make of plane, seems to be that response to a stall warning has to be super fast. Does it?
Generally speaking, no. The time available for an action to avoid stalling depends on how rapidly the stall is approaching. Most airplanes, including the A330 in Normal Law, will have a stall warning margin of the order of 3 percent of the stall speed. In certification tests to determine the stall speed, the speed is reduced at the rate of 1 knot per second until the airplane stalls. If the stall warning is set to occur 5 knots above stall speed, then it would occur 5 seconds before the airplane stalls.
A glance at the AoA, should quiet any fears. IMHO, there should be an independent AoA readout available at all times.
I'm sure any testpilot would agree totally. However, I'm told that, generally speaking, airline pilots have no use for an AoA indicator.

regards,
HN39
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Old 3rd Aug 2009, 21:24
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Time margins

Originally Posted by PJ2
Jeff;
It's very simple and explained very well in Davies. The L/D curve for a jet transport when Davies wrote back then and still today, is very shallow. The airplane will slide along the curve left and right, very easily where as for propeller-driven aircraft the curve is much more "U" shaped mainly due to the thrust and drag offered by the large disc(s) described by the props.
Thank you PJ2, I will try to find your reference. I did a very simple computation a while ago, but I have no experimental data to validate this, do you think it could be plausible ?

This would suggest that it takes a little bit more than 30 s to exceed the MMO, a little bit more than 1 min to reach Mach 0.88, and more than 2mn to reach Mach 0.89 (with a N1=0.95 and a 2░ descent)
Jeff
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Old 3rd Aug 2009, 22:40
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Stall Warning Rationale

Based on responses to my queries, it seems to me that fear of stalling out in cruise is misplaced. One has to wonder where this fear comes from. Training? Were not the stall warning features implemented primarily for approach phases?
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Old 4th Aug 2009, 09:39
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Based on responses to my queries, it seems to me that fear of stalling out in cruise is misplaced. One has to wonder where this fear comes from
Smilin, if I stalled, my main fear would be to see your arse flying in the cabin.
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Old 4th Aug 2009, 09:55
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AoA reference

RE: DJ77 (#4091)



DJ,

I believe industry practice is that the aircraft longitudinal axis (x-axis) is the reference for angle of attack (alpha), flight path angle (gamma) and pitch angle (theta). All angles are positive upwards. The relation between these angles is given by:

theta = gamma + alpha

From the ATSB Interim Report on the A330/QF72 accident:
For an A330, during all phases of flight, the typical operational range of AOA is +1
degree to +10 degrees. In cruise, a typical AOA is +2 degrees.
It may look strange 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.
A calibration curve for the relation between vane angle and airplane AoA is established during flight tests. I assume that this calibration is used in the ADIRU to convert the alpha-vane output to AoA.

Stall warning AoA is normally not affected by airspeed or mach, but it is possible that the calculation algorithm applies a Mach-bias at high Mach.

regards,
HN39
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Old 4th Aug 2009, 10:14
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Not To Worry

Smilin, if I stalled, my main fear would be to see your arse flying in the cabin.
Not to worry then. Except for an occasional trip to the loo, I'd be tightly strapped in.
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Old 4th Aug 2009, 10:41
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re: HN39 (#4112)

HN39, I agree, if the x axis is the reference for AoA then theta = gamma + alpha. Then in level flight [gamma = 0]: theta = alpha. By the way, the choice of a reference for AoA is somewhat arbitrary.

This still leaves AF447 less than 1 deg away from stall warning, if the 4.2 deg AoA triggering threshold is true.

DJ.
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Old 4th Aug 2009, 11:07
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I'll ask again! In ALT2 WHAT is going to trigger the stall warning please?
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Old 4th Aug 2009, 11:48
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Stall Warnings

Acoording to the Air Cara´be ASR, the stall warnings are triggered as soon as the AoA exceeds a threshold which is a function of:
-the aerodynamic configuration (slats/flaps)
-the airspeed (kts) or mach point
-the control law (normal, alternate, direct)
I wonder if this AoA/speed threshold computation is the same as alpha floor & alpha max computations (excepted that the AoA law/alpha prot. will no longer take over to control/protect the plane since it is no longer active, but will only sound a warning as soon as the AoA threshold is exceeded). Does anyone know how these alpha threshold are computed ? (where can we find the AoA threshold tables ?)
Jeff
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Old 4th Aug 2009, 11:52
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I'll ask again! In ALT2 WHAT is going to trigger the stall warning please?
AoA.

BOAC, I may be wrong but I suspect you are mixing up "low speed protection" and "stall warning", the former being some kind of enhanced stick nudger and the later being an audio alarm .
DJ.
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Old 4th Aug 2009, 12:07
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Not to worry then. Except for an occasional trip to the loo, I'd be tightly strapped in.
You better be !

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.
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Old 4th Aug 2009, 12:22
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AoA etc.

RE: DJ77 (#4114)

dj,

By the way, the choice of a reference for AoA is somewhat arbitrary.
Just think of the whole airplane, rather than any particular part of it (wing section and 'incidence' vary considerably along the wingspan). Anyway its not important which reference is used, but you must use it consistently.
This still leaves AF447 less than 1 deg away from stall warning, if the 4.2 deg AoA triggering threshold is true.
With cruise AoA typically 2 degrees, wouldn't that leave 2.2 degrees?

regards,
HN39
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