AF447
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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
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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 ?
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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
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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 ?
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Last edited by Hyperveloce; 3rd Aug 2009 at 19:17.
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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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
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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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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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
You ARE doing something when you intentionally sit there and make no changes.
JD-EE
Jeff;
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.
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 ?
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.
Per Ardua ad Astraeus
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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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Super Fast Response Required?
RE: Smilin Ed (#4102)
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.
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
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?
A glance at the AoA, should quiet any fears. IMHO, there should be an independent AoA readout available at all times.
regards,
HN39
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Time margins
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.
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.
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
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?
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
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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:
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
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.
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.
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
Not To Worry
Smilin, if I stalled, my main fear would be to see your arse flying in the cabin.
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.
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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Stall Warnings
Acoording to the Air Caraïbe 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
-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
I'll ask again! In ALT2 WHAT is going to trigger the stall warning please?
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.
Not to worry then. Except for an occasional trip to the loo, I'd be tightly strapped in. 
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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AoA etc.
RE: DJ77 (#4114)
dj,
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.
With cruise AoA typically 2 degrees, wouldn't that leave 2.2 degrees?
regards,
HN39
dj,
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.
regards,
HN39
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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?
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?