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Old 20th Dec 2012, 07:05   #61 (permalink)
 
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Tyro said
Quote:
The side-stick has artificial spring feel which returns the SS to neutral.
The spring force increases with increasing displacement.
- how is that different to a conventional yoke fitted with artificial feel and hydraulically operated control surfaces? I am not a test pilot but it sounds very similar to me...
There is no artificial spring feel in the Airbus, that I know of. Of course there is feel: that of the spring, and totally unrelated to any aerodynamic variable. In normal law, there is a relationship between load factor and the angle and spring force. That's why it is very natural to fly in normal law. But in direct law, there is absolutely no relation with anything, be it speed, load factor, or whatever. Depending on the circumstances, the same spring force will have very different effects.

Artificial feeling in hydraulically operated airplanes is dependent on speed, one way or another, linearly or square of speed. It does not resemble the feeling of a directly mechanically linked FCS exactly, but enough for a pilot to instinctively feel if they are making too hard an input, and how the airplane should respond to a given input. You don't have that on a sidestick when in direct law.

The B777 went to greater lengths of complexity to achieve a pilot instinctive FCS, being it fully computerized. It is difficult to get the advantages of fbw and still make it feel like a conventional airplane that is very easy to fly. When the B777 reverts to direct law I believe it still retains artificial feeling and then it feels like a conventional airplane, only not so easy to fly, where you can trim stick forces.
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Old 20th Dec 2012, 11:29   #62 (permalink)
 
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Quote:
Originally Posted by DOZY
With Airbus FBW, the protections do not "take over", they limit and constrain attitude.
Why don't you read the thread first ... or is it you're more comfortable in denial mode ... ?
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Old 20th Dec 2012, 12:46   #63 (permalink)
 
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True

It does take over.
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Old 20th Dec 2012, 14:14   #64 (permalink)
 
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Quote:
Originally Posted by atakacs View Post
Bad data is indeed a problem with any FBW design. Not sure if there is really a way around it or if it is the price to pay for advanced flight automation.
Firstly, remember that FBW and automation are two different things. Non-FBW aircraft such as the B757 (Birgenair) and B767 (Lauda Air) have been lost when bad data was supplied to the automation.

@CONF - I'm not in denial - the alpha protections follow pilot input up until the point safety boundaries are reached and will maintain that, in much the same manner as the T7 will progressively counter input through the force-feedback. Of course, as the Mach number increases, the pitch down will also increase, but it's not going to suddenly force the aircraft into a significant dive, in fact it won't descend at all if corrective action is taken.

This recent modification seems to have been an attempt to counter the risk of icing of the AoA vanes, and it would appear that a set of circumstances outside of the certification testing have shown the fix to be inappropriate.
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Old 20th Dec 2012, 14:21   #65 (permalink)
 
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Quote:
Originally Posted by DOZY
@CONF - I'm not in denial - the alpha protections follow pilot input up until the point safety boundaries are reached and will maintain that, in much the same manner as the T7 will progressively counter input through the force-feedback. Of course, as the Mach number increases, the pitch down will also increase, but it's not going to suddenly force the aircraft into a significant dive, in fact it won't descend at all if corrective action is taken.
If not denial ... what is it then ?
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Old 20th Dec 2012, 14:42   #66 (permalink)
 
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Quote:
It's reasonable to presume this is the first incident of this type, as the AoA probes referenced are a new fit item. As such no procedure existed prior (nor was there any need for it).
So it is reasonable to think that if a serious accident had happened (crash) because of this AOA probes problem ... the "no one could imagine that" .. "completely unpredictable" would certainly be in the investigative report ... (this reminds me of something)
Airbus know how work their automation .. and so they were (are) aware of the consequence on the flight automation if this AOA probes problem happens ...
Is that they thought such an event, however, was unlikely to occur?
No procedure needed ?
Even a prayer for good luck?

Last edited by jcjeant; 20th Dec 2012 at 15:03.
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Old 20th Dec 2012, 15:07   #67 (permalink)
 
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jcj - I think you're reading a different meaning into my words than I intended. This problem is a new one that seems to have arisen with a recent modification to the AoA vanes, intending to solve the issue of the icing up of one vane that occurred on AF447. It appears that the fix passed certification and began to be implemented, but the certification tests were not sufficient to catch this new issue (in other words the fix seems to have made the problem worse - with all three vanes freezing in certain conditions).

As with the Thales AA pitot tube design, this incident shows that the certification process needs to be revised based on the evidence presented. EASA and Airbus seem to have been pretty quick in getting the EAD and workaround published once the problem was known.
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Old 20th Dec 2012, 16:38   #68 (permalink)
 
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Quote:
jcj - I think you're reading a different meaning into my words than I intended. This problem is a new one that seems to have arisen with a recent modification to the AoA vanes
I don't think so ...
Older AOA vanes or new AOA vanes can have the problem of this particular incident
It's not unpredictable ... even if certifications are bad or good
All in a aircraft is certified and all can fail one day or another
You can't of course have a procedure for every items who fail in a aircraft .. but it must be procedures when important item fail
AOA vanes are important items necessary for the safe conduct of the flight
The implication of the failure described in this post on the flight automation was predictable by the constructor (like a fail of Pitot for what it was procedure already available .. as it's predictable)
But unfortunately no procedure was available for the pilots at the time of this incident .. and it was a risk that this incident turn in a accident ...
Fortunately .. the pilots not goofed .. Airbus yes IMHO

Last edited by jcjeant; 20th Dec 2012 at 16:45.
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Old 20th Dec 2012, 16:51   #69 (permalink)
 
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jcj - the EAD applies specifically to the modified AoA vanes only. The modified AoA vanes are the only type to have suffered a triple failure. Airbus didn't have procedures for triple failure prior to this EAD and nor did any other manufacturer, so why pick on them?
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Old 20th Dec 2012, 17:35   #70 (permalink)
 
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Anybody knows where we can get the preliminary report ? Merci
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Old 20th Dec 2012, 17:52   #71 (permalink)
 
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Being an EAD bulletin, it's likely that the incident report is still in progress. I did have a hunt online, but nothing as yet. There are a couple of stories on Flight, but some are subscriber-only.
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Old 21st Dec 2012, 03:43   #72 (permalink)
 
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Quote:
Originally Posted by DOZY
The modified AoA vanes are the only type to have suffered a triple failure. Airbus didn't have procedures for triple failure prior to this EAD and nor did any other manufacturer, so why pick on them?
Dual failure is all it takes to do that mess ... and by its own philosophy Airbus is the only one to allow such mess to take place.
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Old 21st Dec 2012, 06:36   #73 (permalink)
 
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Dozy

In case of an AoA failed in such a way that erroneously gives a stall output, the normal law would pitch down to reduece the AoA. Because it would remain fixed, erroneously, FCS would keep pitching down... Or so I believe.

The fbw can take over and we need to be able, firstly, to detect that normal law is no longer desirable and secondly to revert to a safe control law.
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Old 21st Dec 2012, 13:28   #74 (permalink)
 
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"undue" stall warning

but never in 40 years have I ever heard of an 'undue' stall warning....

BEagle, an "undue"stall warning is indeed a real occurence. I flew a lot of L-19 (Birddog) tows (5000 plus) and the stall warning buzzer would regularily go off in turbulence, such as, for example, 15 kn wind and strong thermal conditions. Maintaining attitude always got us (glider and self) through this.

Similarily, when towing through a rotor in mountain wave conditions would cause the same.
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Old 21st Dec 2012, 14:25   #75 (permalink)
 
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I would term that a 'spurious' stall warning. I had a spurious stall warning at about 400 ft on a flapless approach in a VC10 when sticking AoA probes (the jet had been in the desert without a decent wash for too long) jumped, triggering phase advance and a sudden stick push.....

Direct reading AoA gauges should be installed in all airliners. Not some extra thing displayed electronically, just a simple, direct instrument.

The Russians used to have a combined AoA gauge and g-meter in many of their aircraft. What an excellent safety aid that must have been!

Last edited by BEagle; 21st Dec 2012 at 14:36.
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Old 21st Dec 2012, 15:03   #76 (permalink)
 
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In the press .....
Des pilotes Air France exigent le retrait de sondes d'Airbus - Le Nouvel Observateur

Google*Traduction
Quote:
Air France pilots require the removal of probes Airbus
"We can not meet the manufacturer and the authority just by a simple procedure permanently supplement what appears to be a serious deficiency of an element as vital to the conduct of the flight," said Jean-Louis and Barber, president of the AF SNPL Alpa.
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Old 21st Dec 2012, 16:16   #77 (permalink)
 
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Quote:
Originally Posted by Microburst2002 View Post
In case of an AoA failed in such a way that erroneously gives a stall output, the normal law would pitch down to reduece the AoA. Because it would remain fixed, erroneously, FCS would keep pitching down... Or so I believe.
The AoA in Normal Law will be held just short of Alpha Max if Alpha Prot is triggered. An increase in Mach will cause the Alpha Max to decrease - and the pitch angle will follow, but it would be a gradual process - you're unlikely to find yourself in even a shallow descent if corrective action is taken in a timely manner.

Quote:
The fbw can take over and we need to be able, firstly, to detect that normal law is no longer desirable and secondly to revert to a safe control law.
I'm a little fuzzy on what "taking over" constitutes - Alpha Prot should not command significantly opposite the input it is given as that is not the way it is designed to work.

I'm all for waiting to see the incident report before making sweeping judgements. I find the fact that Alpha Floor is not mentioned interesting, as I would have expected it to have been activated if there was a manual pitch-up command given in that circumstance. Equally intriguing is the fact that the crew managed to correctly troubleshoot the situation and return the aircraft safely to the ground despite having no forewarning of the problem. Someone out to bash Airbus at any opportunity probably won't care about such questions, but if we're being open-minded about things it should definitely give pause for thought.

@jcj - No-one's talking about the Airbus ADR procedure being a permanent solution - it is clearly presented by the EAD bulletin as a temporary workaround while the AD is being implemented on the affected airframes.

Last edited by DozyWannabe; 21st Dec 2012 at 16:29.
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Old 21st Dec 2012, 16:22   #78 (permalink)
 
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Four years ago, AF pilots threatened strike if AF did not immediately obey the AD requiring pitot probe replacement.

After 228 people were killed in AF447....

Once bitten. Twice shy...
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Old 21st Dec 2012, 16:27   #79 (permalink)
 
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Quote:
Originally Posted by Lyman View Post
Four years ago, AF pilots threatened strike if AF did not immediately obey the AD requiring pitot probe replacement.
There were a lot more airframes in the AF fleet alone with the Thales AA probes than there are airframes in the worldwide fleet with these AoA vane modifications. I suspect the AD will be implemented with all due haste.
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Old 21st Dec 2012, 17:27   #80 (permalink)
 
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Did the pilots get your memo?
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