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Old 19th Dec 2012, 09:57   #1 (permalink)
 
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How safe is (airbus) fly by wire? Airbus A330/340 and A320 family emergency AD

EASA Airworthiness Directives Publishing Tool

Quote:
An A330 aeroplane, equipped with Angle of Attack (AoA) sensors with conic plates installed, recently experienced blockage of all sensors during climb, leading to autopilot disconnection and activation of the alpha protection (Alpha Prot) when Mach number was increased.
Based on the results of the subsequent analysis, it is suspected that these conic plates may have contributed to the event. Investigations are on-going to determine what caused the blockage of these AoA sensors.
Blockage of two or three AoA sensors at the same angle may cause the Alpha Prot of the normal law to activate. Under normal flight conditions (in normal law), if the Alpha Prot activates and Mach number increases, the flight control laws order a pitch down of the aeroplane that the flight crew may not be able to counteract with a side stick deflection, even in the full backward position.
This condition, if not corrected, could result in reduced control of the aeroplane.
condition on Airbus A330 and A340 aeroplanes.
AoA conic plates of similar design are also installed on A320 family aeroplanes, and installation of these AoA sensor conic plates was required by EASA AD 2012-0236, making reference to Airbus SB A320-34-1521 for in-service modification. This requirement has now been removed with revision 1 of that AD.
To address this condition on A320 family aeroplanes, Airbus developed an “AOA Blocked” emergency procedure, published as a temporary revision (TR) of the Airplane Flight Manual (AFM), to ensure that flight crews, in case of AoA sensors blockage, apply the applicable emergency procedure.
For the reasons described above, this Emergency AD requires insertion into the AFM of the Airbus TR.
This AD is considered to be an interim measure and further AD action may follow.
With multiple frozen/blocked AOA probes and an increasing mach no, the flyby wire system might trigger alphaprot mode. As the system thinks the aircraft is stalling the flybywire system induces a constant nose-down input to the aircraft.
The speed indication however will be normal as the pitot-probes are functioning normal.
The nose-down input is not fedback via the sidestick, nor on the screen. And pilot full nose-up input might not be able to counter the nose-down input of the alpha-prot system.

So the brilliance of engineering instructs us in a procedure to turn off 2 ADR's in order to revert the aircraft to Alternate law, in which no alpha prot exist. Off course first you have to realise what is going on, which could be difficult as there is no feedback from the system to the pilot.

But yes you read it correctly, the system is able to completely override pilot input despite being completely wrong. And the only way to recover from uncontrolable situation is to switch of ADR's.

The procedure:
Quote:
At any time, if the aircraft goes to an unmanageable pitch down attitude despite continuous deflection of the sidestick in the full backward position (in case the flight crew missed the below symptoms or delayed the application of one of the below procedures):
Keep one ADR ON. Turn OFF two ADRs.
And before anyone asks, yes I fly Airbus A320 family. (not 737 as my name might suggest)

Last edited by 737Jock; 19th Dec 2012 at 10:12.
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Old 19th Dec 2012, 10:19   #2 (permalink)
 
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I asked a sim instructor this very same question: what if the aircraft thinks it is in a stall whilst it is clearly not. How do I safely deactivate normal law.

I only got an answer after a lot of huffing and puffing, whilst I think this should have been standard knowledge to all pilots.


Fine. My biggest worry: what if this happens shortly after lift off in IMC? Is there enough time to realize what is wrong and press the appropriate switches?

Last edited by PENKO; 19th Dec 2012 at 10:22.
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Old 19th Dec 2012, 10:34   #3 (permalink)
 
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Hi Penko,
Quote:
My biggest worry: what if this happens shortly after lift off in IMC? Is there enough time to realize what is wrong and press the appropriate switches?
I agree. Why did Airbus never fit a simple "big red switch" like B777 etc.
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Old 19th Dec 2012, 10:48   #4 (permalink)
 
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It's interesting that Airbus advice is to turn off two ADRs, which immediately adds to confusion in the flight deck with only one air data reference available.

Is there any reason why the FACs couldn't be switched off instead to drop the aircraft into alternate law?

Edit: Presumably this only applies to aircraft fitted with a specific type of AoA vane, as we've heard nothing about this for our Airbus fleet.

Last edited by David Horn; 19th Dec 2012 at 10:49.
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Old 19th Dec 2012, 11:50   #5 (permalink)

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737Jock

Big topic which I do not wish to trivialise in the least.

However there are two things involved here the first being the question

Quote:
How safe is (airbus) fly by wire?
and the second the event and all the technicalities that gave rise to the question.

I only feel competent to comment on the question and my response is 'safe enough'. That word 'enough' results from the simple fact of the number of people killed in Airbus accidents compared with those killed in other makes in current use. When I started in the industry wings fell of Electras and fuselages split on Comets in the cruise and that was clearly not safe enough. Today so many people fly so cheaply (and safely) all over the world that their seat on an airliner is just another commodity ruled by market forces. This means (whether we like it or not) that the large amounts of money needed to sgnificantly improve the current safety level cannot be justified commercially and so will not be found. You or I might well be prepared to pay double or treble for a safer seat but you or I are not the market.
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Old 19th Dec 2012, 12:14   #6 (permalink)
 
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A thread on the subject is here also :
http://www.pprune.org/rumours-news/5...oa-probes.html
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Old 19th Dec 2012, 12:28   #7 (permalink)
 
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Yes, safety is a commodity. That means it is negotiable.

The fog enters when the consumer makes a choice. As a commodity, should the consumer be apprised of his/her choices? Chances?

I think after all the recent kerfuffel, that Airbus is mostly guessing at what constitutes a hard floor for safe flight.

Again, pilots of type cannot agree on what is safe re handling the airbus.

Not comforting.
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Old 19th Dec 2012, 12:32   #8 (permalink)
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Correct me if I am wrong, but it drops into Alt2? Wasn't that the mode that killed AF447?
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Old 19th Dec 2012, 12:33   #9 (permalink)
 
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ALT II (b) killed 447.
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Old 19th Dec 2012, 12:42   #10 (permalink)
 
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It is highly debatable that ALT II (b) actually killed 447. Some could argue that misidentification of a stall situation is the result of many different inputs and if the aircraft control law was at fault then Air France wouldn't have got so upset when the safety recommendations were published.

The very nature of our operation in aviation is such that an almost unlimited combination of failures and events can occur, with the likely, unlikely and even remotely conceivable designed for and considered by Airbus.

This AoA vane has been shown not to work after operational experience; the problem has been identified quickly, OEBs have been issued and (as uncomfortable as it is to think our aircraft can behave in erratic ways) the truth is that we as pilots make many more mistakes. How about the incident involving mis-selection of the rudder trim switch instead of the cockpit door lock? What happened after that?

As an A320 pilot, I did not feel comfortable reading the new OEB and background information. However, we can only follow the advice given, and for the rest rely on our training and experience, as we do in any other situation. Our job has inherent risks which we manage. Better we know about it now.

Last edited by The African Dude; 19th Dec 2012 at 12:47.
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Old 19th Dec 2012, 12:48   #11 (permalink)
 
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Not single handedly. Call it a group effort.
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Old 19th Dec 2012, 12:52   #12 (permalink)
 
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Quote:
Originally Posted by BOAC
Correct me if I am wrong, but it drops into Alt2? Wasn't that the mode that killed AF447?
But which option is preferable ?
  1. The computer takes over following erroneous data
  2. The pilot has still the option to discard that computer
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Old 19th Dec 2012, 13:20   #13 (permalink)
 
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It strikes me as extremely counterintuitive to disable a protection?

Especially if one must change the Law to get there?

Last edited by Lyman; 19th Dec 2012 at 13:22.
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Old 19th Dec 2012, 14:22   #14 (permalink)
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Quote:
Originally Posted by conf
But which option is preferable ?
  1. The computer takes over following erroneous data
  2. The pilot has still the option to discard that computer
- 2. You may recall I have long argued for pilots who can fly and a big red button that makes AB into an 'aeroplane'.
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Old 19th Dec 2012, 15:07   #15 (permalink)
 
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Your view is mine too BOAC.
I'd like to see those who think differently how they feel as pax or pilot when their Airbus is going in the mad mode ... I have not seen them commenting yet btw.
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Old 19th Dec 2012, 15:42   #16 (permalink)
 
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"Off course first you have to realise what is going on, which could be difficult as there is no feedback from the system to the pilot."

The OEB does identify the warning signs to look for PRIOR to activation of the ALPHA PROT. So we should realise "what is going on". Admittedly, switching off ADRs is not very comforting.
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Old 19th Dec 2012, 17:19   #17 (permalink)
 
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David Horn...
Quote:
It's interesting that Airbus advice is to turn off two ADRs, which immediately adds to confusion in the flight deck with only one air data reference available.
But that is preferable to seeing increasing nose-down pitch which you cannot correct in Normal Law...

Quote:
Is there any reason why the FACs couldn't be switched off instead to drop the aircraft into alternate law?
Short answer: A330/340 do not have FACs.

Thinking of the A320....?
1. The ADR pb's are located together on the overhead panel, directly above the captain's head, whereas the FAC pb's are one either side of the same panel. Much easier and quicker to use the ADR pb's.

2. Switching off both FACs has more effect on the rudder - it leads to Mechanical Yaw instead of ALT.
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Old 19th Dec 2012, 19:43   #18 (permalink)
 
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Devil

@Conf_iture #12

2! Agreed with BOAC and you!

Waiting that moment i continue to fly my bicycle!

Honest people cannot operate flying machines with random SOPs and call them SOPs and CRM. Automation designers have 100% responsibility, not 99.999...
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Old 19th Dec 2012, 20:17   #19 (permalink)
 
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Agree BOAC. Red switch off to turn the computer driven object back into an aeroplane.

Second, perhaps more important, return to control columns so that either pilot can see what the other pilot is doing.
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Old 19th Dec 2012, 20:31   #20 (permalink)
 
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Emergency electrical config, gear down. Last section of the smoke and fumes drill in the QRH.

Direct law is your friend.
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