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Old 26th Dec 2012, 10:21   #1 (permalink)
 
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B777 Procedure Blocked AOA Probes

In the light of the BR A333 incident, what are the procedures regarding blocked AOA probes on the B77W?
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Old 26th Dec 2012, 10:30   #2 (permalink)
 
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Same as any aeroplane. Power + Attitude!!!

Maui
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Old 26th Dec 2012, 14:33   #3 (permalink)
5LY
 
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Pitch and thrust as Maui says. You should have a basic target pitch and thrust target setting in your head for all realms of flight. They don't have to be precise. If you're in the ball park, you can quite easily keep the blue side up while you find the unreliable airspeed checklist which will take you to a landing. There are many eicas checks that pop up which are mostly a waste of time. Do them, but focus on pitch and thrust.

I ran many of these in the sim in IMC. With a bit of briefing with respect to pitch and thrust targets, I never had anyone fail to land safely.
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Old 26th Dec 2012, 22:26   #4 (permalink)
 
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To be completely accurate, the AoA sensors of the AF 330 never iced up, it was the pitot probes. In my experience of the B777 in identical conditions, it is the engine P1 sensors that ice first. The autothrottle disengages and thrust ref indications disappear, but the A/P stays in. You have time to look up attitude and thrust settings in case the worse happens, but the pitot probes stay clear for 10 minutes or more. As everyone advises, maintain thrust level and attitude and wait to fly clear, its rarely more than a few minutes to do that.
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Old 26th Dec 2012, 23:45   #5 (permalink)
 
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But the question by Kimon is different :
What's happening on a 777 if the AOA probes block at the same angle ?
Is there a possibility for one protection to take over and for the airplane to ignore the pilot's inputs like it did happen on a 330 ?
BTW how many AOA probes on the 777 ?
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Old 27th Dec 2012, 00:30   #6 (permalink)
 
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Quote:
Originally Posted by 777fly View Post
To be completely accurate, the AoA sensors of the AF 330 never iced up, it was the pitot probes.
Ahem... I think you're remembering another dramatic event, there.
What is the topic looks more like related to this.
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Old 27th Dec 2012, 09:42   #7 (permalink)
 
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AZR: Thank you for the correction. Should have read the OP more carefully....
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Old 27th Dec 2012, 12:51   #8 (permalink)
 
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@ AlphaZuluRomeo

Yes, more on that BR (EVA) A333 underreported incident:

« ILS » n

Les dossiers noirs du transport aérien

Last edited by Kimon; 27th Dec 2012 at 12:52.
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Old 27th Dec 2012, 15:31   #9 (permalink)
 
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Cool

Quote:
BTW how many AOA probes on the 777 ?
0..........
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Old 27th Dec 2012, 16:22   #10 (permalink)
 
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Photos: Boeing 777-3B5/ER Aircraft Pictures | Airliners.net

To me, it looks like one in that red circle ... ?
And apparently another one on the left hand side, for a total of two ?
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Old 27th Dec 2012, 20:32   #11 (permalink)
 
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@CONF

Quote:
What's happening on a 777 if the AOA probes block at the same angle ?
Is there a possibility for one protection to take over and for the airplane to ignore the pilot's inputs like it did happen on a 330
Continued activation of the stickshakers & increased column force in the nose up direction.
Don't know the effect on the Flight control computers.

...A330 pilots SS input is not 'ignored', a further nose up input will not be possible if AoA [when blocked] ~ AoAmax.
Roll control will not be affected and SS ND input (if situation permits) will cancel the hi alpha protection
.....but if AoA vanes are still blocked it will reactivate the hi alpha protection on each SS NU input.

Quote:
BTW how many AOA probes on the 777 ?


2 is correct! and like the Airbus of the vane type.
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Old 28th Dec 2012, 02:07   #12 (permalink)
 
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Quote:
Originally Posted by A33Zab
SS ND input (if situation permits) will cancel the hi alpha protection
To the contrary, SS ND inputs will get the airplane deeper into the high AoA protection ... and closer to the VMO/MMO too ...

When a pilot cannot counteract the pitch down ordered by a protection, by pulling the sidestick in the full backward position, I think it is fair to state that his SS inputs are 'ignored'.
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Old 28th Dec 2012, 06:59   #13 (permalink)
 
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Quote:
To the contrary, SS ND inputs will get the airplane deeper into the high AoA protection


FCOM 1.27.20 P 4


To leave the angle-of-attack protection :
  • The sidestick must be pushed more than 8° forward, or
  • The sidestick must be pushed more than 0° forward, for at least 1 second, when α < α max, or
  • α < α prot, if the sidestick has not been deflected since the latest autopilot disconnection.
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Old 28th Dec 2012, 12:29   #14 (permalink)
 
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Cool

They are sensors not probes, probes work in a very different way to AoA sensors.

It may be pedantic, but there is a very big difference between the 2 and how they work.

So there are 2 AoA sensors, but 0 AoA probes.
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Old 28th Dec 2012, 13:16   #15 (permalink)
 
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@ spannersatcx

Good point!
So to recap for clarity's sake:
Pitots are probes.
AOA vanes are sensors.
Static ports are probes?Sensors?
Have static ports ever known to fail?
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Old 28th Dec 2012, 17:59   #16 (permalink)
 
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Cool

Quote:
Have static ports ever known to fail?
in a way, after hangar maintenance or washing tape has been left covering them before, not really a failure of the static port in itself.

In some modern a/c the tubing that was once many many feet long is now very short and goes to an electronic sensor, these can fail, but not that often. Most common failures are the heater parts of probes.
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Old 28th Dec 2012, 18:54   #17 (permalink)
 
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Quote:
Originally Posted by A33Zab
FCOM 1.27.20 P 4
But we're not in the FCOM register here, when everything works by the book, malfunctions included.
We are in the Wild Territory of the Airbus World, where the unexpected was not planned.

Don't forget : Some AoA probes are telling the same lie to the boss ...
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Old 1st Jan 2013, 09:50   #18 (permalink)
 
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B777 ADIRUs

The B777's ADIRU has 6 accelerometers with superior skewed axis combinations and redundancy.
When Pitots fail, any set of 3 sensors will define a solution vector in 3-space.
It follows that 6 non-parallel sensors result in 20-solution vectors.
If there are no errors, the solutions will all be the same within accuracy limits of the system.
If there is 1 faulty sensor,10 of the solution vectors become "wrong".
If a 2nd sensor fails, it also affects ten vectors but 4 are included in the first 10.
That means 2 failures results in 16 "wrong" out of twenty solutions.
The task of identifying the 4 "right" is done!

Last edited by Kimon; 1st Jan 2013 at 09:53.
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Old 1st Jan 2013, 19:27   #19 (permalink)
 
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@Kimon:

A failed sensor is something different then sensing identical but - wrong - information.

In case of a failed sensor the FW bit is set and this enables a system to respond to it, as you described.

But the T7 will sense the same -wrong- information, like the EVA A330 did, because the 3rd comparable value is the average of both sensors.
3 identical AoA is accepted as GOOD information.
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Old 1st Jan 2013, 19:53   #20 (permalink)
 
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Architecture, Modes & Laws

Many thanks, A33Zab!
Given that the brains and voting system are the same whether B777 or A330, the rub is how the architecture of the aforementioned aircraft subsequently handle failures of Pitots, AOA sensors and StatPorts.
Yes, granted horses for courses within a game of two halves but is the B777 overrall back-up and breakdown systems of SAARUs and simplified degraded Modes simply better?
Obviously you can not have a B system in an A and vice-versa.
http://www.coe.pku.edu.cn/tpic/20119263710178.pdf
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