bueb0g
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Why Didn't AF 447 Go Back Into Normal Law?

Fri May 11, 2012 4:52 pm

AF447 reverted to alternate law due to the airspeed disagreement. But after the ice in the pitot cleared and the airspeed became reliable again, why didn't the A330 go back to normal law?

Would the crew have had to manually switch it back to normal law, or would the aircraft simply have stayed in alternate law for the rest of the flight (had it not crashed)?

Or did the fact that the aircraft was stalled when airspeed came back, (or was it earlier?) and so outside its safe flight envelope inhibit a return to normal law?

Thanks all.
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CM
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Fri May 11, 2012 6:09 pm

I assume you are wondering why the AOA/alpha max protections did not kick in and help the airplane recover from its stalled condition. Here's why:

When an Airbus exits the normal flight envelope it enters what Airbus calls the "peripheral flight envelope". In this peripheral envelope, control law protections activate. When the aircraft goes beyond this peripheral area, (i.e. the aircraft is in an extreme upset, as was the case for AF447), the control law protections disable, permitting the pilot full control authority. In other words, AF447 was in an area of the flight envelop where the control law protections are disabled and the pilot control inputs are in no way overridden by the airplane. This is true, even when the airplane is fully intact and operating in normal law.
 
tdscanuck
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Fri May 11, 2012 7:01 pm

Quoting bueb0g (Thread starter):
AF447 reverted to alternate law due to the airspeed disagreement. But after the ice in the pitot cleared and the airspeed became reliable again, why didn't the A330 go back to normal law?

How would the airplane know that the airspeed was reliable again? Once you've flagged data as invalid it generally gets ignored completely. You don't want a bad signal getting tagged as valid just because it happens to pass through a valid range. These types of failures normally need some positive crew action or the end of the flight to reset themselves.

Tom.
 
KELPkid
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Fri May 11, 2012 11:36 pm

Quoting bueb0g (Thread starter):
AF447 reverted to alternate law due to the airspeed disagreement. But after the ice in the pitot cleared and the airspeed became reliable again, why didn't the A330 go back to normal law?

Did it ever? If the aircraft was never recovered from a stalled conditon (as the evidence seems to indicate), then there was never a reliable airspeed indication once the upset occured...therefore, no reliable airspeed indication.
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Starlionblue
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Sat May 12, 2012 12:21 am

I think the questions bueb0g is asking is this: If the aircraft had recovered from the upset and started cruising as normal, and if the speed indication had been "fixed", would the aircraft have gone back to Normal Law. Or more simply, can an aircraft go to Normal Law in flight if it has gone to Alternate Law.

In this case, as Tom says, the speed indication having been invalid would have prevented it. However is there ever any provision for going back to Normal Law?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
KELPkid
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Sat May 12, 2012 12:26 am

Quoting Starlionblue (Reply 4):
In this case, as Tom says, the speed indication having been invalid would have prevented it. However is there ever any provision for going back to Normal Law?

If there were, I'm sure that one of the requirements would be that you are inside the flight envelope. If you're stalled, then by definition, you are outside the flight envelope  
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Starlionblue
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Sat May 12, 2012 2:25 am

Quoting KELPkid (Reply 5):
Quoting Starlionblue (Reply 4):
In this case, as Tom says, the speed indication having been invalid would have prevented it. However is there ever any provision for going back to Normal Law?

If there were, I'm sure that one of the requirements would be that you are inside the flight envelope. If you're stalled, then by definition, you are outside the flight envelope

Hehe. Yes I get that. However what if you were unstalled and back in cruise?

Can an Airbus ever go back to Normal Law in flight if it has degraded to another law?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mandala499
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Sat May 12, 2012 5:33 am

No it cannot. It will remain in ALTN law, and if I remember correctly, direct law once landing gear is out.
The reason being, the computer does not want bad data because it can result in the computer ending up causing the tragedy. It is designed that way.
Once you have unreliable airspeed, you run through the unreliable airspeed procedure, which includes checking which ADR is correct and which is wrong, and then if I remember correctly, switch off the bad ADR(s), and fly on the good ones (if any). If the crew determines that none is good, then they fly with the best one they think is available, but fly on pitch and power only, use the remaining ADR to provide stall warnings only.

The ones with the BUSS installed, if you find no good ADR, switch all of them off, and the Backup Speed Scale (BUSS) will come on, and you fly using that.
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rfields5421
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Sat May 12, 2012 1:10 pm

The way I read some of the other UAS incidents - the aircraft doesn't go back into Normal Law while still in flight.

However, the crew is able to use the Autopilot again after the type of troubleshooting described above and isolating the cause of the UAS.

And as is also obvious, the crew of AF447 never did that troubleshooting.
 
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AirPacific747
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RE: Why Didn't AF 447 Go Back Into Normal Law?

Sun May 13, 2012 9:39 am

Quoting mandala499 (Reply 7):
and if I remember correctly, direct law once landing gear is out.

As far as I know, this only applies to the A32X series.. I heard it is due to lack of computing power but it is not written anywhere. Likely because the A320 is older than the 330/340/380

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