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Old 15th Jul 2010, 00:42
  #1741 (permalink)  
 
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Right Way Up:
If you ever have spare time in an airbus sim try climbing from optimum to somewhere near max alt with blocked pitots. The flight control laws lead to pretty interesting results!!
Are you referring to the Vmo/Mmo protections, or something else??
This is a bit different than a conventional bellows/aneroid type airspeed indicator. The pressure would still be trapped (unchanging) between the pitot blockage and its associated pressure sensor, but the static pressure sensors would be dropping together and would likely pass validation. If the system merely calculates delta P to get the airspeed, then the airspeed would wind upwards pretty evenly between the 3 ADR systems. Normally the pitot side pressure would be dropping if you climb at constant IAS, but would be matched by an equivalent static pressure drop.

The constant pitot pressure, dropping static pressure situation resembles an acceleration and should be amenable to some sort of validity check, because in real life it would be hard to accelerate as you climb, particularly at altitude. At some point, you exceed the aircraft's performance capability.

Last edited by Machinbird; 15th Jul 2010 at 05:59. Reason: add aneroid term, better wording
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Old 15th Jul 2010, 04:14
  #1742 (permalink)  
 
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Machinbird,

As you climb indicated speed/mach will increase, the autopilot/flt directors will command an increasingly high rate of climb to cater for it. With a high rate of climb and with actual speed reducing fast, the increased indicated speed will take the aircraft into the overspeed protections. Just when you want to reduce pitch to normal levels to avoid the stall the aircrafts protection system will stop you from lowering the nose.
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Old 15th Jul 2010, 05:50
  #1743 (permalink)  
 
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Thank you Right Way Up. Another variation on the Vmo/Mmo protection pitchup stall entry.
Looks like Airbus needs to rethink the concept of how they protect against overspeeds.
Were you introduced to this in the simulator, or do you know of a flight incident?
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Old 15th Jul 2010, 06:51
  #1744 (permalink)  
 
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FMS updated by GPS for any airport and it might be extremely accurate
Unfortunately without validated WAAS, you don't know if that GPS update is within 10m or 1000m off the mark.
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Old 15th Jul 2010, 07:58
  #1745 (permalink)  
 
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Machinbird,

In the simulator. As far as I know there is no "known" inflight incident.
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Old 15th Jul 2010, 08:08
  #1746 (permalink)  
 
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Originally Posted by Right Way Up View Post
Sensor validation,

It is possible to tell which is a sole correct pitot reading, as long as you go back to the very basics & fly pitch & power, to corroborate its readings.
To a human pilot maybe, but apparently not for the auto-pilot

If you ever have spare time in an airbus sim try climbing from optimum to somewhere near max alt with blocked pitots. The flight control laws lead to pretty interesting results!!
The classic case where blocked pitot tubes allow decreasing static pressure due to increasing altitude appear as increasing speed. Clearly a significant danger on climb after take-off, which has had fatal results, and an early action of the unreliable speed QRH is to level off.

By why would they be deliberately climbing inside a Cb, no way could they overfly - would conditions be expected to be better at unauthorized FL370?. How about a relatively warm humid low density updraught?
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Old 15th Jul 2010, 11:04
  #1747 (permalink)  
 
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Originally Posted by sensor validation
By why would they be deliberately climbing inside a Cb, no way could they overfly.
The crew could have thought they had left the weather behind, the captain was released by his first officer, which, after having taken stock of the flight’s progress, decided this was a good point to step-climb to FL370.

However, in my opinion the condition of the RTLU doesn’t fit in this scenario. (see mm43#1669 on p.84 and Machinbird #1681 on p.85)


regards,
HN39
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Old 15th Jul 2010, 11:19
  #1748 (permalink)  
 
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Right Way Up:
As you climb indicated speed/mach will increase, the autopilot/flt directors will command an increasingly high rate of climb to cater for it. With a high rate of climb and with actual speed reducing fast, the increased indicated speed will take the aircraft into the overspeed protections. Just when you want to reduce pitch to normal levels to avoid the stall the aircrafts protection system will stop you from lowering the nose.
FCOM 1.27.30 ALTERNATE LAW
(...)
High speed stability

Above VMO/MMO a nose up demand is introduced to avoid an excessive increase in speed.

The pilot can override this demand.

The high speed protection symbol (VMO + 4) disappears.

In addition, the overspeed warning (VMO + 4 or MMO + 0.006) remains available.

(...)
Are you thinking on the possibility of an attentive movement of the crew to change the Laws, from Normal to ALT via switching off PRIM1 and SEC1?
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Old 15th Jul 2010, 11:34
  #1749 (permalink)  
 
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Aquadalte,

My input was from an A320 point of view where in normal law in case of overspeed: (from A320 FCOM)

As the speed increases above VMO/MMO, the sidestick nose-down authority is progressively reduced, and a permanent nose-up order is applied to aid recovery to normal flight conditions
.

So even though you are getting into the low speed regime, the aircraft thinks you are in an overspeed and normal law will not allow you to lower the nose.

If I understand you right you suggest the only initial way to get out of this is to dumb the aircraft down to ALT law. Which I would agree with.
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Old 15th Jul 2010, 12:08
  #1750 (permalink)  
 
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Originally Posted by HazelNuts39
The crew could have thought they had left the weather behind, the captain was released by his first officer, which, after having taken stock of the flight’s progress, decided this was a good point to step-climb to FL370.
Definitely such move would require a clearance before proceeding, even if a step climb was planned on the flight plan.
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Old 16th Jul 2010, 11:04
  #1751 (permalink)  
 
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Hazelnuts,

Aquadalte's quote is regarding the aircraft response in alternate law where your quote is regarding normal law.
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Old 16th Jul 2010, 12:08
  #1752 (permalink)  
 
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Thanks Right Way Up, I misread aguadalte's post and have deleted my post.

regards,
HN39

Last edited by HazelNuts39; 17th Jul 2010 at 06:27.
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Old 16th Jul 2010, 12:48
  #1753 (permalink)  
 
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transition modes with no protection?

HazelNuts39:
We will have to find the recorders to know for certain but I am beginning to see potential Swiss Cheese holes in Airbus system design.
I remember reading the preliminary accident report of the Crash-aerien 30 JUN 1994 d'un Airbus A330-321 F-WWKH - Toulouse-Blagnac Airport where it was stated that, one of the contributing facts for the crash was "the absence of pitch attitude protection in the autopilot altitude acquisition mode [that] played a significant role." [see here]

Since then, I have always kept the idea (although I don't remember having seen it written in the FCOM), that during transition modes (like ALT*, ALT CRZ* modes) protections are lost, even in Normal Law.

Could anybody lead me to the Accident Report of that particular flight? (Can't find it in the web)
Could it be that, after that particular accident, some sort of upgrade was made to Airbus aircraft flight control laws (and that is why I can't find any clues on the subject in current FCOM's?

(please don't be mad at me if I don't reply to your help on this one, soon. I'm flying to LAD this evening and internet there is so slow...)
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Old 16th Jul 2010, 12:53
  #1754 (permalink)  
 
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Overspeed

I am not sure how relevant this is (if at all) but there was an incident in 2000 with a UPS 747-200F (N520UP) which took off from Dublin Airport on a post maintenance test flight.

The aircraft took off with the static drain port caps missing which resulted in the aircraft exceeding its speed limitations due to under reading pitot-static instruments caused by cabin pressure being sensed. The aircraft suffered flap damage caused by the excessive speed.

bizdev

Last edited by bizdev; 16th Jul 2010 at 14:06.
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Old 16th Jul 2010, 14:04
  #1755 (permalink)  
 
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Preliminary report F-WWKH

There is a copy of the préliminary report for the Blagnac A330 F-WWKH (french only) :

Rapport Preliminaire - A330 Toulouse
I think there must be a final report somewhere which has been made by the French BEAD (not BEA), the D is for Defense as the report was made by the military due the flight being a test flight.
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Old 16th Jul 2010, 14:28
  #1756 (permalink)  
 
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Yikes, Right-Way-Up

Shades of Northwest B727 on 1 December 1974.


Goldfish
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Old 16th Jul 2010, 14:38
  #1757 (permalink)  
 
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Very much so goldfish.

The only difference is that in an Airbus if you do recognise the problem once in the "indicated" overspeed it will not let you lower the nose until you are in alternate law either via "abnormal attitude" or you force it into alternate law.

I think Airbus should look at some sort of system that will compare actual performance at altitude compared to calculated performance with the idea that the aircraft will automatically drop into alternate law if there is a large discrepancy.
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Old 16th Jul 2010, 14:45
  #1758 (permalink)  
 
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aguadalte,
The report is nowhere to be found on the BEA site, but PBL put a copy of it on his site :
Rapport Préliminaire A-330 Toulouse 1994
No known english version.
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Old 16th Jul 2010, 15:33
  #1759 (permalink)  
 
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Thanks CONFiture.
I'll read it. Regards.
V.F.
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Old 16th Jul 2010, 22:51
  #1760 (permalink)  
 
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F-WWKH Crash

Aguadalte,

Had you seen:

http://www.pprune.org/questions/4446...nt-1994-a.html

Regards
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