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A320 x A330 Alpha Floor Activation During Cruise

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A320 x A330 Alpha Floor Activation During Cruise

Old 29th Sep 2012, 08:28
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Originally Posted by Microburst2002
High aoa protection pitches the airplane down to prevent aoa from exceeding alpha prot (sidestick neutral)
Not exactly.

During the deceleration to V-alphaprot the airplane is pitching up to maintain lift at decreasing airspeed. The elevator moves NU, the THS follows. That process is arrested at V-alphaprot. The autotrim stops trimming NU, the elevator stops moving NU, the AoA is at Alpha-Prot and stays there, the FPA is level at that point.

As the airspeed continues to reduce below V-alphaprot, the lift reduces as airspeed squared. When lift it is less than weight the airplane starts to descend. It then pitches down to maintain a constant AoA relative to the increasingly downward flight path angle. At some point the trend of decreasing airspeed is reversed, it increases beyond V-alphaprot, lift becomes greater than weight and the flight path starts levelling off. Somewhat later the airspeed trend reverses again and the airspeed reduces through V-alphaprot. That cycle is repeated until the pilot intervenes.

Last edited by HazelNuts39; 29th Sep 2012 at 12:50.
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Old 29th Sep 2012, 13:44
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Well, yes, I just made a sumary. Eventually the airplane potches down and will tend to maintain alpha prot.

But if deceleration rate is very high, the aoa can exceed alpha prot. Somewhere in between that and alpha max, flight augmentation will trigger alpha floor. It can happen, but only if high aoa protection is unable to arrest the aoa increase before.
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Old 29th Sep 2012, 15:25
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Originally Posted by OK465
Or an altitude is reached where available single-engine thrust is sufficient to maintain V-alphaprot and zero VSI.
You think so? Would the phugoid motion not continue, altitude cycling about the ceiling and airspeed cycling about Valphaprot?
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Old 29th Sep 2012, 18:58
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...and how to get rid of alpha floor?

Beside the facts of circumstances which led an Airbus to enter this regime, its not unimportant to know how to leave this flight regime.
Sadly Iberia learnt this lesson on Feb. 7th 2001 in Bilbao on Flight IB 1456.

To state the official Accident Report from the CIAIAC:

"...
De-activation of the AOA protection To de-active the AOA protection the sidestick has to be pushed either to 8 nose down for at least 0.2 seconds or to 0.5 nose down for more than 0.5 seconds with α < αmax.
As a consequence of this logic in the activation of the elevator flight control, imple- mented on the ELAC L80 standard computers, it can be stated that: While in flight, if the angle of attack is high, or its value is increasing fast, and at that specific moment the sidestick is pulled fully backwards, the system will detect a very high anticipated angle of attack and, if it is higher than αprot, will activate the AOA protection. Once this protection has been activated, the system will preclude the aircraft from exceeding the maximum angle of attack but, if it also detects that airspeed is diminishing, it will lim- it even further the possible angle of attack, pitching the aircraft nose down to dampen an eventual phugoid movement..."
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Old 30th Sep 2012, 06:47
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All of which was and is "transparent to the pilot"
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