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This video is about the Airbus A320 High angle of attack protection

At 4:54 - 5:13 why did it pitch down? I know it is trying to maintain Valpha max but if it would have stopped following Valpha max it was at around 200 knots and could have kept climbing at a very good rate. Instead it maintained Valpha max but when it maintained Valpha max the speed decreased and and it pitched down and thus the vertical feet per minute started to decrease.

I thought it was suppose to maintain the highest possible lift and if it stayed at 200 knots with the pitch up the aircraft was climbing at a very high rate but it pitched down and slowed the aircraft so it wasn't climbing at its max performance.

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  • $\begingroup$ Did you mean why did it keep holding Valpha max or why did it not hold Valpha max? $\endgroup$ – ymb1 Jun 16 '18 at 8:20
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You need to look earlier in the video. At 3:59 the sidestick goes full back and full left. The aircraft exceeds the 33deg bank limit on automatic maintenance of pitch and bank, requiring the pilot to hold pitch and bank.

enter image description here

The pilot is in command from 4:54-5:13, not choosing Valpha max. He hits the bank angle and stall limits to demonstrate flight envelope protection, the point of the video.

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  • $\begingroup$ So if the aircraft stayed at a 20 degree pitch angle the speed would have kept decreasing unitl it stalled correct, or would the speed stay constant at a point? $\endgroup$ – Gary Zenger Jun 16 '18 at 7:52
  • $\begingroup$ @GaryZenger The aircraft's flight envelope protection will not let the pilot stall the aircraft. It does not try to maintain a particular attitude with the pilot in control, it just prevents dangerous ones. $\endgroup$ – Pilothead Jun 16 '18 at 7:58
  • $\begingroup$ Suppose it was in alternate law. $\endgroup$ – Gary Zenger Jun 16 '18 at 8:09
  • $\begingroup$ @Pilothead - re title edit, I made the same mistake earlier, op wrote "Instead it maintained Valpha max". $\endgroup$ – ymb1 Jun 16 '18 at 8:09
  • $\begingroup$ @ymb1 At the time specified the aircraft was not at Valpha max, so the question doesn't make sense any other way. You were right the first time. $\endgroup$ – Pilothead Jun 16 '18 at 8:13
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Sequence of events:

enter image description here enter image description here

Full back on the stick plus full left causes the nose to drop.

enter image description here enter image description here

The nose dropping increases the speed. The speed increase with fixed trim (due to trim-up being inhibited and the full back on the stick) causes the plane to nose up until it can't hold the speed anymore.

enter image description here enter image description here

The plane then loses speed, and to maintain the alpha-max it noses down.


From the flight manual:

The pilot can hold full back stick, if it is needed (see windshear), and the aircraft stabilizes at an angle of attack close to but short of the 1 g stall.

Stabilizes AoA, not speed.

As the aircraft enters protection at the amber and black strip. (αprot), the system inhibits further nose-up trim beyond the point already reached. Nose-down trim remains available if the pilot pushes the stick forward.

enter image description here

Holding the alpha-max gives the highest allowable lift coefficient.

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  • $\begingroup$ I don’t think air density plays a role here... too large an effect for too little change. It’s the bank angle driving up stall speed, so the aircraft dives for speed. $\endgroup$ – Cpt Reynolds Jun 16 '18 at 8:41
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    $\begingroup$ @CptReynolds - good point, I've revised the answer. $\endgroup$ – ymb1 Jun 16 '18 at 10:00
  • $\begingroup$ @ymb1 A 1g stall is the red band, correct? $\endgroup$ – Gary Zenger Jun 18 '18 at 2:24

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