Here is Clumsy’s video on that subject:
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Maybe the title was to draw clicks but perhaps you may want to consider changing it as CLB/DES mode in the A320, or “VNAV” as you’ve called it, certainly does not work properly, yet. I’ve noted just a few examples below. It’s improving yet a long way to proper operation.
- I watched twice as you enabled AP1 a massive pitch up as you mentioned. The first time, there was no vertical mode enabled - CLB was still armed (blue) on the FMA. You then disengaged, AP1 yet modes stayed the same. Then, by itself, the aircraft changed to CLB as the active mode. That was problematic for sure. Also, you hadn’t even enable AP1 yet NAV mode was showing as the active mode in the FMA by the green NAV indication in row 1. That is also not accurate. Autopilot can be enabled as early as 400’ so this is still not a properly functioning mode. It shouldn’t “fix itself”.
The second time you tried to enable AP, the aircraft was just about to level at 4,000’ yet the aircraft pitched up even with the FMA speed target set above your present speed. This triggered A.FLOOR which is certainly not good, nor correct. Even after AP was engaged, the aircraft continued to climb away from your target altitude of 4,000’. Not good.
2 The airplane was still showing 4,000’ but somehow decided to float around at about 5,500’ barely climbing, seeming to maintain the set speed, which isn’t how NAV mode would operate.
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It wasn’t until you dialed in 10,000’ and pushed the altitude button on the FCU that the aircraft began climbing in CLB mode. The whole initial climb phase was a mess as you mentioned yet it shouldn’t have been. CLB mode was not at all working as it should have done.
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During the climb to 10,000’ you get an instruction to climb to 16,000’. You dial in FL160, the FMA shows 16,000’ correctly, yet the aircraft stops the climb at 10,000’, your old altitude. That is of course incorrect as you pointed out. That is not CLB mode or “VNAV” mode working, at all.
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For the descent, we still don’t get T/D displayed on the MCDU, when you do start the descent, there is no “yo-yo” displayed showing the flight path deviation, your T/D arrow moves ahead of the plane which is wrong - It should be replaced by a path intercept symbol (blue lightning bolt)
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On the descent, you had a -13,000 crossing restriction in the MCDU. The “-” means “at or below” so technically, the aircraft did perform as instructed. Also, you were in NAV mode yet the aircraft leveled at 9,000’ when you had 4,000’ still dialed in the FCU. No restrictions were present so aircraft should have continued the descent.
Overall, there may be some things somewhat working, yet overall, the CLB/DES mode of the A320 is still very buggy. It’s complex and requires input from multiple systems to function properly - Something that is still a bit off. Very far from VNAV working (please call it CLB/DES mode..VNAV is a Boeing thing. 
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Make sure to post this in the bug section too. It’s 10,000 feet over my head!
No worries! I’ve actually submitted several bugs on this to Zendesk.
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