Why does the plane altitude reset on autopilot when the metar QNH change

Question

QNH set to Transition altitude, 1013 or 29.92, when the metar QNH change on the weather, the aircraft adjusts its altitude.
This is not realisting, QNH should be maintained irrispective of the METAR QNH

Answer 1

Hi, what you describe is normal. It is due to an abrupt change of atmosphere properties when a new METAR is downloaded. Maybe your request is to implement an option so that such an atmosphere change should take effect with a smoother transition...

Comments

Hi
Thank you for the feedback, but to enhance the "realism" of our simulator compared to real world behaviour;
Nothing will happen until you adjust the sub-scale in the altemeter.

Just a thought to keep it "as real as it gets"

Thank you for this awsome product

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Well, if your altimeter setting changes automatically with METAR updates, this is not a behaviour I could reproduce in X-plane 11.02b1 default aircraft. Maybe you have a plugin (or an enabled option which I do not think exists in X-plane default install) that triggers this behaviour. If not, try and delete your prefs file see if it works.

Guillaume

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Now that makes sense, which I will conclude as follow:

• X-Plane does not automatically adjust the aircraft altitude with metar pressure differentials
• This behaviour could be a plugin causing this.

If I understand this right, Thank you

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I think there is a confusion between altimeter setting (ie QNH set on the altimeter) and altitude.

I have never seen X-plane change automatically the altimeter setting (in real life, altimeter setting can only be changed manually)

Let's say you are cruising over A at FL340, which is 34000 feet displayed on your altimeter with an altimeter setting of 1013 HPa (or 2992in.Hg). QNH over A is 1003 Hpa, So your real altitude is 34000 - (1013-1003)*28 = 33720 feet.

Now you reach B, your METAR (ie atmospheric properties) is updated abruptly and QNH is now 1023. In real life, the QNH change wouldn't be so abrupt and while still flying at FL340, your real altitude would now be 34000 -(1013-1023)*28 = 34280 feet.

But in the sim, you are still at a height of 33720 feet so your autopilot wants to reach that updated target height of 34280 feet in order to display 34000 on the altimeter again, which still has the setting of 1013 Hpa.

So in this theoretical case, before METAR update, your altimeter would display 34000. After METAR update, it would display 33440 and your autopilot would go up by 560 feet in order to reach the updated FL340.

It is more a problem of abrupt atmospheric property transition. Is that what you observe?

PS : I am not affiliated with Laminar Research.

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here is the scenario:

QNH 1013.25hPa for fl125, local metar QNH 1011 hPa.
Now maintaining fl125, local metar jump to 1020 hPa.

In real life the plane will maintain altitude, but in X-Plane it will adjust the aircraft altitude, as if to recalculate the delta between ISA and real metar pressure, adjusting the aircraft altitude to the new delta. As you are flying to a high pressure in my scenario, the aircraft will climb and settle at a higher altitude before the metar update.

As you have mentioned above, this is just a sumary of that in short.

This is not true in real life, what is true is that your ground clearance will adjust with metar pressure and not your aircraft altitude, X-Plane seems to maintain your altitude, based on the delta of the altemeter subscale and the real metar pressure.

Now can this be fixed, or should we live with this?