Interesting thing I found;
When the aircraft reaches below [x : 63 [around water level], the Fog mod will incorrectly use "cave/underground" fog instead, turning the otherwise blue sky into gray. (This probably isn't the suitable GitHub repo to post this on, but just in case it is...)

To Reproduce:

• Install IMB11's Fog
• Install Immersive Aircraft
• Go to a shore (floor must be level with the sea) with an Airship (the smallest one).
• If correct, you should see the fog transition to gray.

With and without Shader (Miniature Shader):

Additional Refs: (Stepping out of the Aircraft, on a horse, and below the Aircraft's level)

Could it be that when sitting in the aircraft, the position of the player is just slightly below the threshold? Does a boat work fine? (I assume the boat is just a tad higher)

Now that I tried it with boats... it seems to be the fog's issue, not the aircraft.

This should be enough to close the report.

Thanks for testing, please forward the boat hint to Fog