I deleted that pilot and started a new identical Campaign with RNAS 8, again.
This time the whole flight got up and I didn't disable any mods!
Mystery...

Odd one - something on the PC maybe got corrupted in memory or on disk.

Glad its working again Elephant. 8 RNAS is one of my favorite campaigns to fly (or, should I say attempt, I never last very long!)

Well, It was just the mission type (Airfield attack) and the fact that I was leading the flight.
In the next mission the problem emerged again...
I disabled the AI weights Mod and...ta-daaa!!!
The rest of my Sqn was airborne again.
Thank you all for the help and concern!

This is interesting, because there was no AI weight mod available when I experienced the refusal to take off in the R.E.8 squadron. I was unable to find the old thread in the CA forum (it seems the forum search doesn't remember my posts for more than a year), but I'll keep looking.

I think I didn't have the problem in every mission I flew, just every now and then. Still, it was annoying enough to force me to switch squadrons.

Elephant, please keep flying in RNAS 8 and let us know if the problem comes back.

When I have time, I'll try flying in the same squadron. I don't have the AI weight mod, so we'll see what happens.

Takeoff problems for some few combinations of squads/aircraft types/mission types
were known problems since very long time.
They seem to occur only very rarely, and I guess they will be eliminated in WOFF.

Yes we are creating all brand new occasional random issues
lol

Seriously it could also be down to things like the AI code being stopped or interfered with (AntiVirus etc or other background task? or modded FM/aicraft weight etc.

Best exit the manager and load and try again or reboot etc first.

I'm a bit late to the party but I started a RNAS-8 campaign this week and after 4 hours flight time I've had no issues. I started right from October 1916 and it's been a joy so far. I'll post back if anything 'wonky' happens.

Thanks Shiloh, I think it's just a very rare occurrence as we'd have seen many reports of it over the years otherwise and we haven't. It is possibly due to something crashing in memory or the mods affecting the AI flying. As always if anyone sees an oddity like that usually making a new pilot, or exit OFF and restart of the PC should fix.