I'm pretty sure it was from Microsoft. I may have even the paper instructions, i'll search for them :-) Wikipedia: "FS1 Flight Simulator is a 1979 video game published by Sublogic for the Apple II. A TRS-80 version followed in 1980. FS1 Flight Simulator is a flight simulator in the cockpit of a slightly modernized Sopwith Camel.[2] FS1 is the first in a line of simulations from Sublogic which, beginning in 1982, were also sold by Microsoft as Microsoft Flight Simulator."

I played it on the 48K.

I can't believe there was a version that run on the 16K ZX81 as well!

48k of course, not 32k :-)

Rubber keys?

yep (wikipedia.org/wiki/ZX_Spectrum)

I had the spectrum plus. Still 48k, but a better keyboard