The ZX81 balloon "sim" reminded me of what was certainly MY very first "flight sim".

In 1977 I was in grad school for optics and I bought one of the first pocket-size programmable scientific calculators. I wanted HP-65 but could only afford the cheaper Texas Instruments SR-56. It had a single line red-LED display and 100 lines of program space (perhaps 400 bytes, or 0.4 kbytes, or 0.0004 MB) which was NOT saved when you powered off, and it had no storage medium either. So you had to key in the 100 lines of assembly-like program code every time you wanted it.

For class work there were useful things like numerical integration, but I spent a lot of time with the TI-supplied lunar lander program (a friend and I made some modifications to this program, being engineering students and all) . The display was something like this:

-17.095

... which meant that you were descending at 17 fps at 950 feet above the lunar surface.

Flight Sims on Calculators

Your input was a series of "burns" at one second intervals (not real time), meaning you entered a number like 50 to represent burning 50 lbs. of fuel that second (basically your throttle). Starting height and fuel load were selectable, so it was not always the same sequence of burns to get down safely (you could also run out of fuel). A successful landing was 0.0000, a crash left the impact velocity flashing in the display. This shows you that people will try to do "flight simulation" on virtually anything that can calculate numbers!