First I apologize for resurrecting this forum post. After many hours searching google for anything related to this specific request and made an account just to bump this with my support (on the behalf of many of the silent lurkers).
I saw a hotkey binding called "Toggle Jetpack Engine" and thought it was for IC2.
It turns out the hotkey I thought was IC2 was actually for a similar mod, Simply Jetpacks.
I was stunned that there was a hotkey binding to control the jetpack hover mode, but not the engine itself.
Backstory:
It's minecraft 1.12 on the direwolf20 pack and it's been awhile since I've last played minecraft.
I maxed out a full set of EnderIO Dark Armor (Elytra, speed, jump boost) and then added 5 additional vanilla enchantments (thorns 3, protection 4, etc.) on all pieces. The gear is maxed right? Right.
So I figured I'd throw a jetpack on there as a finishing touch. However, the jetpack was dead and my Thermal Expansion "Energetic Infuser" and "Flux Capacitors" (items that auto charge one's inventory) refused to charge it. That's when I learned only IC2-related stuff would charge it.
Excited to use the jetpack's "hover mode" feature to do some sky work, I built a machine that would charge the jetpack. That's when I realized that the "Toggle Jetpack Engine" hotkey I saw earlier was actually for Simply Jetpacks. I can't turn the jetpack off at all, just let it die out (It lasts awhile on a charge though).
Now I'm unsure what to do because I can't remove the jetpack and I don't have nearly enough materials to remake the chest. I feel, for me at least, a hotkey binding would fix this dilemma, as I could turn the jetpack on only when I want to use it's hover mode feature (the only reason I attached it to my chest to begin with).
TL;DR
A hotkey binding to toggle the jetpack engine on/off (like the Simply Jetpacks mod) would be to players that desire precision with the jetpack. In addition to those people, others may have accidentally attached the jetpack, realized you can't remove it, and would rather turn it off than make new gear. Adding this hotkey option certainly won't cause anyone harm, and let's be real, what engineer would design a jetpack in real life without an on/off switch? 