If you are to use aircraft instructions, do not use any symbols or non-ASCII text. This could corrupt your build.

Source:planeflightx>>>
If you have lost your aircraft because you entered symbols into your aircraft instructions, please do as follows!
If you have already put a new aircraft in your design editor and haven't saved your aircraft before putting your aircraft instructions in, unfortunately your craft may be lost. If not, continue.

Open this folder on your desktop:
C:\Users\yourwindowsusername\AppData\LocalLow\Jundroo\SimplePlanes\AircraftDesigns
Open editor.xml with your notepad
Scroll all the way down to the bottom of your file
Remove anything including the word "instructions", this also includes any / or <> on the same line
Add at the very bottom of the file this exact line:
</Aircraft>
After that, load Simpleplanes and open the builder.
If your aircraft does not load, it's probably an issue other than the Aircraft Instructions.

again, credit to planex for helping me figure this out in the comments

((this was originally a post asking for help but i think i'd share on a forum post how to recover your aircraft from the major flaw in code, and it's a waste of resources to make a new forum post anyway)

since this is a bug post i will go ahead and say why this happens
The game's aircraft primarily use XML code to tell the game what your design looks like and how it works, Using a symbol that is also used in XML(or a symbol in general, no clue) can seriously screw up the actual code used to make your plane work and the game tries to fix this by deleting the improper bits but in turn deletes things that are important to the design working rather than just clearing the instructions.