Where I live plumbers make $40/hr, so about $60,000/yr.

Note, though, that those are starting salaries. They will approximately double for an engineer with 5-10 years of experience.

Hardly, unless you make it into management. This is more typical:

5-10 years

Those ranges were in the 80's to 100's, which is about double the starting salaries. In my own experience, everyone I work with had doubled their salaries by the 10 year mark at the latest. I did it in 4 years, but only because I accepted an international assignment which comes with several premiums.

$40 is not starting salary! In VT it takes 5 yrs to be a master plumber. And guess what, you'll likely start at $12! After 5 years, if you pass the test, you're still no where near $40, if you work for someone else.

My comment was that the engineering salaries are starting salaries. What is being compared is an established plumber's salary versus a starting engineering salary - an invalid comparison.