What qualities must a good engineering supervisor possess?

I presume you are refering to an Engineering manager. First an engineering manager has to know the scope of the project as they will discuss your work to both internal or external stake holders as well as do the marketing necessarly for your work if you have done well. Some work speak for itself, some doesn't and it is necessary for your success to have someone who is invested in yours. 

The other side, or the behavior factors have already been addressed by Edmund.

-APPRECIATION SKILLS

-GOOD JUDGEMENT

-GOOD ANALYTICAL SKILL

-POSITIVE SPIRITED

-PASSIONATE ABOUT YOUR JOB