Question: My PhD supervisor (in the field of artificial intelligence) invited me to several meetings with another PhD student. Since I have more knowledge than my supervisor in some topics, I made many suggestions about how this student could improve his methodology. In total, I went to 5 meetings of 3–4 hours each. I could have used this time to do my own research.

When the other student started to write his manuscript, I asked to be an author, since I had contributed directly to the research, but he felt I only deserved to appear in the acknowledgments. So I asked my supervisor to be on the paper and even volunteered to help writing it. My supervisor said “Don't worry about his paper, you have your own things to do, and he is not going to submit this work anytime soon”.

One week later, they submitted the paper to a major conference without my name on it. I felt really insulted that they used my time and skills with no credit to me.

I didn't actively do the research or help write the paper, but I felt I was supervising the student since I have the most experience on this topic. Many academics in my field get their names on papers by just giving suggestions.

What should I do? I am planning on taking it to the head of department.

Answer: 
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> When the PhD student started to write a paper about it, I asked him to be on the paper, since I felt I contributed directly to his research. But he said I just gave him suggestions and he could add me in the acknowledgments, not as co-author. And he said I should ask my supervisor if I deserve to be on the paper.
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As a general rule, contributing ideas, providing feedback, inspiring others does not qualify a person for authorship. During such discussions, we think and state our own views and ideas without paying much heed to the legwork involved for implementing the same. 

Making those ideas come to life are an entirely different thing. 

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> One week later, they submitted the paper to a major conference without my name on it. I felt really insulting that they used my time and my skills with no benefit for me.
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You have gained plenty from these discussions. You got to parade your knowledge and mind before others. You draw inspiration from them and later on you may gain a formal collaboration. Also, you gained an acknowledgement. Anything more, would be injustice to the authors involved in the paper.

Science rarely progresses without discussion with people from diverse backgrounds. That is what makes science inter-disciplinary.
The other answers cover most of the points but I’d just like to add that submission doesn’t necessarily equate with acceptance. They may have rushed this paper slightly to meet a conference deadline and neglected you in the rush to submit. They may be planning a journal submission after the conference and would like to collaborate with you on this. The supervisor/student may have decided to "take a punt" on this conference, knowing that the paper will be reviewed quickly and that reviewers' comments will be useful even if it is a likely rejection.

Bottom line: don’t take it personally. Consider if you **want** an authorship on a paper that you have had no direct control over.