I sign an employment contract stating that I would work on a rotational bases . I am an aircraft mechanic and I would work overseas for a extended period of time ,and then be off for the same time in this case they told me a (3 months) rotation. which was

I sign an employment contract stating that I would work on a rotational bases . I am an aircraft mechanic and I would work overseas for a extended period of time ,and then be off for the same time in this case they told me a (3 months) rotation. which wasn't on the contract.
I was a full direct employee; with health benefits.
I worked mt first 3 months rotation at their US branch.
While keeping correspondence with mt immediate Boss; I was off for a full duration of 4 and a 1/2 months. I then received a call by my Boss to set up a meeting with him and the HR person.
They proceeded to tell me that I was fired for 'terminated for unable to perform the duties of my Job.
I can argue weather they should or should not have fired me , but the fact that they waited to tell me after 4 1/2 months is the problem I have. Question:
Am I entitled to Back Pay? Or a Severance Pay for their lack of communication? and who do I see about this matter?