It sounds like you were hired to do one job, and that job was 'cancelled,' and then you were given a different job (doing the same thing at a different site). It is not unusual for different jobs to pay at different rates (and the change in location can make a difference in the job responsibilities that justifies a different wage).
There are a number of places you could look to better ascertain your potential rights.
Do you have an employment contract? That may provide you with certain rights. A letter offering the job might, in some jurisdictions, reflect an offer that could form the basis of the employment contract. But of course, the change in job locations could reflect a mutual alteration in that contract.
Is there an employees' manual? That may also provide you with certain rights.
In today's economy, it is not unusual for wages to be lowered when a company needs to make a choice between laying-off some employees to maintain wages, or reducing wages across the board to prevent lay-offs. Many companies in the aviation sector have experienced higher costs, customers that do not pay on time, tighter credit terms, and other factors that make it difficult to continue doing business the way that they have done it in the past, so it is possible that you have found yourself 'caught' in your employer's dilemma. You may want to talk to other employees who have been there longer to see what the norm in your company is. You may also want to talk directly to the employer and find out why you were assigned a different wage when you were transferred to the different site (it is also possible that it was a mistake that could be corrected!).
After you've done some due diligence you may have the answers you want; but even if you do not have the answers that you want, you will have a lot more information to provide to a local lawyer who might be able to help you analyze the law as it applies to your specific set of facts.