Ideally you need to qualify as a solicitor or barrister. You do not need a law degree but with a degree in any subject you can do a one year "conversion" course before either taking the solicitor's exam course or the bar course. That would be followed by two years articles (paid) or one years pupillage (poorly paid). These days you need a good degree (1st or upper 2nd) to have any prospect of getting articles or pupillage and so there is some mileage in taking a subject that you enjoy and are confident of getting a good degree in. Then you have to hope you can achieve similar in the following law exams.
Which way you go rather depends upon what you mean by aviation lawyer. If you are looking at things like regulatory work, licencing or aircraft leasing/purchase agreements then I would suggest the solicitor route and hope to get into one of the few firms doing this type of work. Alternatively there are in house jobs with airlines. It is a relatively small and specialist field.