I don't know the difference between "available" and "accessible". These words have the same meaning; "You can use or get it." What's the difference? I know "available" can be postpositive, but I don't know whether "accessible" can be postpositive or not. For example; "I'll leave here on the first flight accessible." Is this sentence correct?
A thing has to be existent to be accessible or inaccessible.
Consider these sentences:
There is no flight available.
It can mean there are flights, but they are booked out or otherwise - well not available. It can also mean, there are no flights at all.
The building is not accessible.
It means that some measure or circumstance precludes me from entering the building. I cannot enter.
Otherwise dictionary entries will tell you that accessible means in the relevant meanings:
accessible 1 (of a place) able to be reached or entered.
1.1 Able to be easily obtained or used.
available 1 Able to be used or obtained; at someone's disposal.
1.1 (of a person) not otherwise occupied; free to do something.
Thus, available is more about being there, not occupied, ready to be used. On the other hand accessible in its primary meaning is more about getting to something or in our age also being allowed to do something.
The website was available for browsing but the private section was inaccessible due to password restrictions.
If access to something available to others is not allowed to me personally, it is inaccessible and unavailable to me. I can not avail myself of the opportunity to get access to it.
If a plane is available to me, it's not necessarily accessible to me (I can avail myself of the plane only if I get access to it). But once I'm allowed access to the plane (and I'm able of course to ascend the ladder and seat myself in my chair), it will be available to me and I can "leave here on it".
I would resort to using "I'll leave here on the first flight available". Whether you have access to the plane or not (somebody has blocked the entrance or you are unable to get into the plane for some reason) is your private issue.
There exists something, so it is available. But whether you can get it or not makes it (in)accessible to you.
For example: The encyclopaedia is available. That is, there is at least one existing in some library. If you can get it, use it, refer to it, it is accessible to you. But if you cannot get it, it is far from you, or it is in another city/library, it is inaccessible to you, although it is available.