You ask about turning the following conditional into reported speech:
The plane can make tight turns if the wind is weak.
The use of present tenses marks this as a real conditional. In other words, this is talking about something that is in the realm of the real, grammatically speaking1. We are not talking about unreality or irrealis here. Note that the modal can here refers to ability. You can also is able to.
If you want to maintain this type of conditional in reported speech, you change the present tense to past tense, resulting in
He told me that the plane could make tight turns if the wind was weak.
Note that could here retains the sense of ability. You could also write was able to.
Introducing the additional information that the plane crashed does not change the type of conditional sentence that you have, so the verb tenses are the same as without this clause:
He told me that the plane, which crashed, could make tight turns if the wind was weak.
It's interesting that the plane crashed, but this fact changes nothing about the type of conditional sentence you have. You could just as well have introduced the fact that the plane landed safely.
Were is the past tense of are, not is.
Both were and was are also used in unreal conditionals, but your sentence is not an unreal conditional.
1 Whether planes in general or this specific plane can actually make tight turns in weak wind has nothing to do with the grammar of the sentence; nor does it matter if the speaker is wrong or even lying about the plane: grammatically this is a real conditional. For the forms of conditional sentences, see, for example, mindmeister.