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Not sure if "is" becomes "was" or "were" when passing from direct speech conditional to indirect speech.
Direct speech:
The plane can make tight turns if the wind is weak.
Indirect speech:
He told me that the plane, which crashed, could make tight turns if the wind was (or were?) weak.
It is not the change to indirect speech which dictates the change in verb form; rather it is the use of the verb could to describe the possibility of an outcome contrary to what occurred, and the introduction of information about that outcome in the form of the parenthetical which crashed.
He told me that the plane, which crashed, could make tight turns if the wind were weak.
This is the subjunctive mood; it is revealed when we use a conditional like could to introduce a speculative event which is contrary to fact. That the plane crashed provides a hint that the wind was not weak.
In this case, "was" vs. "were" is simply singular vs. plural, and based on the subject they are enacted by.
if the wind was weak. : "wind" is singular => "was".
if the winds were weak. : "winds" is plural => "were".
As a side note, both "the wind was" and "the winds were" are idiomatically the same.
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.
Additional information:
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.
