If the leading zero is NOT used and you hear only ONE digit, then you have
no way of knowing whether you missed something, or not.

What might I have missed if I hear,"Podunk traffic, Waco niner eight zero
one victor, entering downwind runway two, stop-and-go, Podunk."

"Podunk traffic, Waco niner eight zero one victor, entering downwind
runway two ZERO, stop-and-go, Podunk."

He's actually coming the other way, head on to you who hears "runway
two, stop and go..."

How do I know the digits haven't been transposed?

I suppose you don't, and that's the argument for omitting the leading
zero. Further, you could hear it correctly and transpose it in your own
mind.

Which one trumps the other? I don't think either is trump. Do what
other pilots expect to hear, whether by local practice or AIM.

Jose
--
Money: what you need when you run out of brains.
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