MOSCOW—Russia, once a global aviation power, has become the most dangerous country in which to board an airliner.

Investigations of nine commercial plane crashes this year, including one that killed an entire professional hockey team, found a raft of gross violations and errors, such as drunk or sedated flight crews, forged safety documents and panicked pilots. In one crash, the navigator used the wrong guidance equipment and aimed his jetliner at a tree, far from the runway.

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