The Japan Airlines Flight 123 plane crash is the worst single plane accident in its history.
ご質問ありがとうございます。
"The Japan Airlines Flight 123 plane crash"＝「日本航空123便墜落事故」
"is the worst single plane accident in its history."＝「は飛行機単機の史上最悪の事故だ。」
追加："They had accidently used not two, but only one for an essential part of the plane's tail."
ご参考に！
アメリカ合衆国 
