The plane had to make an emergency landing due to engine failure.

emergency 是名词，由前缀 e-（表示"向外"）和词根 merg（"沉没"）加上名词后缀 -ency 组成，原意是"突然浮现的状况"，现在指"紧急情况、突发事件"。 emergency landing 指飞机在遇到紧急情况时采取的迫降措施。