nosedive

noun

1. A headfirst fall or jump.
2. A drop in altitude with the nose of the craft angled downwards.
3. (usually)A rapid fall, e.g. in price or value.

   “Byers was frequently at odds with Rail Regulator Sir Tom Winsor, but quit after 11 months - his career took a nosedive when he admitted false claims for £125,000 expenses, and he was banned from the House of Commons for two years.”

verb

1. (intransitive)To dive down in a steep angle.
2. (intransitive)To perform a rapid fall in price or value.

   “With punctuality nosediving and both overcrowding and cancellations on the rise, a new simplified timetable was introduced in December amid threats from the Mayor that he would ask the Secretary of State for Transport to strip Abellio of the franchise.”