let down vs fail

let down and fail both are verbs.

Word	Noun	Adjective	Verb	Adverb
let down	No	No	Yes	No
fail	No	No	Yes	No

As verbs, fail is a hyponym of let down; that is, fail is a word with a more specific, narrower meaning than let down:

Other hyponyms of let down include betray, come short, fall short, disenchant, disillusion.

let down (verb)	fail (verb)
fail to meet the hopes or expectations of	stop operating or functioning
move something or somebody to a lower position	deteriorate
	prove insufficient
	disappoint, prove undependable to; abandon, forsake
	become bankrupt or insolvent; fail financially and close
	be unsuccessful
	fail to get a passing grade
	fail to do something; leave something undone
	judge unacceptable
	be unable
	fall short in what is expected