let down vs disappoint

let down and disappoint both are verbs.

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

As verbs, let down and disappoint are synonyms defined as:

let down (verb)	disappoint (verb)
fail to meet the hopes or expectations of	fail to meet the hopes or expectations of
move something or somebody to a lower position