low level flight vs terrain flight

low level flight and terrain flight both are nouns.

Word	Noun	Adjective	Verb	Adverb
low level flight	Yes	No	No	No
terrain flight	Yes	No	No	No

As nouns, low level flight and terrain flight are synonyms defined as:

low level flight (noun)	terrain flight (noun)
flight at very low altitudes	flight at very low altitudes