It's only natural to be nervous when the airplane takes off.
Literal
Airplane [subject-が] take-off time [at-に] get-nervous [nominalizer-の] [topic-は] natural [masculine-さ].
当たり前 means 'natural, expected, a given.' The sentence-final さ is a casual, slightly masculine assertion particle — lighter and less forceful than ぞ or ぜ. ~のは...当たり前だ is a useful pattern for normalizing feelings.