For example, it was the librettist's suggestion to incorporate the "Navy Hymn" into the opera, because the story concerns a US navy pilot, Dodge (played by William Burden). Hagen took McFall's suggestion and ran with it. Aunt Helen (played by Jane Eaglen) sings the McFall/Hagen arrangement of the Navy Hymn at a dramatic moment in the opera (Act Two Scene Two); but Hagen has included musical fragments of the tune as far back as the first scene, so that when Eaglen sings it for real, it will crystallize what we've been hearing all night long.
The text of the hymn, as sung by Aunt Helen:
Lord, guard and guide all those who fly
Through the great spaces of the sky.
Be with them always in the air,
In darkening storms or sunlight fair;
Oh, hear us when we lift our prayer
For those in peril in the air.
Aunt Helen's text is based on the second verse of the hymn (the first verse, "Eternal Father, strong to save", concerns sailors more than pilots). Many renditions of the hymn are available on YouTube; my favorite, musically speaking, is embedded below. They singers get to the second verse at 1:55.