Though Brits still say aeroplane, the word airplane replaced aeroplane as the standard term in the United States in 1916. So GN’s use of the older term must have been merely for nostalgic effect, or perhaps to cast a posh, British air (aer?) over its Glacier Park hotels.
This brochure, which has a colorful map on one side and black-and-white photos and text, with brown trim, on the other side, is undated. One photo shows a bus that is clearly older than the Glacier Park red jammers, which were introduced in 1936. So I would date the brochure to sometime between 1930 and 1935.