BETWEEN terminal or intermediate airports, intermediate landing fields are provided at approximately 30‐mile intervals.
(1930), "Air Route Organisation in U.S.A.: Intermediate Landing Grounds at 30‐Mile Intervals and Route Beacons Every Ten Miles Are the Features of American Night‐Flying Airways", Aircraft Engineering and Aerospace Technology, Vol. 2 No. 1, pp. 16-16. https://doi.org/10.1108/eb029225
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