Pirates with Wings

First published in Sky Fighters, Winter 1949. Reprinted in the collections Night Over The Solomons and The Collected Short Stories of Louis L'Amour - Volume 4: The Adventure Stories.
The year is likely 1948. Turk Madden is back in South America flying a survey run for Tropco, an oil exploration company in the Matto Grosso area of Brazil. The crew he is piloting are using a new state-of-the-art device called a fluxgate magnetometer which will help find likely petroleum deposits. A rival company, Petex, is wanting the mission to fail.