In 1937 his company, United Air Transport, provided regular airmail and transport services to the Grande Prairie and Peace River areas. In April, McConachie decided to expand to take advantage of the growing consumer demand for good, reliable air service. He announced that he would increase services by flying between Edmonton and Whitehorse once a week, with stops in Grande Prairie, Fort St John, Fort Nelson, and Lower Post, located on the British Columbia and Yukon border.

In 1939, a local air company, Peace River Air, was also established due to the market demand. McConachie expanded his service again and renamed his company Yukon Southern Transport.

He used a Ford tri-motor to perform at summer fairs, and made a profit transporting businessmen across the Rockies to Vancouver when they needed to get there faster than a train could travel.

When the Second World War started in 1939, conditions changed for McConachie’s Yukon Southern Transport. He faced stiff competition and spare parts were suddenly difficult to obtain with factories focused on producing wartime supplies.

At the end of 1941 and the beginning of 1942, he sold Yukon Southern Transport to the newly formed Canadian Pacific Air Lines limited.