April 10, 1917, a furious air battle was taking place as troops assaulted the eastern slope of Hill 145 near La Folie wood during the Battle of Vimy Ridge. Despite a plague of German fighters, the Royal Flying Corps (RFC) kept observation planes aloft, taking vital photographs of the front, helping the artillery pinpoint enemy guns that were dug in and camouflaged, and reporting on enemy strength, position and movement. And because they flew so low, RFC pilots, a quarter to a third of whom were Canadian, were also in danger from machine-gun and rifle fire from the ground. In just the first week of what they called Bloody April, the RFC lost 131 aircraft. It was not unusual when an aircraft with the RFC roundels flew low over the battlefield. But it was unusual when it began st...