Roll
From MechWarrior: Living Legends Wiki
Roll is a control for maneuvering VTOLs and Aerospace. By pressing [a] or [d], the aircraft can rotate left or right about the Z-axis. By rolling to one side and pitching up or down, an aircraft can make a much sharper turn than by using yaw.
For additional information on how to use roll to maneuver an aircraft see the Air Combat Guide.