The use of air vehicles as reconnaissance platforms dates back before the invention of the airplane to the use of hot air balloons by the armies of nineteenth century Earth. The advent of the airplane increased the usefulness of aerial reconnaissance. Spy satellites with their detailed photographs and relative invulnerability temporarily reduced the importance of aerial reconnaissance, but the standard spy plane never went out of style. Now scattered throughout the Human Sphere, almost every world has recon planes to look for the enemy, or just to watch the migration patterns of the local wildlife.
         Most propeller-driven planes can cruise at just below the speed of sound, though of course breaching that barrier would destroy the props. Those planes with rocket boosters or powered by jet engines can of course break that barrier though such planes are less common.