There was a time when cargo holds on commercial aircraft were made accessible during flight, so crew members could manually extinguish fires in a usual event. Newer, larger, class D cargo compartments limited the in-flight accessibility, driving the need for more stable and automated fire systems. Larger cargo holds also posed a significant risk of onboard fires, particularly due to large volumes of oxygen. Cargo holds on modern airliners are built with specialized composite linings to minimize fire risk. Moreover, more effective fire suppression systems are installed to detect, warn, and suppress in-service fires.