Post-war piston airliners such as the Lockheed Constellation (1943) extended the technology to civilian service. The piston engined airliners generally relied on electrical compressors to provide pressurized cabin air. Engine supercharging and cabin pressurization enabled planes like the Douglas DC-6, the Douglas DC-7, and the Constellation to have certified service ceilings from to . Designing a pressurized fuselage to cope with that altitude range was within the engineering and metallurgical knowledge of that time. The introduction of jet airliners required a significant increase in cruise altitudes to the range, where jet engines are more fuel efficient. That increase in cruise altitudes required far more rigorous engineering of the fuselage, and in the beginning not all the engineering problems were fully understood.