The lifespan of an airliner is not truly measured in time. Instead, it is counted based on pressurization cycles. Each time an aircraft is pressurized during a flight its fuselage is subjected to stress. The “lifespan” of an aircraft is reached when there are certain metal fatigues and cracks which may pose danger. The “service life of 20 years” is generally expressed by approximate figures of 51,000 flight hours and 75,000 pressurization cycles for most aircraft. If an aircraft is used on long haul routes it experiences relatively few pressurization cycles in its “life” and can remain airworthy far beyond 20 years.