Flight Hours and Flight Numbers in Aviation

In aviation, flight hours and journey (cycle) counts are subject to specific limitations for aircraft maintenance, safety, and efficiency. These limitations are regulated by international aviation authorities EASA (European Union Aviation Safety Agency) and ICAO (International Civil Aviation Organization) like. These regulations aim to ensure air transport operations are conducted safely and sustainably.

Flight Hours

Flight hour refers to the duration an aircraft spends in the air. It is one of the fundamental components of airplane maintenance cycles and directly influences maintenance programs and aircraft performance.

• EASA defines flight hour as the time between takeoff and landing. It is one of the most critical parameters triggering aircraft maintenance schedules. EASA establishes various rules based on flight hours; for example, when an aircraft reaches a specified flight hour limit, certain maintenance procedures must be performed. (EASA Part-145)
• ICAO provides a general framework for flight hour regulations but does not specify detailed maintenance requirements. ICAO emphasizes the safety significance of flight hours and their role in aircraft maintenance cycles. Flight hour calculations are integrated into aircraft maintenance programs according to established standards. (ICAO Annex-6)

Cycle Count

Cycle count refers to the operational sequence from takeoff to landing. One cycle includes one takeoff and one landing and also plays a important role in determining aircraft maintenance cycles.

• EASA: Cycle counts are critically important for assessing structural integrity and wear of specific mechanical components. Aircraft are subject to maintenance requirements once they reach a specified cycle limit. EASA sets a defined cycle limit for each aircraft type, and aircraft undergo inspections and maintenance upon reaching these limits.
• ICAO: With regard to cycle counts, ICAO ensures that aircraft maintenance procedures comply with international standards. When cycle limits are exceeded, ICAO provides guidelines to determine maintenance needs and operational safety requirements.

Additional Considerations for Multiple Takeoffs and Landings in a Single Flight

If an aircraft performs multiple takeoffs and landings during a single flight (for example, during training flights, touch-and-go operations, or short long-haul flight segments), each takeoff-landing sequence is counted as a separate flight cycle (FC). For instance, if an aircraft performs six takeoffs and landings during a single flight session, this is recorded as six flight cycles. This distinction is important because certain aircraft components—particularly landing gear, pressurization systems, and fatigue-sensitive structural parts—are directly affected by the total number of flight cycles, not just flight hours.

Components with life limits based on cycles may be monitored according to cycle counts rather than flight hours. For the same component, limits may be imposed based on flight hours, cycle count, calendar or calendar time. In such cases, maintenance and replacement programs are planned according to whichever criterion is reached first, ensuring alignment with the component’s real operational loading. This is particularly significant for life-limited parts (LLPs) and body fatigue monitoring, as it ensures compliance with manufacturer and regulatory guidelines and carries substantial importance importance.