To quote page 6
(I) REFERENCE CYCLE (Ng): Theoretical cycle used a reference for setting service life limits. Includes one start, one power acceleration corresponding to the "takeoff power" and one shutdown.
(ii) OPERATING CYCLES: A flight may consist of two types of cycles:
- Complete Cycle: An engine operating sequence including one start, one power acceleration and one shutdown.
- Partial Cycle: An engine operating sequence corresponding to a power decrease followed by a significant increase without engine shutdown. It may be defined as the number of hover events and landings.
(iii) Free Turbine: All components. One flight = One cycle
Gas Generator Equation: N = K1 + (n x K2)
K1 = Coefficient corresponding to maximum Ng (see table over page)
K2 = Coefficient corresponding to minimum Ng (see table over page)
n = Number of partial cycles carried out between start and shutdown
Those of us that can read will have noticed
(iii) Free Turbine: All components. One flight = One cycle
Which obviously means as well - NO Flight = NO Cycle.
Ezy Air,
You may understand "how" to work out the Ng cycles but here is the "why".
The design of the Arriel and some other TM engines involves a unique feature called the "injection wheel". This wheel is an integral part of the rotating assembly of the Gas Generator. It is subject to some fairly severe low cycle fatigue as it has the added issue of relatively cold fuel flowing through it and combustion occurring very close by and it links the gas generator turbine to the compressor so probably has about ~ 4-5 thousand horsepower going through it. The fuel and Ng speed are linked so the range of Ng is directly related to the temperature cycle and speed cycle that the injection wheel and other LCF affected components are subjected to.
The injection wheel probably has the lowest "cycle life" of all the rotating components.
Low Cycle Fatigue is real as some have found to their detriment. The partial cycle count was never an issue with this engine until someone found out the hard way.