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Maximum Continuous Rating
This is the highest engine rating that can operate continuously at approximately 90 to 95% of the maximum power. It is more than what is required for climbing at a good fuel economy. Operational demand in this rating arises from specific situations – for example, a very fast climb to altitude (mainly in military use). Typically, a climb is accomplished at around 85% power to reduce stress on the engine and to achieve better fuel economy. For this reason, the maximum continuous rating is not included with some engines and rather is merged with the maximum climb rating.
Maximum Climb Rating
The climb schedule is accomplished at approximately 85 to 90% of the maximum power. A typical climbing time for a civil aircraft is less than 30 to 40 minutes, but it can run continuously.
Maximum Cruise Rating
This rating is approximately 80 to 85% of the maximum power matched to the maximum cruise-speed capability. Unless there is a need for higher speed, typical cruise is performed at a 70 to 75% power rating, called the cruise rating. This gives the best fuel economy for the LRC. In a holding pattern in an airport vicinity, engines run at still lower power, barely maintaining altitude while waiting for clearance to proceed. The rating depends on the weight of an aircraft; at the end of cruise (lightweight), an approximate 65% rating is sufficient.
Idle Rating
This rating is at approximately 40 to 50% of maximum power and is intended for an engine to run without flameout but also produces practically no thrust. This situation arises at descent, at approach, or on the ground. During a descent, it has been found that better economy can be achieved by descending at partial throttle, at about a 60% power rating. This results in a shallower glide slope to cover more distance and consume less fuel.
Representative engine performances of various types at takeoff, maximum climb, and maximum cruise ratings are given in this section for an ISA day. Engine manufacturers also supply performance data for non-ISA days, which is more critical for hot and high-altitude conditions when engines produce considerably less power. To protect engines from heat stress, a fuel-control system is tuned to cut off power generation to a flat-rated value (at an ISA-day engine rating) up to a hot day that can be 20° C above the ISA day. This book does not address non-ISA-day performances; in the industry, they are supplied.
