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ENGINE RATINGS
An engine rating specifies the thrust that an engine can (or is allowed) to develop in a particular operating mode. For commercial certification, these ratings are defined as follows.
Takeoff (Wet) This is the maximum thrust available for takeoff for engines that use water injection. The rating is selected by actuating the water injection system and setting the aircraft throttle to obtain the computed “wet” takeoff thrust. The rating is restricted to takeoff, is time limited to 5 min, and has altitude and ambient air or water temperature limitations. Takeoff (Dry) This is the maximum thrust available without the use of water injection. The rating is selected by setting the aircraft throttle to obtain the computed takeoff (dry) thrust for the prevailing conditions of ambient temperature and barometric pressure. The rating is time limited to 5 min and is to be used only for takeoff and, as required, for reverse thrust operations during landing.
Maximum Continuous This rating is the maximum thrust that may be used continuously, and is intended only for emergency use at the discretion of the pilot.
Maximum Climb Maximum climb thrust is the maximum thrust approved for normal climb. On some engines, maximum continuous and maximum
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Full throttle may not be used below flat – crated temperature Part throttle, rated thrust Figure 6.28 Thrust curve for a flat-rated engine. |
climb thrusts are the same. For commercial engines, the term formerly used, normal rated thrust, has been replaced by the more appropriate term, maximum climb thrust.
Maximum Cruise This is the maximum thrust approved for cruising.
Flat Rating
Engines that must be operated at “part throttle” at standard ambient conditions to avoid exceeding a rated thrust are referred to as “flat-rated” engines. This refers to the shape of the thrust versus the ambient temperature curve. For example, the General Electric Company’s CF6-6 high bypass turbofan engine is flat rated up to an ambient temperature of 31 °С at sea level, or 16 °С higher than a standard day. Thus, its thrust as a function of ambient temperature varies, as shown in Figure 6.28. At full throttle, the thrust is seen to decrease with increasing temperature. Therefore, by flat rating an engine out to a temperature higher than standard, one is able to maintain rated thrust on a hot day.
