Originally Posted by allison
Recently, my company told me that the engines have been derated to 22k. And that if required, i can simply disconnect the auto throttle and manually set, via the relevant tables, the 23.5k N1 setting. The options are not available to me on the FMS, only 22k is available.
Change of company might be the smartest idea here...
Back on topic: if the CFM56-3C1 engine is fixed derated to 22K by FMC pin programming, that's the maximum thrust you can use for takeoff. The engine will and should be able to achieve 23.5K (and more in case of firewalling thrust levers) thrust rating even if derated to 22K, but you invalidate quite couple of things.
First, the -3C1 derate to 22K on -400 is usually done to comply with Chapter 3 noise certification. Second, the engines might be derated to prolong their on-wing life. In this case, the EGT margin should be enough for 22K operation, but you could find yourself busting EGT limits on takeoff with 23.5K rating, just because engine condition is insufficient for 23.5K rating operations. Third, if the company has complied with the SB for engine thrust reduction, than you should get new manuals from Boeing with performance data only for 22K (and perhaps 20K, but not 23.5K).
If you really need the 23.5K derate, just complete an action in the same SB that is used for derating the engine and you get your maximum thrust rating back - but I suppose there are some reasons why you have 22K rating and company is "suggesting" use of 23.5K with (by Boeing) unapproved methods.