Originally Posted by
Dave Therhino
TD: The 737NG is different from the Everett airplanes. It has a thrust lever position (can't remember if it's detented, but I think it is) at which the purchased thrust rating is commanded. Several degrees above that thrust lever position is another position just short of the lever stop at which the maximum thrust approved for that airplane minor model (-600, -700, -800, etc.) based on Vmc testing will be commanded. This extra thrust, when available, is referred to as performance-reserve thrust. If the operator has purchased the maximum thrust approved for the airplane model, that region of lever travel will result in no additional thrust (unless you are in alternate mode, in which case you can overthrust the engine). Boeing designed it that way to provide operators with additional thrust for use in an emergency if the crew decides it is needed on airplanes on which the higher thrust rating is approved, but was not purchased. All of this is covered (except my inability to recollect if a detent exists at the purchased thrust position) in the engine control system description document prepared by your former counterparts in Renton, and it's covered in the FCOM.