Originally Posted by
StopStart
Selecting the mysterious THR button post acceleration will then engage CLB and a change to THR REF?
Selecting the THR button will adjust thrust as appropriate for the situation in terms of engines operating. For all engines operating, after takeoff whether in VNAV or FLCH the thrust will be changed to the armed climb limit. The climb limit was armed on the ground as CLB, CLB1 or CLB2. Which one is selected? it depends on the situation. Heavy departure and a high initial altitude or close-in minimum crossing height and CLB(full climb thrust) is a likely choice. Light weight with a low altitude level off or you don't want fuel pump lights on due to a high pitch attitude, then CLB2 is better. Either way, selecting it selects the armed climb thrust. But why do that when it will happen automatically anyways from the FMC entry on the takeoff ref page. In other words, it doesn't get pushed. I have never touched it once in the aircraft. In FLCH, it is a different story as you are operating at a lower automation level and so if you were to a FLCH takeoff(perhaps for QFE ops where VNAV is not allowed), pressing THR selects climb thrust. After a go-around, pushing THR selects CLB thrust.
If you have one engine inop on takeoff, The FMC is smart and knows this and knows that you don't want climb thrust. Once again, while in VNAV, there is an automatic thrust change, but this time to max continuous thrust at the appropriate speed when the flaps are up which is Vref+98 . But if in FLCH, you manually select MCT with the THR button at around Vref+100. This I have done in the sim due to a screw up in setting up the FMC during repeat exercises where VNAV was leading to a descent so FLCH is selected. I think it was due to not setting a cruise altitude in the FMC. After a go-around, pushing THR selects MCT.
In other pitch modes such as ALT or V/S, pushing it can give you THR REF for the appropriate reference thrust limit.
In other words, the button never really gets pushed.
At least I think this is how it works.