I thought that for long-range cruise, as the aircraft gets lighter due to fuel burn off, you step-climb gradually, and slow down. I saw graphical proof in a text book once.
Actually, for "modern" aircraft, if you conduct the absolute optimum cruise climb at MRC (or lower CI), the Mach Number remains remarkedly similar, with only minor variation. MRC cruise at a constant level DOES (after an initial small increase) demonstrate decreasing Mach No. (I didn't see it in a book, I see it every time that I do a cruise climb, which is often, yeah, it's me asking for the block levels).
I understand Airways now use CI of 0 in the climb and 40 in the cruise on the 767, but don't quite understand how this impacts on efficiency. Could someone explain please? ta.
Damned good idea. The climb is the most fuel-critical stage of any flight, and a poorly managed speed schedule during climb will induce fuel penalties which cannot be recovered even on a long 10 hour or so sector. "Pushing it" a little faster on cruise has far less effect on overall fuel burn than an "off-optimum" climb schedule.
Beware "the coffin" with modern jobs, though...
Perhaps so for older types, but typical modern aircraft offer best fuel economy at a level about where 1.5G to 1.6G can be tolerated, and 1.6G is the definition of moderate turbulence. Of course, if a positive wind gradient indicates a level higher than the still air optimum, buffet boundaries could be a problem.
Just my 3 cents worth..........
Regards,
Old Smokey