why flying lower than RFL...!?
- RFL is based on assumed TO-weight by dispatchers. more extra-fuel or pax or cargo -> higher gross weight -> lower level
or: running late, flying fast (M.8) -> unable for higher level which would have been good for M.78/.79
or: crew wants more buffet margin, maybe due to expected turbulence. 738 fmc gives you info about optimum and max LVL (in hundreds of feet) for act weight and desired speed. e.g. opt: 368 max: 381
now u could go up to FL380. but the red bars on the speed tape will get VERY close to your speed bug from both sides. one bad bump and you can't hold altitude anymore. plus your climb performance is pretty miserable.
or: radiation: crews don't feel the need to fly in FL400