At the danger of making heads spin yet another way of looking at it:
As has been pointed out PFD FMA “101” is that the right hand FMA simply tells you what the elevators are trying to control and how.
VNAV PTH means the elevators are trying to fly the VNAV programmed path (a really important need to monitor/need to know when you get to using VNAV for approaches).
VNAV SPD means the elevators are controlling speed. TBH most commonly seen when the desired vertical path has (perhaps only temporarily) gone out of the window... sometimes seen if you have to reprogram the FMC during a descent, e.g. for a direct, or change the VNAV descent speed since it takes time for VNAV to recalculate the path, also often seen if you’ve gone high on the profile - appropriate use of speedbrake (horror of horrors to some) should get you back on path (at which point you should get automatic transition back to VNAV PTH again). It’s not unusual to see the Pitch FMA shuttle between PTH and SPD in a descent....which leads into the final point...
Above all and regardless of the fancy pitch modes mentioned above the big health warning in all this is to make sure somebody is monitoring the FMAs and really importantly the speed ..
Last edited by wiggy; 19th Mar 2018 at 07:56.