ppos holding
hello boeingdream,
if you fly lnav/vnav & for some reason (atc, weather, operational, whatever... .), you want to hold at present position, on the cdu, you select "hold" mode, then line select the "ppos" prompt(> or caret as it is called), execute & the aircraft will start a righthand turn(standard holding pattern), reduce to the holding best speed as calculated by the fmc( circa 210-220kts = min drag speed) and do this until the fuel is gone. it will give you the hold time available as an advisory, by calculating the fuel backwards : reserve fuel from map + fuel required to fly from ppos hold to map via the programmed route.
you have indeed speed protection modes when flying with all the automatics, AFDS(Autopilot Flight Director System) & A/T(autothrottle) engaged. the aircraft will not fly too fast nor too slow & will respect flap & gear placard speeds. in some flight regimes, speed & A/T modes will not be sufficient & a "performance reversion mode" will automatically activate. e.g. if a high V/S(vertical speed mode) cannot be sustained in climb/descent, without a speed reduction below/acceleration above a minimum/maximum speed, the the system will revert to level change, the nose will pitch down/up as to keep the speed just above the limits. it will also work in vnav path descent(reversion to level change descent), but it(the reversion mode) will not work in our example when flying level in vnav. normally the A/T will keep the FMC calculated speed, but, if for some reason(failure or pilot intervention), the A/T reduces thrust & disconnects, you will have a speed reduction & stall.
last point, the automatics are a terrific tool & help, if well understood, but the thinking & management must be done by the crew. fmc & autopilots are just sofisticated computors obeying to sod's law: "garbage in/garbage out"
hope this helps.
bm