Oh, in that case, we still owe you the answer.
On a jet, the most fuel efficient way to descend is of course to stay in the optimal cruising altitude as long as possible, then cut the thrust and glide with a certain glide speed to the destination deceleratate for the approach, and, ideally you only advance throttles as far as the outermarker to spool up for go-around.
Simply put, the speed (i.e. steepness of your glide/dive
) calculation is based on economic fuel to flight time ratio (the cost index). So all you have to do is make a good decison about the top of descent point observing all the speed and altitude constraints ahead. Which is, obiously the tricky part.
For this regime the autopilot interface has a mode selector named Level Change (B) or Open Descent (A).
Should you find yourself short of the field, either you slow down (shallow your descent) - not good as you accumulate extra time on the flight, or add thrust - fuel penalty, the lower the worse.
If, on the other hand, you are coming in 'long' you can dive instead of glide, but this brings extra speed / energy you need to dissipate anyhow (search hot and high) or you can use the speedbrakes / lower the gear early which is definitely bad for your masculine ego (by many considered by far the least acceptable option of all above) . Either way, you've been uneconomical because you could have started descent earlier - saving fuel by cutting throttles sooner.
To help pilot with this task (the irony of the word) manufacturers designed a feature in their flight management/guidance computers. On "A" called Managed Descent on "B" V-Nav. One should stuff the machine with all sort of available data, e.g. constraints, altitude winds and temperature, transition altitude and QNH (1033 gains you about 550 ft which at late stages could make or break your day). Also the expected use of anti ice (some engines would spool up injecting energy to the airframe). What you get, is a glide trajectory the aircraft should follow on the precomputed profile.
As opposed to the LVL CH / OP DES method, main guidance here is the descent angle instead of speed and that is quite off the piloting basic rules, takes some time getting used to but works fairly OK at the end of the day. This should take you to FAP / FAF.
Yet, the what ifs remain. In the classical" way you fear of the altitude, in VNAV you fear of the speed. Abrupt disturbances in Open Descent fill pilots brain lobes, in Managed, passeneger barf bags (overstated on both examples, of course).
Cheers,
FD
(the un-real)