...could it be the flightcrew?
I mean, they do seem to fly the aircraft around a bit...
OK, assuming that the question was more along the lines "
What determines the descent rate of an aircraft", it can be several things:
- The crew can select to fly a desired vertical speed, or ATC can request them to fly a specific vertical speed.
- The FMC can generate a descent path, and the aircraft will fly the vertical speed necessary to maintain this path.
If ATC keep us high
, we fly level & wind the speed back - so when they let us decend further, we can do a "dirty dive", i.e. burning up some of the excess potential energy by converting it to kinetic (speed) energy. On the other hand, if ATC ask us to descend earlier than would be best for fuel economy, we go down at a reduced vertical speed, sometimes at little as 500 feet per minute. The "dirty dive", in comparison, can produce descend rates of over 4000 fpm, and if we throw in landing gear & speedbrakes as well, we can make the -700 drop like a bleeding stone
So I think the answer to your question lies in the co-operation between ATC and flightcrew. If there were no other aircraft in the sky, we'd descend at a fairly constant rate with engines idle, only slowing down to configure the aircraft with gear & flaps and then re-introducing thrust about 1500 ft. above the runway in order to maintain the final approach speed.
Hope this helps - cheers fm
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