Boeing 737-700 desent rates
 

Hi all,

Another question for you. Who decides the decent rates of the aircraft?

It seems to vary quiet a lot. On a flight to Gothenburg a couple of months it seemed that we were dropping like a stone where as in other instances, the decent was quite gentle.

Any help would be apprecaited as usual! :o)

Uuuhhhmmm....
 

...could it be the flightcrew? :D 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 :E

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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Great stuff. When you mean the engines are idle are they powered right down so that they are no longer producing any thrust? Also what is the average decent rate? I know it could vary but does -1800 feet per min sound about right?

I know what you mean about the -700 dropping like a stone, it's like being on the "vomit comet"! :D

paulkinm,

At idle, the engines still produce a small amount of thrust, in the region of 2500 lbs a side, but yes, the general idea is to descend in glider-mode (starting off around 2300 fpm, if memory serves), initially at a speed around M.78-.79, then transitioning to 275-290 KIAS, slowing to 240 below FL100 and then finally reducing the speed while configuring, all the time with the engines in idle. Only when we're at F30 or 40 do we add thrust again to maintain final approach speed.

However, this being far from an ideal world, compromises must be struck so ATC can fit all the aircraft in. Therefore, we only fly to a handful of places where there genrally will be no or very limited ATC interference with the descent-planning. REU is one of them, LPA is another (at least at the time we get there!) Oh, one mustn't forget the top-notch guys in CPH who once (via CTRL) cleared us for a visual approach 22L while still 120 NM out :ok:

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