When is the right time for initial descent?

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When is the right time for initial descent?

Postby as136roy » 21 Dec 2009 02:30

I'd like to know when is the right time to start descending from cruise level to initial approach altitude (12,000ft)? How far (in nm) from the arrival airport?

Scenario: I'm flying a A330 @ FL330 and I'm planning an initial descent to 12,000 to prepare for my approach. My usual descent rate @ 1800fpm.

Thanks in advance for the help... :wink:

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Re: When is the right time for initial descent?

Postby clipper759 » 21 Dec 2009 07:04

There are alot of ways to do that Roy. I've always prefered the simple ones as opposed to the higher math.

For the top of descent just estimate 3NM per 1000 feet of altitude to lose...from your example 33000-12000=21000. 21 x 3 = 63NM on a 3* descent. The easy way to determine a 3* descent is Groundspeed x 5 = Vertical speed..so at say 450Kts ( 450 x 5 = 2250 or ~2300FPM).

Obviously wind and speeds will change as you descend so this is not an exact science..but it is a reasonable estimate.

Hope that helps.

Randy
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Re: When is the right time for initial descent?

Postby John Iverson » 17 Mar 2010 09:12

A very good real world pilot with 23,000 hrs plus, now retired told me he used 150 miles from destination as a rule of thumb. I started doing it that way... like it a lot.

As far as I'm concerned, simple is good.
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Re: When is the right time for initial descent?

Postby vic365 » 18 Mar 2010 01:52

When FS9 ATC tells me too? :lol: :P :wink:

Just kidding but, that is how I fly, with FS9 default ATC (yes, I DO hate it but...I'm not sold on RC yet).

There are many ways to determine your TOD and, if your only flying modern airliners, the ones which result in a 3* descent angle, like the one clipper759 mentioned, will work fine. If you fly any classic propliners or other aircraft you may have to adjust considerably.

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Re: When is the right time for initial descent?

Postby scmahan » 15 May 2010 13:02

When ATC tells you really is the right answer. One can always request a lower altitude from ATC and if traffic allows they will normally clear you down to a lower enroute altitude. Often however because of traffic volume ATC, either Center or Approach control will keep you high until the last possible moment, in order to sequence traffic into the approach sytem. When traffic is heavy at your destination expect to arrive high and fast.

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Re: When is the right time for initial descent?

Postby Phillip Speer » 15 May 2010 18:40

Actually the pilot is responsible for his flight profile including the time to request descent. ATC may give you clearance to descend when ready to a certain level or to a particular level by a particular point, but they won't necessarily know when you need to descend. Remember though not to descend without ATC clearance to do so.
Randy's tip above at 3x1000ft (plus a bit to slow down as well) is a good one and qorks pretty much every time.

Phillip

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Re: When is the right time for initial descent?

Postby vic365 » 17 May 2010 00:14

Interesting Phillip...I never knew that. I always thought ATC had complete control over speeds, altitudes, etc.? Of course...it makes no difference when using FS9 default ATC :P . I'm not sure if it makes any difference when using ANY addon ATC, like RC?

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Re: When is the right time for initial descent?

Postby Phillip Speer » 17 May 2010 20:17

IT's one of the many deficiencies with default ATC. Not sure about RC (not used for a long time) as most my experience is with online ATC. Basically pilots and ATC have a "contract" to fulfil their aims.
Pilots tend to want to get to their destination the quickest/cheapest way possible.
ATC provide a service to allow them all want to do the same thing at the same time without bumping into each other.
Pilots provide the basis of their routing in their flight plan (and can include crossing restrictions etc), but things like TOD will depend on aircraft type, weight and other performance factors specific to the flight that ATC will be unaware of. In quiet periods then block levels can be given eg descend when ready but in higher traffic situations then ATC may need to provide tighter seperation.

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Re: When is the right time for initial descent?

Postby zfly2sky » 02 Sep 2010 05:46

FOR FS9 ONLY!

Hi, I know there are a bunch of freeware calulators for descent figuring
but for FS9 fliers that have FS Navigator, you can do a real kool thing
on the FSNav map. At about half way through your flight or 200 nm out
or so from your destination airport, set your Auto Pilot ALT at the level
for catching the ILS (i.e., 2800 ft). Then set your Vertical Speed (VS)
at -100 fpm. Zoom in on your dest airport and increase your VS until
you see an "arrow" appear on your flight path. Now decrease your
VS until the arrow is just before the outer marker of the dest airport.
That arrow means that your aircraft will attain that altitude (2800ft)
where the arrow is pointing on your flight path. Play around with VS
in order to see the arrow because it disappears if it is way out there.
But when doing this, watch for mountains in your path (FSNav
shows peak hights on map) and check out your dest airport altitude
and add (i.e. 2800) to that total and you are good to go.

Image
Last edited by zfly2sky on 14 Jan 2011 05:19, edited 1 time in total.

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Re: When is the right time for initial descent?

Postby AS455 - Don » 27 Dec 2010 18:15

Current Altitude X 3 + 10 = Miles out for Decent (drop 3 decimal points)
12,000 X 3 + 10 = 73nm TOD

It could happen!

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Re: When is the right time for initial descent?

Postby zfly2sky » 14 Jan 2011 05:23

Thanks Don,

Now that's a simple formula to figure decent rate...
Must be an actual "Real World" formula I bet.
Thanks alot for that one Don!!

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