Stupid controllers...

[DaveSims]

What airspeed are you using during the descent, that makes a difference.  The slower you go, the less ground you are covering.

[Scorp64]

Ok, I think I'm finally starting to figure this out.    When I'm ordered to start the descent (from FL380) I'm at cruise speed (.85 mach) with speed hold on.  The speed hold reduces power to maintain the speed, but that's maintaining .85 mach.  Instead I need to disengage speed hold, pull the throttles to idle, and then start the descent.  That's what you're saying right?

Big learning curve for me, the last flying game I played was B-17 Flying Fortress (or was it Aces Over Europe?).

[DaveSims]

For starters, in real life there is a speed limit of 250 kts below 10,000 feet.  I usually have my autopilot set on Mach hold when cruising in a jet, then when I start my descent switch it over to airspeed.  Whether you realize it or not, indicated airspeed changes in relation to Mach at different altitudes.  This is due to the fact that the air is thinner up top.   To make a long story short, up high Mach .8 could be only 250 kts airspeed, or less.  By switching the autopilot, the airplane kinda automatically slows down as you descend.  By the time you reach 10,000 you are already below the speed limit.  Traffic coming into a large airport is usually required to slow to 200 kts once they enter the airport's airspace as well.

[Scorp64]

I did notice that ias changes at different altitudes at the same mach speed.  However, I did't know that there was a speed limit.  I'm trying to do all this as realistically as possible, so thank you for the info.

[BAW19]

Don't know how helpful this will be but there are two rules of thumb I use when I make a descent :- the 'rule of fives' and the 'rule of threes'.

The first one helps you judge a vertical speed that gives a nice 3 degree descent profile and works all the way down until your on the ground. It's keyed on your ground speed so tune to a VOR and check your GndSpd on the Nav Display. Mutliply this by 5 and that's your V/S.

500kts x 5 = 2500 fpm
250kts x 5 = 1250 frm
140kts x 5 = 700 fpm

Like I said, this will give you a 3 degree glide slope which is generally speaking regarded as a comfortable descent profile. It's also the slope that the next rule expects when it calculates how high you should be at any point from the airport.

Again you can tune to a VOR with DME to get your distance. Just multiply that by 3 to get your Flight Level. OK, OK, flight levels are only supposed to be used over 10,000ft. I'm using them here as an illustration of the maths. In reality just think in feet and drop the last two zeros.

100nm x 3 = FL300 (30,000ft)
40nm x 3 = FL125 (12,500ft)
10nm x 3 = FL030 (3,000ft)
3nm x 3 = FL009 (900ft)

With practice you can use both rules to juggle your speed and v/s to get you down when you want to be.
The Rule Of Three is probably most useful as you can quickly and easily work out how high you should be at any point in your descent and adjust your v/s to keep yourself on the profile.

In reality if you're flying VFR and plan ahead using both rules you'll need very little adjustment to stay on the ideal descent profile.

If you're flying IFR (say with VATSIM or IVAO) your V/S will largley be dictated by the altitudes given to you by controllers and the path they've asked you to fly. But you can still use the so the Rule of Three to check they're being reasonable in giving you shallow enough approach profiles.

Sounds complex now I write it down but it's really quite simple!

All the best

Ian