How to know when to descend

[jewell45 4]

Hey guys, I recently bought the Coolsky DC-9 and it is great. But I am so use to computerized flight systems and because of this I have no idea when to start my descent in this thing (Kind of embarrassing). So as simple as it might be, I need a bit of help.

[N60JBravo 27]

((cruise altitude - descend to altitude)/1000)*3=TOD in nm

 

I am cruising at FL490

 

Destination IF altitude is 4000'

 

49,000 - 4,000 = 45,000

 

45,000/1000 = 45

 

45*3 = 132

 

I want to begin my descent no closer than 132 nm from the IF.

 

Typically I will add 10 nm.

 

regards,

Joe

[Big_Duke 11]

Try this rule of thumb. If you'd like something more, try searching the AVSIM File Library or Google "descent calculator" and you'll find plenty, many of which are freeware.

[woodreau 0]

In general, you can use the 3-to-1 rule to determine when to descend. OR 3 miles to descend every 1000ft for a 3 degree descent.

 

You usually want to be at 10,000ft 30 miles from the airport.

 

So if you're at 30,000ft you start descending at 90nm from the airport.

 

An alternate method I use when I'm flying an aircraft and I descend in V/S Mode, is I plan to descend at 3,000fpm. I look at my GS. 600kts = 10 miles a minute. 420kts = 7 miles a minute.

 

So I start with 10000ft, whatever altitude I'm at say 37,000ft = 37-10 = 27,000ft to lose / 3,000ft/min = 9 mins.

 

If my ground speed is 420kts, then I start my descent at 9 x 7 = 63 miles from the point where I want to be at 10000ft 30 miles from the airport or 63 + 30 = 93 miles.

 

 

So that's the mental math I use to double check the FMS.

[DaveCT2003 1,820]

You could go old school for the old school aircraft...  3 nautical miles (nm) per 1000ft.  Just work it backwards to figure your Top of Descent.

 

Should do the trick for you.

[jerry allen 3]

3 times altitude.  If at FL350 = start down at 105nm from a sea level airport for a straight-in landing.

5 times ground speed.  If GS is 400 kts, then descent rate should be 2000fpm.

These values will change on the way down, so recalculate every 3-5 thousand feet during descent.

 

jetjerry

[jewell45 4]

Thanks guys I also found a cool website that does it for you http://aerotexas.com/desplan/desplan.php

[hmsdreadnought 12]

I know a guy who flew the old Fokkers for Horizion - and the 3/5 rule was his lifesaver.  That plane had minimal auto flight capability and absolutely no moving map - just a couple of VOR recieivers and an ADF.  He swore by it.