Calculating decent rate / distance

CVA7769 · Posted: Wed Feb 23, 2005 2:40 pm

Hi All,

I'm wondering if there is an easy way to deal with decents when flying to a destination airport?

Quite often I'll find myself flying along happily at FL350 or so and think to myself Hmmm....I should probably start decending soon. However, at what speed, have many ft per minute, etc is always the problem for me.

Is there an easy rule of thumb to follow here? Something that can be calculated easily on the fly (no pun intended)

Thanks!

Darryl
CVA7769

kgreenwood · Posted: Wed Feb 23, 2005 3:07 pm

well Daryl - my basic 'rule of thumb' is 3 times the altitude +10

f'rinstance - as you're happily flying at FL350 - 35 X 3 = 105 + 10 = 115 -- at 115nms from destination start a 1500-2000 fpm descent

As to speed that, of course, depends on your aircraft type. If your in a jetliner (B737/A320/CRJ etc) normally you fly your mach number to a transition speed. For a 737, let's say you're cruising at m.78. Maintain m.78 until your indicated airspeed (IAS) reads 310 knots. Maintain 310 knots on the descent until you reach 10,000 feet. Slow to 250 knots below 10,000 feet. As you approach destination slow to 210 knots at approximately 20-30 miles - then 180 for localizer intercept then final approach speed once established.

For a turboprop (DH8/B190) - the 3X+10 rule still works but your speed will be different. At altitude you, normally, will be cruising in the 200-210 kias range. You can allow that to slide up on descent to 235-245 and maintain that all the way in to 20 miles, then again slow to 210/180/final speed.

Note that turboprops are much easier to slow down that jets - basically pulling the throttles back on a DH8 or 1900 will give you an instant speed reduction but a jet is far slipperier - it cannot descend quickly and slow down at the same time (without use of the speedbrakes) - proper planning is a must

hope this helps a bit -- cheers

CVA8077 · Posted: Wed Feb 23, 2005 3:22 pm

What all them guys said!!! :p

Bill Haystead · Posted: Wed Feb 23, 2005 4:18 pm

Yep, Ken Greenwood does.

Cheers Bill

Dave Shaw · Posted: Wed Feb 23, 2005 4:18 pm

There are many ways to calculate descents. Ken's is an excellent method that is perhaps one of the easiest to remember.

There are some more methods at Michael Oxner's "Topic of the Week" column. Check this out:

http://bathursted.ccnb.nb.ca/vatcan....ic.html

Quote:

Now the kicker with this is, or at least it seems to me is, you always FREAKIN speed up when descending

LOL Rick....mean old Mr. Gravity is getting to you again I see.

Cheers!

cva227 · Posted: Wed Feb 23, 2005 5:04 pm

It's also at item 5.1 on our Info page

Michael

CVA7769 · Posted: Wed Feb 23, 2005 6:47 pm

Thanks for all the replies gents!

I'm gonna give it a shot hopefully tonight. Will try to be online for the Multi

Thanks!

Darryl
CVA7769

DS · Posted: Wed Feb 23, 2005 6:48 pm

Righto Michael - how are you? Long time no post.

Fromthe INFO Page...

Calculating your Descent

Okay, so you're at 33,000 feet and you've got to cross the ABCDE Intersection at 12,000… great, how the #### do I figure out when to start my descent? If you're not running FMS (or it craps out, as it often does) when do you start your descent? How far from the ABCDE ISEC should you start your descent?

Here's the formula:
(3 X FL) + 10 = approximate nautical miles distance to 0 feet (at 1800 ft/min)
Let's take 33,000 feet as your cruise altitude, and 12,000 as your desired crossing altitude…
1. (33-12=21) 33,000 (or just 33) and subtract 12,000 (or just 12)… okay that should equal 21,000 (or just 21)…
2. (21X3=63) Multiply 21 by 3… okay that should equal 63…
3. +10 Now add 10 to 63… okay and that should equal 73!
4. =73 Start your descent to 12,000 73nm from the desired ISEC, VOR, FIX, or NDB.

28,000 feet to 13,000…
1. (28-13=15)
2. (15X3=45)
3. 45 +10=55
4. 55nm Start your descent to 13,000 55nm from the ISEC, VOR, FIX, or NDB.

Rate of descent
Thanks to Michael Morrison (CVA227) for this handy tip from the FS Official Strategy Guide by Nick Dargahi

1. Distance to Airport: 90 miles
2. Speed: 350 Knots
3. Altitude: 28,500 ft
4. Airport altitude: 500 ft

The Formula:
1. miles / (knots/100*1.66) = X (rounded)
2. Change in Altitude / X = rate of descent (ft/min)
Therefore, using the above as an example:
1. 90 / (350/100*1.66) = 15.5 = 16 rounded
2. (28,500 - 500)/16 = 1750 ft/min.

cva227 · Posted: Wed Feb 23, 2005 11:39 pm

Hi Daryl:

Doing fine. Been flying mostly offline and always monitoring the forum.

I'm still cleaning the bathrooms with the toothbrush that Lloyd gave me way back when. I forget, are you supposed to change the toothbrush once the blue on the bristles disappears? On the other hand, I've been using that toothbrush for five and a half years now and the bristles are really wearing down. As a senator I'm allowed to ramble. Darn, can't find my way out of the YWG Cargo Hanger. Alyn, Steve, Help!!

BTW, do you know Larry McLean? He produces and directs DOC and Sue Thomas FBI

Michael

CVA222 Bob · Posted: Thu Feb 24, 2005 1:48 am

Excellent replies guys.... I'm amazed at the response.

Of course, the aircraft you are piloting and it's characteristics are the determining factor. Flying the 737ng's (without the FMS reference) I'll make sure to start down about 90-100nm of the field. And again, if that field is way above sea level, I'll delay that. Also, the direction of landing has a lot to do with this decision. Arriving from the east and landing on the 8's at YVR will delay your descent significantly.

Without trying to calculate, you would be prudent to evaluate your vertical at 100nm of the field. Flying right means using the above information to determine your best time to descend.

Calculating decent rate / distance

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