Simviation Forum - When to start descent

Dec 19^th, 2008 at 12:26am

juanca Offline
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Hi! can You please tell me how to calculate wahen to start my descent from cruise altitude and how to calculate my vertical rate!

Thanks Cool

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Reply #1 - Dec 19^th, 2008 at 12:48am

RitterKreuz Offline
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one easy one i have always heard. not exact science but it is a rule of thumb quickie method

take your altitude, subtract field elevation and multiply the first number by 3.

example cruising at 5,000 feet, and the elevation of airport is 1,000 feet

5,000 minus 1,000 = 4,000 (first digit of 4,000 is 4)

so... 4 x 3 = 12

begin descent 12 miles out

to calculate rate of descent you should have...

take your ground speed and divide by 2 and add a zero to the end of the result

for example, closing on the airport at 120 knots ground speed

120 divided by 2 = 60

add a zero to the end of 60 to get 600

descent at 600 feet per minute

thus... at 12 miles distance from the airport begin a 600 foot per minute descent

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Reply #2 - Dec 19^th, 2008 at 10:38am

Brett_Henderson Offline
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RitterKreuz, suggests an excellent rule-of thumb for light GA.. and since you're asking about something basic, it's safe to assume you're flying something basic. You'd have long since understood descents, and rates of descent before finding yourself at FL340 in a jet Wink

It is just a guide-line though, intended to help you visualize the whole process. Kinda like calculating exact headings and fuel burns when you're learning. They'll rarely apply to the real flight, but they do help get you to understanding it all. A 30 knot head or tailwind will throw a big wrench into a planned descent at 120kias. It's a full 25% Shocked

And GA descents are usually a one-step deal (as opposed to a descent from 38,000msl in a 747..lol wrought with airspaces, speed restrictions and huge variations in winds aloft). However, it starts out on a similar rule-of-thumb... i.e. 10 miles for every 3,000 agl (start your descent from 30,000agl at 100nm out (give or take)).. and adjust it as needed..

Every descent is unique... the only real way to get proficient at executing them, is to practice, practice, practice.. Cool

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Reply #3 - Dec 19^th, 2008 at 12:27pm

Anxyous Offline
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An exact way, which isn't exactly easy to do by head, would be:

Altitude in feet to descend*3/1000=X

Altitude in feet to descend/(60/(GS/X))

&&&&

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Reply #4 - Dec 23^rd, 2008 at 11:39pm

-Crossfire- Offline
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If you have a Garmin 530, use the VNAV function. It calculates everything for you. Grin

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Reply #5 - Dec 24^th, 2008 at 4:11am

beaky Offline
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RitterKreuz wrote on Dec 19^th, 2008 at 12:48am:

one easy one i have always heard. not exact science but it is a rule of thumb quickie method

take your altitude, subtract field elevation and multiply the first number by 3.

example cruising at 5,000 feet, and the elevation of airport is 1,000 feet

5,000 minus 1,000 = 4,000 (first digit of 4,000 is 4)

so... 4 x 3 = 12

begin descent 12 miles out

to calculate rate of descent you should have...

take your ground speed and divide by 2 and add a zero to the end of the result

for example, closing on the airport at 120 knots ground speed

120 divided by 2 = 60

add a zero to the end of 60 to get 600

descent at 600 feet per minute

thus... at 12 miles distance from the airport begin a 600 foot per minute descent

That's a good one... but I've always done it the hard way, for some reason:
Assuming 500 fpm for the descent (no need to rush it unless you're milking a tailwind or have high obstructions on the way)...

Cruise alt minus pattern alt (more important to bug-smasher pilots than jet jockeys) , divided by 500 (fpm descent); this gives you minutes required for descent; figure this time against your target groundspeed during the descent. The result is the miles you'll use getting from cruise to TPA.

As I said, it's not a quick deal for those who aren't math savants, but I manage fine with that if I figure it all out before I fly. Grin

In a pinch, I can do it on the fly well enough, especially with my trusty "whiz wheel". Grin

I'm pretty ignorant about instrument flight planning, particularly with turbines, but it seems to me that the end-of-descent target for those ops should be the altitude of the FAF, not the runway(?). Gives you a little wiggle room, maybe?

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Reply #6 - Dec 24^th, 2008 at 6:59am

RitterKreuz Offline
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beaky wrote on Dec 24^th, 2008 at 4:11am:

I'm pretty ignorant about instrument flight planning, particularly with turbines, but it seems to me that the end-of-descent target for those ops should be the altitude of the FAF, not the runway(?). Gives you a little wiggle room, maybe?

Flying turbines was pretty much the easiest work i have ever done.

ATC was always pretty good about getting us down to a perfect altitude even for a visual approach.

even when given a crossing restriction i just estimated the required descent rate and adjusted it up or down the closer i got to the fix.

on particularly lazy days i would just command the autopilot down 1,000 feet per minute and if i needed more later or less later it was a matter of adjusting the vertical speed wheel a few clicks up or down.

but thats the good thing about a pressurized airliner... if you suddenly end up needing a 2500 feet per minute descent... cabin rate of descent might be only 750 feet per minute so nobodys ears really hurt.

In an unpressurized airplane... 2500 FPM down is 2500 FPM down any way you slice it Cheesy

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Reply #7 - Dec 24^th, 2008 at 9:06am

Brett_Henderson Offline
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I rarely cruise at over 6000agl, so it's never an issue Cheesy

I just keep the 3000 feet for every ten miles in the back of my mind (i.e. try to be at TPA+3000 ten miles out).. and it works out well Cool

So much of it depends on the airplane too... If you're flying a 250hp Mooney... you not only want to be at TPA +6000 at 20 miles out.. you want the manifold pressure down to 20 inches at 20 miles out.. while not reducing it by more than an inch per minute.. and NEVER letting it fall below 15"... There's more to a descent than its rate Wink

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