3:1 rule

[lookin4better]

where can i find a good explaination of the 3:1 rule?

thanks

 

[svcta]

You talking about descent profiles? 3 miles per thousand feet of altitude? If so, it works well. Keeps you higher longer and is still a managable vertical path for jets without the introduction of drag for speed problems. Shoot for 10000' @ 30 miles from the place where you're going. That puts you @ 3:1 Run the math and you can get a quick idea about where you should be(alt) vs. how far away you are. All of the jets I've flown will readily out descend a 3:1 path, but there's usually no need to stay up like that. It's an easier descent on the pax(body angle) and ATC generally uses the same target.

 

[lookin4better]

yup......learned it YEARS ago but have been WAY out of practice and cant find it...is there a rate of descent or ground speed thing involved too? ....Thanks

 

[Rxflyer]

To me, staying "on profile" using 3:1 means starting down from FL310 at 93 miles, being at FL200 at 60 miles, etc. Works for us in a Citation 500.

 

[Princedietrich]

Yeah but it's more fun to be at 410 60 miles out from destination in a learjet, pull it to idle, pop the boards, fuel computers off, and HANG ON!

 

[lurker]

yup......learned it YEARS ago but have been WAY out of practice and cant find it...is there a rate of descent or ground speed thing involved too? ....Thanks

I use a descent rate of 5X ground speed, which seems to work well.

i.e. ground speed 400 kts, v/s = 2000 fpm

 

[Baronman]

3:1 Works well...

Step 1
To calculate how far in advance you have to descend you multiply the altitude you need to lose by 3. Example...Cross XYZ VOR at 10,000 while you're at FL180. 8 (thousands of feet to lose) x 3 = 24 miles.

Step 2
The vertical speed you will use depends on your groundspeed. The higher the groundspeed the higher your vertical speed to maintain 3 degrees and vice versa.

Groundspeed x 5 works well.
500 kts x 5 = 2500 fpm
400kts x 5 = 2000 fpm

OR
Groundspeed divided by 2
Example on an approach at 180kts
180 divided by 2 = 900 fpm
140 divided by 2 = 700 fpm

 

[avbug]

The 3:1 rule in aviation, simply stated, means only that out of every three marriages you try, one won't drain you completely dry. The 6:1 rule is a better rule, because out of six marriages tried, one might work.

Why would you turn the fuel computers off in the learjet?

 

[Princedietrich]

Why would you turn the fuel computers off in the learjet?

Spool the engines down even lower to get a greater rate of descent. A lear will plummet pretty darn fast if the fans are spinning at 30% as opposed to 75% at idle.

And no, I'm not the one who turned them off. The captain I was flying with did, and believe me it scared the bajeezus out of me until I saw that it actually worked and we didn't die.

 

[astroglider]

The definitive 3:1 answer.

There's a great explanation of the 3:1 rule and other Handy Rules of Thumb for aviation found in pages 227-229 of "The Turbine Pilot's Flight Manual".

All concepts are clearly written and easy to understand.

cheers