3:1 rule

[knma'91]

Has anybody heard of this? 3:1 rule used for a planned descent? :ugh:

[Murray_NN]

This is a rule of thumb used for descent planning.

You lose 1000ft for every 3nm travelled over the ground.

eg: you are at 30000ft you want to descend to be level at 10000ft

you should start your descent at 60 miles before your target altitude of 10000ft. (20*3=60nm)

Hope this explains it.

[redsnail]

Use it all the time to ensure that the profile that ATC is forcing on us will work.

The FMS and VNAV is a wonderful tool until ATC give you a heading. Then you either do a lot of programming or fly the machine using the 3x profile and common sense.

:D

[wileydog3]

Works very well even with FMS in that you can back up the magic and you exercise some brain cells.

A variation is twice the altitude ABOVE 20,000 and then three times 20 and below. As another noted, there are certain gates you want to be aware of for energy.

60 miles at 20,000ft
30 miles at 10,000ft and 250kts or about 40miles, 10,000ft and 350kts.
Next gate is 10 miles and 3000ft near initial flap configuration speed
finally, 3 miles, 1000ft and nearing landing config

The 3:1 theme would begin at 105miles for 35,000ft or the second technique at 90 miles (60 + twice above 20k or another 30 miles for total of 90)

Then too, you can use your Mach to figure the actual rate (VSI) you will need in the descent OR you can just hit your numbers for a vertical ladder. Works nicely..

[knma'91]

Thank you for clearing that up for me

[smith]

Using this rule, in light twins anyway, usually do 3xaltitude to lose plus 5nm. Rate of descent is 5x groundspeed.

eg Descending from 10,000ft to 7,000ft at g/s of 140kts

10-7= 3, x 3 =9nm,+5nm =14nm therfore descent would start at 14nm out.

R.o.D =g/s x 5= 140x5 = 700fpm

[Delta Wun-Wun]

I was taught to use the same. Divide the amount of height to lose by 3= The distance required to lose the height, then 5 times the ground speed for the rate of descent.
Works on a turbo prop.
One of our more senior trainers used the same to check descent when he flew the Airbus...worked there as well....not that he didn`t trust the computers you understand:ok:

[potatowings]

That's exactly what I use, it works well in all a/c, the important thing is that you need to understand that mathematically it almost works. If you multiply your RoD by 5, you need to add a bit to compensate for it almost being right, if you are losing large amounts of height, ie 30,000' or more.

If you add an extra 10% to RoD, you will iron out the inaccuracy ie.

Groundspeed = 450 kts.
5 times 450 = 2250 feet per minute.
+ 10 % = around 2500 feet per minute.

This compensates for wind direction too.

Excellent technique. Easier maths than the time to descend technique, but worth knowing both for those days that you end up in a different place or very high on the profile.

[PleaseSayAgain]

Just my 2p worth, but as some training captain recently pointed out to me, using 3x your distance for working out descent profiles works out even better, because it might give you that extra leeway to decelerate, etc. Meaning that if you have 20 trackmiles to go to your center fix, you would want to be at around 6000ft, whereas using the other method you would still be alright at 18 miles. I've tried it, it works beutifully and just gives you those couple of contingency miles, good for a CDA into Gatwick where it might pay off to be on the cautious side... ;)


Happy Flying!
PSA :)

[Turbine King]

That's brilliant Stuart taught me this at one of my visits to CATS during Air Law of all subjects when we were discussing arrivals

Now I know why