3 to 1

[maremare505]

Greetings fellow aviators: ( I may have send this message before)
But can somebody provide a reference (doc) or explain the 3/1 descent calculation.
Tks,

 

[Andy Neill]

I presume you're referring to the notion that if I have 10,000 feet to lose, I would begin my descent 30 miles out. In more general terms, for an altitude loss of x thousand feet, begin the descent 3x miles out.

 

[Huggyu2]

I believe that many airliners descend at about 3 - 3.5 degrees nose low in idle. So for fuel efficiency, airlines shoot for this.
Can any of you airline pilots verify/debunk this? I don't know this for sure.

 

[con-pilot]

The 3/1 ratio has worked for me in every jet aircraft I have flown with the exception of the 727, 3.5/1 worked much better for that aircraft.

The 3/1 ratio works for descents from FL 51.0 to any crossing restriction down to figuring a descent rate for a runway that does not have any type of visual guidance system.

The biggest/most common mistake that new jet pilots make is not adding in enough mileage to slow down to 250 knots at 10,000 feet (or whatever altitude for speed reduction). A good rule of thumb for most jet aircraft is ten miles, less for slower VNE (325 kt. range) and higher for higher VNE (390 kt. range).

Now with high head/tail winds more adjustments must be made. However, with all the new FMS systems with vertical guidance software it takes all the mental guesswork redundant.

But, being the old fart I am, I still run the 3/1 rule in my mind on all descents no matter how sophisticated the FMS.

 

[scoot]

I use the three to one ratio to check my altitude when approaching an airport, especially an unfamiliar one at night. It gives a normal 3 degree glideslope to the runway. DME X 3 = how many feet above the airport elevation you should be if on a 3 degree glideslope. i.e. three miles out times three equals 900' AGL. ... 5 miles out you should be 1,500' AGL, etc. Works great!

 

[ptarmigan]

It is based on 300'/mile, which is pretty close to a 3 degree GS if you are using NM's. Well, not too far off if you're using SMs either, as the ratio is really 300'/approx 5700'.

Doesn't quite work the other way (multiplying the altitude by 3). 10 miles = 3000' by the above, but if you multiply 3000 by 3 you get 9, and the error increases, but there is usually enough variability in the wind or your choice of speed vs. weight, etc, that it is close enough.

 

[SiuDude]

I believe that many airliners descend at about 3 - 3.5 degrees nose low in idle. So for fuel efficiency, airlines shoot for this.
Can any of you airline pilots verify/debunk this? I don't know this for sure.

My airline now asks that we use a 4 degree glidepath on an initial descent from cruise.

 

[Guitar rocker]

I agree with the 3:1 rule. With headwinds and tailwinds......2 miles per 10 kts of wind always worked well. If yah got a 50 KT tailwind....2X5=10....start your descent 10 miles sooner than normal....do exact opposite for headwind.

 

[HawkerF/O]

That 3-1 rule is one of the best in aviation. Personally I don't use the VNAV feature unless I'm told to cross a fix and I have other fixes before I get to that one, as the FMSs I use wont easily give you the distance to a fix unless it's the next one in the Flight Plan. Doing this stuff in your head keeps you sharp and you wont have to rely on automation for something simple like this.

Everyone does it differently, so I'll share with you what I do, and you can take it for what it is worth.

As has been previously said, take the altitude that you have to loose and multiply that by 3. That is the distance which you'll need to start your decent for exactly a 3 degree decent. As simple as that is, it's not really that simple. There's more.

Speed needs to be taken into account, but only on some decents. If you are descending to an altitude at or above 8000ft MSL, you have to consider your speed. Below 8000ft MSL, it's no so important. Here's why:

It's as simple as 250 below 10. If you are descending to an altitude below 8000, you'll have to slow, and below 8, you will have slowed early enough to make the restriction.

At 250 Knots, you need 1250 FPM to maintain 3 degrees. That # is obtained by multiplying your ground speed by 5. That is your rate of decent.

I take the distance I have to go in the decent and add 20 miles to it. That gives me 10 miles to get set up and stable and another to 10 miles to be at 2000-3000ft above the field 10 miles out. That tends to always work out well if ATC keeps you coming down.

As for the Speed, if you need to descend 30,000ft and you are doing 450knots Ground Speed (GS), then 2250 FPM down at 90 miles from the fix is what is required to make EXACTLY 3 degrees. That means at 90 miles out you go right into a 2500FPM decent and 30,000 feet later you are still at 2500 FPM, but once you get there, you pull up to stop the decent. We all know how rough and impractable that would be, so that is why you add the additional distance, in my case, the 20 miles. So if you have a strong tailwind, coming down at 500 Knots and 2000 FPM will leave you 500 feet short for every minute you descend. After 4 miuntes you are already 2000 feet short. The angle is too shallow. Does that make sense?

Now, as you come below 10K, you'll have to slow. When I go below 10K, I sneak the power back to idle at 13K and arrest the decent if need be to 2000 FPM. That will slow me to 250 at 10K like clockwork. Since I am now doing approx 250 KNots (depending on the winds), I only need 1250 FPM, but I keep the 2000 FPM to 5000ft MSL, then it's 1000 FPM to the FAF. With that decent schedule, I've slowed enough (laterally and vertically) to erase any deficit that I acquired in the decent.

Do the math in your head and just play with it a little bit and it'll all make sense. You can also use the VNAV as a back up to check your mental math.

 

[LAFrequentflyer]

excellent info - thanks...I'll have to practice it next time i fly...

-LAFF