lmml 14/32
Topic Author
Posts: 2358
Joined: Tue Jan 30, 2001 2:27 am

Rate Of Descent Formula

Fri May 17, 2002 9:19 pm

Will somebody please remind me what the rate of descent formula is? Something to do with your ground speed?
Posts: 256
Joined: Thu Jun 07, 2007 11:46 pm

RE: Rate Of Descent Formula

Fri May 17, 2002 9:32 pm

rule of thumb:
to stay on a 3° glideslope ( = 5% descend),
the rate of descend in ft/min should be equal to 5 times the groundspeed in knots.

Was this what you were looking for?
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Joined: Wed Nov 29, 2000 1:54 am

RE: Rate Of Descent Formula

Fri May 17, 2002 10:37 pm

Another formula for determining when to start a decent, for say a crossing restriction:

when to start decent = 3 x altitude to lose
rate of descent = 1/2 Groundspeed

Lets say you are told, Cross XYZ VOR at 10,000'. You are at FL350 and your groundspeed is 400 knots.

25,000' to lose, multiply 25 x 3 and you get 75. Start your descent 75 miles away from XYZ VOR.

400 Knots groundspeed divide by 2 and get 200. Put a zero at the end and get 2000, and decend at a rate of 2000 FPM.

This formula will work perfectly everytime, but I usually add 5 miles to the distance so you can do thing smoothly for the passengers.

Keep checking your progress along the way. as you get to 30,000' you should be no less than 60 miles away from XYZ VOR. If you are within 60 miles, increase your decent a little and check again at 25,000' to see if that helped.

Adjust your Rate of Decent also for Groundspeed along the way. As the winds change as you go lower, you may need to adjust for that as well.

lmml 14/32
Topic Author
Posts: 2358
Joined: Tue Jan 30, 2001 2:27 am

RE: Rate Of Descent Formula

Fri May 17, 2002 11:06 pm

Thank you both for your replies. That is exactly what I was looking for. However on my last flight I noticed something. We were cleared from FL350 down to FL150 with no restrictions. We started descent some 10 miles before the desired TOD point and were doing 436Kts with a 28Kt tailwind. According to the formulas our rate of descent should have been approx 2700FPM. We were on the profile according to the scale on the ND of our B733 but were doing 4000FPM.
My question is this: What was the computer calculating
a) our descent profile down to FL150
b) our descent profile to touchdown.
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RE: Rate Of Descent Formula

Sat May 18, 2002 1:03 am

oh my god!
this topic is so helpull
i allways fly the cessna around on FS2002 and i sit there thinking about the maths. like how far away from the airport.
and i allways end up with to many digits oin my head and it confuses me.
when u say that though it all makes such simple sense.
when u say times it by three jetpilot is that coz ur going at about 3 nm a minute?

RE: Rate Of Descent Formula

Sat May 18, 2002 1:40 am

LMML14/32, the FM computer calculates it a little bit different from the geometric method. It worries about fuel saving so it calculates the profile assuming idle thrust. In my experience, on average, it's roughly the same, in particular flight plan segments, calculation varies slightly.

Cosync, the multiply by 3 has nothing to do with speed. It's a good number/rule to get you the altitude/distance relationship of 318ft per 1nm, based on a 3 degree descent path, the easy way  Smile/happy/getting dizzy.

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Joined: Sat Jun 30, 2001 5:58 pm

RE: Rate Of Descent Formula

Sat May 18, 2002 1:56 am

Just remember that if using a typical constant speed descent you will require more distance to get down if you're heavy than if you're light.
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RE: Cosync

Sat May 18, 2002 4:43 am

About your speed question, an approximate way to tell nautical miles/minute is multiplying the Mach number by 10. Mach .80 X 10=8 NM/min. So a car doing 60nmph on a highway is going at M.10.

lmml 14/32
Topic Author
Posts: 2358
Joined: Tue Jan 30, 2001 2:27 am

RE: Rate Of Descent Formula

Sun May 19, 2002 7:18 pm

Thanks MD11Nut. It says in your profile that you were a Flight Test Engineer. What did your job entail exactly?

Cosync: for your Cessna it is best to use the ground speed formula rather than the altitude one.

RE: Rate Of Descent Formula

Sun May 19, 2002 11:06 pm

Hi LMML 14/32,
As you know when an aircraft is built or modified by an aircraft manufacturer, it would be going through an exhausting flight test program to validate the design and then a shorter, formal flight test program to certify it with the FAA and JAA. Of course, there would be literally hundreds of people involving in the flight testing, most are engineers but they are not flight test engineers (FTE). There are only 2 or 3 designated to be FTEs for a typical flight test program. On the ground the flight test engineers (FTEs) are not the ones in charge but are the technical focal points. Only one FTE goes on a test flight, and he would be the test conductor, conducting the tests from the observer seat. He is also responsible for planning the tests, consolidating test plans, generating the test cards. The two flight test pilots fly the plane and worry about evaluating the handling qualities of the aircraft, the flight test engineer (FTE) worries about the rest.

The FAA has about of couple of dozens flight test engineers (and flight test pilots) also. These guys go on FAA certification flight tests. The FAA FTEs have the right to conduct the FAA tests but most of the time they let the OEM FTE conduct the tests with them watching over his shoulders.

I'm not sure I'm making any sense. I'll be glad to chat some more if you have other questions.


RE: Rate Of Descent Formula

Sun May 19, 2002 11:51 pm

Back in my major airline days, I learned a slightly different version of the same formula:
Descent point = 3 X Altitude (FL410 to 3,000'MSL = "38" X 3 = 114 NM prior)
Descent Rate = 6 X Ground Speed (500 KTS GS X 6 = 3000 FPM Descent Rate)
The formulas work extremely well. They work for Citations as well as B747s. They would even work well for piston-powered light aircraft, but there are perhaps simpler ways to calculate the descent point for them. For something to do, we frequently would bet dinner and the person who missed his decent had to buy. I've never had to buy a dinner.

These formulas used be "need to know" and actually still are, but in today's world of FMSes they are seldom used any more. Now days, you simply plug in the crossing points and altitudes into the FMS and watch things happen.

lmml 14/32
Topic Author
Posts: 2358
Joined: Tue Jan 30, 2001 2:27 am

RE: Rate Of Descent Formula

Mon May 20, 2002 1:10 am

MD11Nut you had a very interesting job. I'm sure you have many a hair raising story to tell ! So this means that you also got to fly many different aircraft in your career. I have always wanted to go on a test flight. I work for an airline and have often made requests to ride on the jumpseat of a flight following a major maintenance procedure that requires a test flight. However I have always been turn down due to, I am told, insurance complications. All the engineers I talk to about this think I must be off my rocker. They say that it is a stomach churning experience and they do not flock to go themselves.

Jetguy: so you are saying that these formulas have not been simply invented by pilots "on-the-job" as a rule of thumb. They can actually be test questions, if you know what I mean? I have noticed that only "mature" pilots use these formulas. Newer guys rely a lot on the FMS and shrug off these multiplications.

RE: Rate Of Descent Formula

Wed May 29, 2002 4:56 pm

LMML 14/32...
Sorry to take so long to answer the questions you have asked me. Somehow I missed your post.

These formulas are on no test or examination that I know of. I believe that they are simply "rules-of-thumb" that pilots came up with prior to FMS and VNAV. You can do the math, these formulas do a reasonable job in approximating a 3 degree descent angle. Because we frequently encounter clearances which incorporate altitude crossing restrictions it is very important to have a way of determining descent points and rates that allow us to meet those restrictions in the most efficient and economical manor. Starting a descent too early and/or descending too quickly increases fuel consumption. Starting down too late and/or too slowly often causes ATC "heartburn" or requires us to use the speedbrakes - both are things that we like to avoid whenever possible.

As far as "new" pilots not using these formulas, I've noticed that also. FMSes and VNAV are very nice things to have. Personally I use them alot (99% of the time); however, there are times when things can get very busy and programing the FMS can be very distracting. It's nice to be able to instantly figure out a descent point and rate that will work. Also, FMSes have been known to fail. I am our company's EFIS and FMS training captain. I make sure that all of our pilots are thoroughly versed in the operation of the FMS, but I also make sure that they can fly without them. It's called airmanship.

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Joined: Wed Aug 30, 2000 8:10 pm

RE: Rate Of Descent Formula

Wed May 29, 2002 11:15 pm

It is always a good idea to back up the FMS and make sure the info your seeng looks right. That's what we do, set it up in the FMS and figure out if the numbers its using are accurate.

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