Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Rate Of Descent Formula  
User currently offlineLMML 14/32 From Malta, joined Jan 2001, 2565 posts, RR: 6
Posted (12 years 5 months 1 week 6 days 8 hours ago) and read 32767 times:

Will somebody please remind me what the rate of descent formula is? Something to do with your ground speed?

13 replies: All unread, jump to last
 
User currently offlineErasmus From Italy, joined Jun 2007, 0 posts, RR: 0
Reply 1, posted (12 years 5 months 1 week 6 days 8 hours ago) and read 32767 times:

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?
Regards,
Erasmus


User currently offlineJetpilot500 From United States of America, joined Nov 2000, 78 posts, RR: 0
Reply 2, posted (12 years 5 months 1 week 6 days 7 hours ago) and read 32767 times:

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.

Enjoy,
Jetpilot500


User currently offlineLMML 14/32 From Malta, joined Jan 2001, 2565 posts, RR: 6
Reply 3, posted (12 years 5 months 1 week 6 days 6 hours ago) and read 32767 times:

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.


User currently offlineCosync From Mexico, joined Nov 2001, 556 posts, RR: 0
Reply 4, posted (12 years 5 months 1 week 6 days 4 hours ago) and read 32767 times:

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?


User currently offlineMD11Nut From , joined Dec 1969, posts, RR:
Reply 5, posted (12 years 5 months 1 week 6 days 4 hours ago) and read 32767 times:

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.

Regards,
Nut


User currently offlineSeagull From United States of America, joined Jun 2001, 340 posts, RR: 1
Reply 6, posted (12 years 5 months 1 week 6 days 3 hours ago) and read 32767 times:

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.

User currently offlineGoboeing From United States of America, joined Jun 2000, 2706 posts, RR: 15
Reply 7, posted (12 years 5 months 1 week 6 days 1 hour ago) and read 32767 times:

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.

Nick/PHL


User currently offlineLMML 14/32 From Malta, joined Jan 2001, 2565 posts, RR: 6
Reply 8, posted (12 years 5 months 1 week 4 days 10 hours ago) and read 32767 times:

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.


User currently offlineMD11Nut From , joined Dec 1969, posts, RR:
Reply 9, posted (12 years 5 months 1 week 4 days 6 hours ago) and read 32767 times:

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.

Regards,
Nut


User currently offlineJetguy From , joined Dec 1969, posts, RR:
Reply 10, posted (12 years 5 months 1 week 4 days 5 hours ago) and read 32767 times:

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.

Jetguy


User currently offlineLMML 14/32 From Malta, joined Jan 2001, 2565 posts, RR: 6
Reply 11, posted (12 years 5 months 1 week 4 days 4 hours ago) and read 32767 times:

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.


User currently offlineJetguy From , joined Dec 1969, posts, RR:
Reply 12, posted (12 years 5 months 1 day 12 hours ago) and read 32767 times:

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.

Jetguy


User currently offlineCV640 From United States of America, joined Aug 2000, 952 posts, RR: 5
Reply 13, posted (12 years 5 months 1 day 6 hours ago) and read 32767 times:

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.

Top Of Page
Forum Index

Reply To This Topic Rate Of Descent Formula
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
How Can We Maintain A Constant Rate Of Descent? posted Mon Apr 19 2010 13:26:52 by KELPkid
B744 High Rate Of Descent posted Sat Apr 2 2005 13:57:56 by SevenHeavy
What Is The Fuel Burn Rate Of B777-200LR/300ER posted Tue Apr 26 2011 08:44:03 by philippelouis1
Calculating Top Of Descent (TOD) posted Sun Dec 20 2009 21:19:15 by Wardialer
What Determines Top Of Descent? posted Mon Dec 7 2009 05:52:08 by Jgarrido
Rate Of Climb For 737, A310, AN-72 posted Sun Aug 17 2008 04:07:46 by SkyGazer
Calculation Of Rate Of Climb posted Wed Mar 22 2006 00:30:19 by Jetflyer
Rate Of Climb: A333 Vs A343 posted Sun Jan 8 2006 19:58:41 by HT
Rate Of Climb Versus Angle Of Climb posted Wed Feb 23 2005 16:13:58 by Kiwiineurope
Rate Of Climb posted Sun Jun 6 2004 09:38:15 by KBUF737

Sponsor Message:
Printer friendly format