I need help calculating the `Rate of Descent`. I use Little Navmap to plan my flights. It gives me a clear ToD but I'm not sure at what rate I should begin descending. I usually just estimate between 1000-2000 no matter the aircraft, although I switch between the TBM9, B748, and C172. I try to follow the angle shown in the `Flight Plan Elevation Profile` in Little Navmap, which helps but probably not efficient.

Should I continue estimating my Rate of Descent or is there a more accurate method?

Thank you for your time.

Useful rule of thumb for a 3 degree descent is 5x groundspeed.  Thus 200kt groundspeed requires 1000fpm. 

BTW you don't have to depend on LNM to get the TOD 🙂   Another useful rule of thumb is 3x altitude (10000 ft requires 30 nautical miles), but then you have to factor in any significant head/tailwinds yourself.

Edited February 25, 20215 yr by lzamm

Thank you. I need to get in the habit of using the rule of thumb technique. 

For quick calculations on the fly, try this:

https://descent.now.sh/

Most aeroplanes in the simulator have avionics which will give you a time and distance to a waypoint, and if you have that, you can work an approximate descent rate out quickly without any need for any maths other than some really basic division, which is handy if you're doing all the other flying stuff yourself. For example:

Let's say you're 20 miles from a waypoint where you need to be 8,000 feet when overhead. You're currently at 20,000 feet and your avionics reckon you'll be at that waypoint in eleven minutes at your current speed. So, you round eleven minutes down to the nearest decimal (i.e. ten minutes), and round the 12 thousand feet you need to descend to the nearest decimal too (i.e.10,000). This makes quick calculations in your head whilst doing other stuff easy to work out. So now we've got a basic figure of needing to drop 10,000 feet, in ten minutes, i.e. 1,000 fpm, so you set that in your autopilot or start flying that by hand if you prefer.

This isn't intended to be spot on initially, but it will be fairly close while you fine tune things. So now you're at 1,000 fpm descent rate and you have some avionics which will help you find out whether that's too high or too low a rate...

Observe how likely the trend is indicating that you'll be at around 14,000 feet when 5:30 minutes out from your target (i.e. the halfway mark for time and altitude). If that looks likely, your rough guess at 1,000 fpm is good, but if it looks like you'll make it early or late, roughly estimate by what percentage you'll be off, then add or subtract from your current descent rate to get a nearer estimate. This should then be pretty good, but you can rinse and repeat again if you like.

You'll be surprised how well this 'ballpark it with nice round figures, then tune it a bit' method works.

Edited February 25, 20215 yr by Chock

There's an excellent web-based utility that I use very often, that gives you the Rate Of Descend. It also gives you the Indicated AirSpeed (IAS) during the descent, which is very useful. I think it follows the known principle of 3 degree descent path, which is generally used. Look here:

http://aerotexas.com/desplan/desplan.php

Cheers, Ed

 

As others have said, I use 3x or 4x the altitude to lose to figure out distance required. 4x is for slippery jets like the ones I fly and easily accounts for the time needed to shallow to cross 10,000 at 250. That technique in most jets will get you between 2000 to 3000 FPM at idle, higher at higher altitude and lower at lower altitudes. The other technique to go along with that is your MACH number. If traveling at .80, that wag is 8 miles a minute. So if it's going to take 80 miles to lose that altitude, you are looking at 10 minutes to do it. Once you have the time, you can figure out what descent rate is needed to do it. If you have 10,000 to lose in 10 minutes, 1000 FPM. Of course, keep in mind that the current altitude will be a factor in what the jet will give you clean. This is also helpful with crossing restrictions. If ATC says, descend and maintain 9,000, cross 30 miles South of CVN at 13,000, you can quickly figure it out without touching the box. If your calculated required descent rate will exceed what you know your jet can give clean or with the brakes out, you can inform early on that you can't make that restriction. Now, you can easily touch the box and run a what if scenario if your box has that. Or, you can alter descent angles and descend now/direct to altitude and see what it says also. I'm just old school 🤣.

Edited February 25, 20215 yr by G550flyer
omission

So much helpful advice - thank you!