# descent Rate

Vin4evr First Officer

hey Folks !
Does anyone know how to calculate the descent rate? thanks

michlin First Officer

I cannot remember who originally posted this but it was on this board by a member

Rule of thumb for the descent is:

If you are flying at 18000ft at a ground speed of 200kts and you need to descend to 3000ft, the difference is 15000ft. Multiply 15 x 3 = 45 miles out you must begin your descent. Half your ground speed is 100, add a zero, and 1000 ft per minute is your rate
of descent.

Matthew Shope (mypilot) Chief Captain

That quote was from the learning center. 😉 I remember reading that.

michlin First Officer

That's the guy! 😀 Learning Center

Vin4evr First Officer

Ron had another way to calculate rate of descent in one of the fliying lessons in 04 sim. Does anyone know which section is that fliying lesson?i looked for it..cant find it.

Vin4evr First Officer

lets say you have 20 miles for touchdown and you are at 7000ft at 190kts. how would you calculate rate of descent with the known distance rather than descenting at a certain distance?

renesis Trainee

Another slight variation on what was given above.

JAA ATPL training material gives the following as a guide to cross-ref FMC:

3 x Height (in 10's thousand feet - so 30,00ft is "30") + 10 (for deceleration)

= descent distance out from a given point.

Such that:

You're flying at 33,000 feet and would like to descend to a sea level airport. So, 3 x 33 = 99 (don't forget to add the 10 for deceleration) = 109. You start your descent at 110 track miles from your destination.

In reality, this figure should give you a close approximation to what you FMC would compute - assuming you use an FMC of course.

lionlicker First Officer

f = feet above airstrip
N = Nautical miles from airstrip threshold
G = ground speed as shown on DME or GPS in Knots
fpm = foot per minute descent

fpm = (f * G) / (N * 60) . . . . or . . . f / N / 60 * G . . same thing.

will give you the glideslope that aims directly at the threshold.

ft/Nm = foot per Nautical mile = f / N . . . . gives you the decline (glideslope) angle.

300 ft/Nm is a common final glideslope angle.

Prozac919 Trainee

Calculate how much altitude you have to lose (ex: flying at 25000' but the airport is at 1000'. You have 24000' to lose). Next, figure out how fast you are going in miles per minute--basically take your groundspeed and divide it by 6 (ex, our groundspeed is 300 mph which means we are traveling 5 miles per minute). Now, if I know that I am traveling at 5 miles per minute, I can figure that it will take me 120 miles to lose the altitude at 1000 fpm decent rate, 80 miles at 1500 fpm, 60 miles at 2000 fpm, 48 miles at 2500 fpm, or 40 miles at 3000 fpm.

The 3 x your altitude to lose works well when to start a descent, however, it doesn't tell you how fast you need to descend.

chris Ingham (teenflon5) Trainee

It depends entirley on the plane and what type of decent you are doing. From my PPL Flying Training book, ( I use a C152 like the C172 in fs), In a cruise decent (200-300rpm), you maintain the cruise speed (around 90 knots).

"In this type of decent you can use a somple rule of thumb to gauge at what point to start the decent toward an airfield. Read the height above the ground level in thousands of feet, multiply by 3, adn the result is the distance required (in nautical miles) to decend to ground level, i.e. at height 5000', start the decent 15 nautical miles from the destination

Be careful- all sorts of factors such as groundspeed, aircraft type, prevailing wind etc. will affect this rule of thumb, but it will give you an appreciation of where to stary your decent." (AFE, The Private Pilot's Licence Course, Flying Training PPL 1 by jeremy M Pratt)
It completley depends on the aircraft type and the type of decent.