Several months ago I read someones formula for calculating the number of miles from your landing location you need to begin your descent, using 500 fpm rate of descent, so you will arrive at pattern altitude without having to slow down, speed up, reduce the rate of descent or increase it. I don't know if it was on this forum or if I read it in a popular aviation magazine. I have one formula, but it requires straining the brain or using a calculator. The one I read (and promptly forgot) was so simple even I could do it with no stain, no pain. This one involved taking your present altitude, subtracting the pattern altitude of the airport you were going to land at and somehow dropping the zero's and wala--the result was the number of miles from the airport to begin your descent.
Does anyone out there remember seeing such a formula or do you know of a "simple" one?
Frank
Distance to Airport Descent Rate
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zero.one.victor
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- Joined: Tue Apr 23, 2002 12:11 am
I don't know about everybody else,but I navigate "gps direct". My Garmin Pilot III has user-selected data fields on several of the nav pages. I've set up one of these to indicate "time to destination",both to answer the inevitable "how long til we get there?" questions as well as to help me plan my descent.
For example,If I need to descend from 6500' cruising altitude to 1000' pattern altitude,that's a 5500' change at 500 fpm so I need to start my descent about 11 minutes out.
As far as planning based on miles,figure a power-on cruise descent is good for close to 120 mph,that's 2 miles a minute. For the same 5500 change in altitude,at 500 fpm down you'll need 11 minutes,that's 22 miles.
It never hurts to arrive at your destination a little high,it gives you the oportunity to spiral down overhead & observe the windsock & any action on the field.
Eric
For example,If I need to descend from 6500' cruising altitude to 1000' pattern altitude,that's a 5500' change at 500 fpm so I need to start my descent about 11 minutes out.
As far as planning based on miles,figure a power-on cruise descent is good for close to 120 mph,that's 2 miles a minute. For the same 5500 change in altitude,at 500 fpm down you'll need 11 minutes,that's 22 miles.
It never hurts to arrive at your destination a little high,it gives you the oportunity to spiral down overhead & observe the windsock & any action on the field.
Eric
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Eric Leclercq
- Posts: 14
- Joined: Wed Sep 18, 2002 4:33 pm
Distance to Airport Descent Rate
Hi,
where I work, 500 fpm is our standard rate for non-pressurized aircraft. To answer your question, when I started flying the ATR-42 guys showed me a clever little formula
It seems to be the one you are referring to... 
It is quite simple, you take the altitude you need to lose, remove the zeroes at the end and multiply by a factor appropriate to your aircraft type, take the result and perhaps remove another zero, depending on your aircraft.
Example: (Lets take Eric's numbers)
5500 feet, remove the zeroes, gives you 55, multiply by aircraft factor 4 (which seems to be appropriate for the 170), which results in 220 miles in this case. You know by experience that you need to remove the last zero to get to 22 miles. In my language I would put the C-170 in a 4 to 1 category. The ATR being faster was a 3 to 1 or 2 to 1 depending on the rate of descent we would use.
I hope what I just tryed to explain makes sense...
If not, please ask and I will try my best to help.
Eric
where I work, 500 fpm is our standard rate for non-pressurized aircraft. To answer your question, when I started flying the ATR-42 guys showed me a clever little formula
It seems to be the one you are referring to... It is quite simple, you take the altitude you need to lose, remove the zeroes at the end and multiply by a factor appropriate to your aircraft type, take the result and perhaps remove another zero, depending on your aircraft.Example: (Lets take Eric's numbers)
5500 feet, remove the zeroes, gives you 55, multiply by aircraft factor 4 (which seems to be appropriate for the 170), which results in 220 miles in this case. You know by experience that you need to remove the last zero to get to 22 miles. In my language I would put the C-170 in a 4 to 1 category. The ATR being faster was a 3 to 1 or 2 to 1 depending on the rate of descent we would use.
I hope what I just tryed to explain makes sense...
If not, please ask and I will try my best to help.
Eric
Pressurized airplane drivers use cabin-rate to compute exactly the same method regardless of the airplane's altitude. If you've got an 8,000 foot cabin it'll take 16 minutes to get it to sea level at 500 fpm. If you're travelling 400 knots that's almost 8 miles/minute meaning you've got to start down 128 nm out to reach sealevel (8 X 16) with a descent rate of 2,000 fpm from FL 330 which puts you at 1,000' above the S.L. airport. If you're at a higher cruise altitude it's a simple matter to either start down sooner (which only puts the cabin down a little farther from the destination). In other words, when the DME or GPS says you're 16 minutes to destination, ... then it's time to be headed down.
The 170 travels at 2 miles/minute so a descent of 500 fpm means to multiply your req'd altitude loss (say for example 8,000') by 2, which equals 16 minutes, times two (120 mph) which equals a descent started at 32 miles. Or you can shorten it by multiplying the 8K by 4 to get the same thing (since 2 {minutes to descend 1000 feet results in a 500 fpm descent} X 2 {miles travelled per minute} = 4.)
The 170 travels at 2 miles/minute so a descent of 500 fpm means to multiply your req'd altitude loss (say for example 8,000') by 2, which equals 16 minutes, times two (120 mph) which equals a descent started at 32 miles. Or you can shorten it by multiplying the 8K by 4 to get the same thing (since 2 {minutes to descend 1000 feet results in a 500 fpm descent} X 2 {miles travelled per minute} = 4.)
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
Rate of descent Formula
Thanks to all who contributed to my "memory failure". I think now I will write this down so I don't forget it. Just hope I don't forget where I put what I have written down! May have to use Eric's formula---fly at pattern altitute all the time!!!!!!!!
Ole Pokey
Ole Pokey