rules of thumb

A

azaviator08

New Member
You are at 13000’ and need to cross a 7dme fix at 5000’feet. You are doing 150kts and want to descend at 700fpm. How far from the fix should you start descending?

Can anybody help me calculate this using rules of thumb?

Also- How would you calculate when you were 3 minutes from a fix using rules of thumb. Say you were doing 150kts, how do you know when you will be 3 minutes from a fix?
 
program the fms, imo...
What I would do is this: 13-7 = 8k feet to descend, 700fpm is roughly 12 minutes, going 2.5 miles/ min so that would bring you to a grand total of 28 miles. I always err on the side of caution, hence why i used 12 minutes instead of the 11.4ish or so. I would also probably start my descent at 30 miles too.
 
azaviator08 said:
You are at 13000’ and need to cross a 7dme fix at 5000’feet. You are doing 150kts and want to descend at 700fpm. How far from the fix should you start descending?

Can anybody help me calculate this using rules of thumb?

Also- How would you calculate when you were 3 minutes from a fix using rules of thumb. Say you were doing 150kts, how do you know when you will be 3 minutes from a fix?
Click to expand...

Rule of 3. If you are at 13,000 and need to descend to 5000' that's 8,000' of altitude to be lost. Take 8 multiply by 3 will give you 24 NM out start your descent.

If you are traveling at 150 kts add a 0 to it and divide by two. 1500/2= 750 fpm rate of descent.

In your scenario I would descend at 750 fpm rate of descent at 24NM from your fix.

I teach my guys this, and it works out pretty good.
 
E6B or easy numbers and interpolate.
Example: you have 8000 feet to lose. At 500 fpm(an easy number) that would take 16 minutes. At 1000 fpm (another easy number) that's 8 minutes. 700 is about halfway in between, so it'll take you about 12 minutes (halfway between 8 and 16) to get down. Every 60 knots is a mile per minute, 150 knots is 2.5 times 60 , so 2.5 miles per minute. 2 miles a minute times 12 minus is 24 miles, .5 miles a minute times 12 minutes is 6 miles, 6+12=30 miles out you start down. Takes way less time to think through than it does to type or read.

Pro tip: round your groundspeed up to the nearest 30 knot increment. Makes all your miles per minute calculations easy to figure.
 
You must log in or register to reply here.
Back
Top