Do you chop and drop? Descent planning.

How early do you plan your descends?  If you are not planning this well in advance, you are subjecting the engine to shock cooling, your passengers to discomfort, wasting fuel and likely coming in too high for the pattern and for landing.

Say you are at 8000 ft. (IFR flying in a westerly direction) and the fix for the IFR approach is at 3000 ft, you have 5000 feet to loose - when should you start your descent?

Method 1

Descents are usually done at a 3 degree glidepath, so the quick rule of thumb is to take altitude to lose, divide by 1000 and multiply by 3 for the distance before your desired arrival fix you should start to descend by. 

If you want to lose 5000 ft., so you should descend (5000/1000)*3 = 15 miles out.  What should your vertical speed be?  Multiply 5 by your ground speed... so 130 x 5 = 650 ft./min. 

And if 130 kts is your ground speed, that means you have to start your descent 8 minutes out (calculated with your flight computer)

Method 2

If you have a target vertical speed you want to descend with, say 500 feet per min., with 5000 feet that's 10 minutes out.  In 10 minutes, you'd cover 13 miles with your 130 ground speed, so you have to do your descent 13 miles out. 

For both methods, mechanically, to keep speed constant you typically need to remove 100 RPM per 100 feet / min vertical descent, so take out 500 RPM keeping speed constant to achieve your 500 feet per min. descent rate.

(more methods here)

Using the EFB

What if ATC or weather conditions delay the start of your descent?  With the distance and time available what should your vertical speed be and how would you adjust your ground speed if you had a target vertical speed?  For these calculations you need to pull out your trusty EFB, but would have to enhance it with manual tick marks.  Details here.