ILS Glideslope Speed RNAV

For ILSILS —Instrument Landing System glideslope speed questions, there are three methods to solve the problem:

1. Use trigonometry. This gives the most accurate answer, but is time consuming and requires converting between knots and feet per minute — a difficult conversion, and easy to get wrong.
2. Use $RA = 60D$. This is slightly quicker and slightly less accurate, but also requires converting units.
3. Know that on a 3° glideslope, your rate of descent in feet per minute is 5 times your groundspeed. This is the fastest and most efficient way.

Note that if you are given a TASTAS —True Air Speed, you must first apply a head or tailwind correction.

To use the fastest and most efficient method:

1. Multiply your groundspeed by 5.
2. Multiply the answer by your actual glideslope.
3. Divide by 3.

This will give you your rate of descent in knots.