ILS Glideslope Speed Change 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=60DRA = 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.

This formula also applies to a change of groundspeed. So multiply the change in groundspeed by 5, then convert for the new glideslope by multiplying by new glidepath and dividing by 3.

Finally, think about which way to apply the correction. The faster your groundspeed, the faster you have to descend. So an increase in tailwind/decrease in headwind requires a faster rate of descent, and vice versa.

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Prefer multiple choice questions

What change in rate of descent is required for a 32.3 kts decrease in headwind on a 3.6° approach?