Rule of thumb: descent rate for a 3-degree glidepath with 80 knots groundspeed is:

• A. 350 FPM
• B. 400 FPM
• C. 450 FPM
• D. 500 FPM

Answer: (B)

Descent rate on a glidepath is found by turning the glide angle into vertical speed: vertical speed ≈ horizontal speed × tan(angle). For a 3-degree glidepath, tan(3°) ≈ 0.0524.

With a groundspeed of 80 knots, convert to feet per minute: 80 knots is about 1.333 nautical miles per minute, which is roughly 8,101 feet per minute. Multiply by tan(3°): 8,101 × 0.0524 ≈ 424 ft/min.

Using the common rule of thumb of about 318 ft per nautical mile for a 3° glidepath, and 1.333 NM per minute at 80 knots, you get 1.333 × 318 ≈ 424 ft/min.

So the descent rate is about 424 ft/min, and the closest practical value is around 400 ft/min.