What expression is used to determine the glide-slope descent rate from ground speed?

What expression is used to determine the glide-slope descent rate from ground speed?

• A. GS/2 x 10
• B. GS x 2
• C. GS + 10
• D. GS - 5

Answer: (A)

Glide-slope descent rate scales with how fast you’re moving horizontally. For a typical glide path around 3 degrees, the vertical speed you need is about five times your horizontal speed when you express horizontal speed in knots and vertical speed in feet per minute. The expression GS/2 x 10 is just another way to write five times the ground speed (GS), which matches that rule of thumb. So at 100 knots, it gives about 500 fpm, aligning with the usual glide-slope target of roughly 500–550 fpm. The other forms don’t preserve that proportional relationship: doubling the ground speed, or adding/subtracting a constant, would not yield a consistent glide-slope descent rate.

Glide-slope descent rate scales with how fast you’re moving horizontally. For a typical glide path around 3 degrees, the vertical speed you need is about five times your horizontal speed when you express horizontal speed in knots and vertical speed in feet per minute. The expression GS/2 x 10 is just another way to write five times the ground speed (GS), which matches that rule of thumb. So at 100 knots, it gives about 500 fpm, aligning with the usual glide-slope target of roughly 500–550 fpm.

The other forms don’t preserve that proportional relationship: doubling the ground speed, or adding/subtracting a constant, would not yield a consistent glide-slope descent rate.