Quick Math‎ > ‎

Sink rate for a stable approach.

Your approach ground speed is one hundred knots and you want to maintain a three degree glide slope.  What should your vertical speed be in feet per minute? (Use Pi = 3, and 1 NM = 6000 ft.)

Ans: 500 fpm

This is a two part problem.

First convert the ground speed into the desired units of ft/min.  One hundred knots is six hundred thousand feet in sixty minutes.  (10,000 fpm)

Next convert 3 degrees to radians:  Three degrees is 1/60th of 180 degrees. (180 deg = Pi radians) One sixtieth of Pi is approximately 1/20 rad.

For small angles, the sine, the tangent, and the angle in radians converge.  Multiplying the horizontal ground speed and the glide slope in radians: 10,000 fpm X (1/20 rad) = 500 fpm.

(Since three degrees is the standard ILS glide slope, we don’t have to do the math from scratch each time.  Just divide your ground speed in knots by two and then tack a zero on the end.)