Vertical navigation: slopes, descent rates and top of descent

Navigation · Chapter 8 · DGCA CPL and ATPL

How high you should be on a slope, how fast to come down, and where to start the descent.

Remember

What a 3° approach looks like

Lined up with the runway • On final approach you fly the runway's extended centreline (the faint white line) • The nose points at the runway; the ground comes up slowly
From the side: the slope • The pink line is the glide path: a 3° slope down to the runway • Every NM closer, you must be 300 ft lower (gold posts)
Its true shape • Drawn true, 3° is very flat: 6 NM along, only 1 800 ft up • That is why the other pictures stretch the heights
Where it meets the runway • The path aims at the touchdown zone, about 1 000 ft past the threshold • So you cross the threshold at about 50 ft: the threshold height in the sums
PAPI: two red, two white • Four lights beside the touchdown zone tell you if you are on the slope • On it: 2 white, 2 red. Too high: more white. Too low: more red
Too high, too low • 0.6° high: 4 white ("all white, too high")
Feet per minute • At 140 kt you cover 2.33 NM a minute: 2.33 × 300 = 700 ft/min • Watch the readout: the height falls steadily, the rate stays the same
From the cruise: top of descent • Cruising at 9 000 ft: start down 30 NM out (9 000 ÷ 300)

Height on a slope

One degree, 100 feet per mile • The 1-in-60 rule on its side: 1 ft up in 60 ft along is 1° • 1 NM ≈ 6 000 ft (really 6 080: about 1 % out) • So 1° = 6 000 ÷ 60 = 100 ft per NM
A 3° glide path • A 3° glide path = 300 ft per NM • At 5 NM: 5 × 300 = 1 500 ft
Add the threshold • Threshold crossing height 50 ft
Backwards: how far out? • 2 100 ft on a 3.5° path