How fast should you descend during an non-precision
approach?
3 steps:
1) Find the VDP
Knowing the MDA, how far from
the runway do you need to be at the MDA?
2) Find the Time from the FAF to the VDP
Knowing your ground speed, how
long will it take you from the FAF to the VDP?
3) Find the Descent Rate
Knowing the time to the VDP and
the altitude to lose, how fast do you have to lose it?
- Determine where you need to be at the MDA? (how far
from the end of the runway)
- You need to make a normal descent from MDA to the
runway (~3.0 - 3.5° glideslope)
- This is usually the VDP, but the VDP isn't always
on the IAP.
- Usually
- The higher the MDA, the farther out you need to
see the runway.
- Note the trend in the numbers below. Just
remember one line and then add or subtract 0.25 mile for every
100' MDA above or below the one you remember.
MDA in AGL |
VDP |
300 ft |
~ 1 mile |
400 ft |
~ 1.25 miles |
500 ft |
~ 1.50 miles |
600 ft |
~ 1.75 miles |
700 ft |
~ 2 miles |
- Example:
- The MAP is at the approach end of the runway
- The MDA is 600' AGL
- So the VDP is 1.75 miles from the end of the
runway
- It's 5 miles from the FAF to the MAP (end of the
runway)
- So subtract 1.75 miles from 5 miles = 3.25
miles to get from FAF altitude to the MDA / VDP
- Guess at how many minutes it will take you to
travel 3.25 miles at your groundspeed (example 90 knots)
- Put the Whizwheel triangle on your speed
(90), 3.25 miles on outer ring shows 2.17 minutes on inner
ring.
- Let's just say the FAF Altitude is 1500' and the
MDA is 300' or 1200' to lose between the FAF and the MDA /
VDP
- Spin to put 2.17 under the altitude to lose
(1200)
- Read the FPM required over the the inner ring
"10" (550 FPM).
Or, use the
chart |