Non-Presision Descents

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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

 
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 Dick Fechter
(507) 775-7686
FPS at 44RF.com (at=@)
139 13th St NE
Byron, MN 5592
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