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Climb Gradients. As you might suspect, computing a climb gradient is really no different than the
enroute descent calculations, but let's run an example to see how it's done. Let's say you are getting
ready to fly a Departure Procedure requiring a climb gradient of 350 FT/NM to 8000'. So, we need to
climb out at a 3.5° angle. Our climb airspeed will be 155 KIAS. The airport is 3000 MSL.
·
First, we need to calculate our TAS. Because our TAS increases as we climb, we will be
conservative and use our TAS at 8000'. In this case, 155 KIAS at 8000 MSL works out to
180 KTAS. Dividing this by 60 will give us our speed in NM/MIN.
·
Now for VSI:
VSI = (Pitch Angle) X (NM/MIN X 100) = 3.5° X (3NM/MIN X 100) = 1050
FT/MIN
Calculating a Visual Descent Point (VDP). The first step to computing a VDP is to divide the Height
Above Touchdown (HAT) from the IAP by your desired descent gradient. Most pilots use a 3°
(300 ft/nm) glidepath for landing. Here is the formula to use:
HAT / Gradient (normally 300) = VDP in NM from end of runway
Now that you know how far the VDP is from the end of the runway, you may add this distance to the
DME at the end of the runway to get a DME for your VDP. Armed with this information, it is easy to
compute the distance from the FAF to the VDP. This distance is important in computing the descent
gradient necessary for final approach. Using the FAF altitude, the MDA, and the distance from the
FAF to the VDP, you can compute a descent gradient from the FAF to the VDP along with a target
VSI to ensure you are meeting the desired descent gradient.
Example: HAT = 420 FT, MDA = 840 FT MSL, DME at the end of the runway = 0.5 DME,
FAF = 6 DME,
FAF altitude = 2500 FT MSL, desired landing gradient = 300 FT/NM, Approach
airspeed = 150 kts GS.
VDP = HAT / Gradient = 420 / 300 = 1.4 NM from end of runway
VDP DME = DME at end of runway + VDP distance = 0.5 DME + 1.4 DME =
1.9 DME
Descent Distance = FAF DME - VDP DME = 6.0 DME - 1.9 DME = 4.1 DME
Altitude to lose = FAF altitude - MDA = 2500 - 840 = 1,660 FT
Descent Gradient = altitude to lose / distance = 1660 / 4.1 = 405 FT/NM (4°
VSI = Angle (NM/MIN X 100) = 4 (2.5 X 100) = 1,000 FT/MIN
With this information you can depart the FAF maintaining a 4° descent gradient (400 ft/nm). Your
target VSI is 1000 ft/min. Each mile you should lose 400'. At 5 DME, you should be at 2100', at 4
DME, 1700', etc. Continue this descent gradient until reaching the VDP at 840' MSL. Hopefully, at
the VDP, you'll have the runway in sight. Adjust your descent to a 300 ft/nm gradient and pick up