Summary of the 60-to-1 Rule and Other Rules & Formulas - Cont'd - P-5530119

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Example: For 150 KTAS and a 2° pitch change
TAS/60 = NM/MIN150/60 = 2.5 NM/MIN
VSI for 1° pitch change = NM/MIN X 100 = 2.5 X 100 = 250 FT/MIN
VSI for 2° pitch change = 2 X (NM/MIN X 100) = 2 X (2.5 X 100) = 500
FT/MIN
Precision Glide Path. The glide path published for an approach will be the same for every aircraft.
Therefore, a pitch change equal to the published glide path can be made on the attitude indicator when
intercepting the glide path. Aircraft speed has no effect upon the amount of pitch change required
when intercepting the glide path. Speed only affects the time required to fly the final approach
segment and your rate of descent (VSI). Prior to intercepting the glide path, compute the target VSI
for your planned groundspeed. (There's also a chart in the back of the approach plate that does this for
you.) When you intercept the glide path, crosscheck your actual VSI; it should be close to your target
VSI. Using the previous formulas, some algebra, and substituting GS (groundspeed--which is TAS
corrected for wind) we get the following formulas to compute your target VSI:
VSI for a 3° glideslope = (GS X 10)/2 or "Half the groundspeed and add a zero"
Example: 130 KIAS; 10 kts headwind; GS = 120 kts
(120 X 10)/2 = 600 FT/MIN VSI
VSI for a 2½° glideslope = [(GS X 10)/2] ­ 100
Example: 130 KIAS; 10 kts headwind; GS = 120 kts
[(120 X 10)/2] ­ 100 = 500 FT/MIN VSI
Descent Gradients for Approaches or Enroute Descents. Now let's look at another real world
application. You are flying along fat, dumb, and happy when ATC directs you to cross the ABC
VORTAC at 12,000'. A quick glance inside shows you are 25 nm from the ABC VORTAC. You are
at FL 270 and you are cruising at 165 KIAS or 255 KTAS (no wind). What descent gradient is
required and what VSI should you expect?
·
First, you need to know what your descent gradient has to be. You can find the descent gradient
by applying the 60-to-1 relationship of 100 ft/nm.
Required Gradient = Altitude to Lose / Distance to Travel
Descent Gradient = alt to lose / distance in nm = 15,000 / 25 = 600 ft/nm
To lose 15,000' in 25 nm, you'll need a descent gradient of 600 ft/nm or about a 6° pitch change.
NOTE: For practical applications, each 60 kts of wind will change pitch 1° (a 60 kt tailwind will require an
additional one degree lower pitch, and vice versa)
·
Now that you know what descent gradient is required, you can compute what your VSI should
be if you make a pitch change of 6° (using the formula from above).
VSI = (FT/NM) X (NM/MIN)
VSI = (600 FT/NM) X (4.25 NM/MIN) = 2550 FT/MIN
If you maintain a constant IAS throughout the descent then your TAS will decrease as you get lower
meaning the VSI required to maintain thedescent gradient will slowly decrease as you descend. If
you hold 2550 ft/min all the way down to 12,000', you will get down early. The most important part of
the equation (which remains constant no matter what speed you are flying) is the descent gradient. You
must descend at 600 feet/NM (or about 6°) in order to make the altitude restriction at the ABC
VORTAC.
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