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

JOINT ADVANCED MULTI-ENGINE T-44A

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 2 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 the 6 descent 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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RADIO INSTRUMENTS STAGE