calculations

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buffalopilot

Well-Known Member
Given a glide slope angle just say 5 degrees and a ground speed how do you calculate the FPM (this is not a 3 degree GS so dont tell me times 5)

Alo on a DME arc, how do you calculate distance traveled

thanks
 
buffalopilot said:
Given a glide slope angle just say 5 degrees and a ground speed how do you calculate the FPM (this is not a 3 degree GS so dont tell me times 5)

Alo on a DME arc, how do you calculate distance traveled

thanks
Click to expand...

Simple rule of thumb for DME arcs is -

20 DME = 3 radials per NM
15 DME = 4 radials per NM
10 DME = 6 radials per NM

So, if you were on a 10 DME arc and had 40 radials to cross you'd take 40/6 to give you approximately 6.5 NM.

To calculate descent rate, take 1/2 your GS. But this is for a typical 3 degree GS. So, if flying an approach at 100 knots, you'd be approximately 500 FPM.

Who's your interview with?
 
Descent rate in feet per minute=(tangent of glide slope angle)x(ground speed in nautical miles per hour)x(6076 feet per nm)/(60 minutes per hour)

For example:

5 degree glide slope
120 knots

Descent rate = (tan 5)x(120)x(6076)/60=1,063 feet per minute
[note: tan5 = 0.087489]


Example #2:

3 degree glide slope
[tangent of 3 degrees = 0.052408]
100 knots ground speed

(tan3)x(100)x(6076)/60=530 feet per minute descent required, close enough to merit's quick calculation.
 
Actually you can't use tangent in that case because the Earth is not 100% flat and thus shape formed by the vectors is certainly not a triangle :)
 
Chris_Ford said:
Actually you can't use tangent in that case because the Earth is not 100% flat and thus shape formed by the vectors is certainly not a triangle :)
Click to expand...

I'll bet it's 100% accurate within any reasonable number of significant digits. Innacuracies in actual groundspeed and descent rate measurements will be orders of magnitude higher than any error induced by approximating the arc of the earth's surface as a straight line over the distance covered in any normal aircraft descent.

:p
 
SteveC said:
Descent rate in feet per minute=(tangent of glide slope angle)x(ground speed in nautical miles per hour)x(6076 feet per nm)/(60 minutes per hour)

For example:

5 degree glide slope
120 knots

Descent rate = (tan 5)x(120)x(6076)/60=1,063 feet per minute
Click to expand...

Or,

VSI= (NM/min)(pitch degrees)(100)

So for the example of a 5 degree glideslope at 120 knots (2nm/min):

VSI=(2)(5)(100)
VSI= 1,000 fpm

You have to know your speed in NM/min, but you don't need to know the tangent of anything. Just another quick, pretty accurate estimation.
 
SteveC said:
I'll bet it's 100% accurate within any reasonable number of significant digits. Innacuracies in actual groundspeed and descent rate measurements will be orders of magnitude higher than any error induced by approximating the arc of the earth's surface as a straight line over the distance covered in any normal aircraft descent.

:p
Click to expand...

prove it :)
 
Sources reveal that "Who Wants A Wendy's Frosty?" didn't do well in the preliminary polls. Lack of sufficient incentive cited as primary reason.

:crazy:
 
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