APPROACH DESCENT RATE - GLIDE SLOPE

[navigator44]

I am just beginning study for the Instrument written. My question is: at 90 knots, would 500 FPM be about right for the glide slope? I think most glide slopes are pitched at about 3 degrees. Is this correct?

Is there any chart available that shows various descent rates at various approach speeds? One I coluld placard to the instrument panel?

Thanks
Jim

 

[X3 Skier]

I am just beginning study for the Instrument written. My question is: at 90 knots, would 500 FPM be about right for the glide slope? I think most glide slopes are pitched at about 3 degrees. Is this correct?

Is there any chart available that shows various descent rates at various approach speeds? One I coluld placard to the instrument panel?

Thanks
Jim

If you look in the front of the IR test figures book, there is a table of speed and descent rate for various glide slope values. I forget the number but it covers the whole page. You can use during the test. The glide slope is shown on the IAP.

Cheers

 

[Ghery]

And at 90 Kts ground speed (not air speed) 500 fpm is going to be close.

 

[dmspilot]

If you look in the front of the IR test figures book, there is a table of speed and descent rate for various glide slope values. I forget the number but it covers the whole page. You can use during the test. The glide slope is shown on the IAP.

Cheers

It's also on the inside back cover of FAA approach chart booklets.

Or you can use trig:
sin(3) * 90 = 4.71 knots * (6076ft/nm) / (60min/hr) = 477fpm

Or use the rule of thumb:
divide Knots GS by 2 and add a zero, 90kts => ~450fpm

 

[navigator44]

Thanks, you all have been a big help.

 

[gismo]

And at 90 Kts ground speed (not air speed) 500 fpm is going to be close.

As Ghery pointed out, descent rate is proportional to groundspeeed not airspeed. The relationship is fairly simple if you use feet/minute and Knots because 3° equals a 20:1 slope and 1 Kt is almost exactly 100 ft/minute. Put the two together and you get 100 fpm descent for every 20 Kt of groundspeed. So for 100 Kt GS you need 500 FPM, 120 Kt needs 600 FPM, and 90 takes 450 FPM.

 

[timwinters]

Or use the rule of thumb:
divide Knots GS by 2 and add a zero, 90kts => ~450fpm

That's definitely expressed in right brain terms.

In left brain terms...5 times the ground speed.

 

[Everskyward]

That's definitely expressed in right brain terms.

In left brain terms...5 times the ground speed.

Uh yeah. That's what I've always used. I guess I must be left-brained (and right-handed).

 

[midlifeflyer]

That's definitely expressed in right brain terms.

In left brain terms...5 times the ground speed.

Why is GS divided by 2 and add a zero (or for that matter add a zero and divide by two) any more or less right brained than multiplying by 5?

Thinking "Oh it's about this" and nailing it would seem to be more right brained than either.

 

[timwinters]

Why is GS divided by 2 and add a zero (or for that matter add a zero and divide by two) any more or less right brained than multiplying by 5? .

For the same reason that concrete subcontractors who failed math in high school take the area of a floor and divide it by 81 to get the cubic yards of concrete needed to pour a 4" thick slab.

They have no idea why this works, just that it does. It's their "rule of thumb". And if they failed more subjects yet, then they just give the concrete dispatcher the dimensions and let him tell them the quantity needed. (true stories, happens everyday on jobsites)

Mathematics is not rote. You must understand why or you'll be forever lost.

As Gismo pointed out "The relationship is fairly simple if you use feet/minute and Knots because 3° equals a 20:1 slope and 1 Kt is almost exactly 100 ft/minute. Put the two together and you get 100 fpm descent for every 20 Kt of groundspeed. And since dividing by 20. So for 100 Kt GS you need 500 FPM, 120 Kt needs 600 FPM, and 90 takes 450 FPM."

These all equate to multiplying by 5...not dividing by 2 and adding a zero.

You'll never find that latter description in a math book.

 

[Everskyward]

For the same reason that concrete subcontractors who failed math in high school take the area of a floor and divide it by 81 to get the cubic yards of concrete needed to pour a 4" thick slab.

They have no idea why this works, just that it does. It's their "rule of thumb". And if they failed more subjects yet, then they just give the concrete dispatcher the dimensions and let him tell them the quantity needed. (true stories, happens everyday on jobsites)

Mathematics is not rote. You must understand why or you'll be forever lost.

As Gismo pointed out "The relationship is fairly simple if you use feet/minute and Knots because 3° equals a 20:1 slope and 1 Kt is almost exactly 100 ft/minute. Put the two together and you get 100 fpm descent for every 20 Kt of groundspeed. And since dividing by 20. So for 100 Kt GS you need 500 FPM, 120 Kt needs 600 FPM, and 90 takes 450 FPM."

These all equate to multiplying by 5...not dividing by 2 and adding a zero.

You'll never find that latter description in a math book.

Wait a minute, I always thought rules of thumb were used so you didn't need to go through all the math.

Then there's the issue of trying to explain what a "rule of thumb" is to someone who is not a native English speaker.

 

[azure]

Or even a native English speaker, for that matter...

Strictly speaking though, even Lance's method involves a "rule of thumb", since a 20:1 slope is slightly shallower than 3* (about 2.86*). 19:1 is actually closer to 3*. It's close enough for g'ment work though, when you consider that the FAA also recommends ROTs that amount to saying pi = 3.

 

[dmspilot]

For the same reason that concrete subcontractors who failed math in high school take the area of a floor and divide it by 81 to get the cubic yards of concrete needed to pour a 4" thick slab.

They have no idea why this works, just that it does. It's their "rule of thumb". And if they failed more subjects yet, then they just give the concrete dispatcher the dimensions and let him tell them the quantity needed. (true stories, happens everyday on jobsites)

Mathematics is not rote. You must understand why or you'll be forever lost.

As Gismo pointed out "The relationship is fairly simple if you use feet/minute and Knots because 3° equals a 20:1 slope and 1 Kt is almost exactly 100 ft/minute. Put the two together and you get 100 fpm descent for every 20 Kt of groundspeed. And since dividing by 20. So for 100 Kt GS you need 500 FPM, 120 Kt needs 600 FPM, and 90 takes 450 FPM."

The rule of thumb has absolutely nothing to do with how good your understanding of math is. It's meant to be as easy as possible to calculate quickly and accurately in your head.

These all equate to multiplying by 5...not dividing by 2 and adding a zero.

They are identical. 5x = 10x / 2.

 

[Apache123]

These all equate to multiplying by 5...not dividing by 2 and adding a zero.

You'll never find that latter description in a math book.

Unless you're reading about optimizing arithmetic for embedded systems which are inefficient at division/multiplication operations so you use bit-shift operations for performance. Add three zeros to the original value and add that to the original value with one zero added to multiply by ten much faster than multiplying directly by ten.

 

[Captain]

One need not understand WHY the formula works to use the formula. I think you're just showing off with the whole 20:1 equals 3 degrees thorium behind the rule of thumb.

In the cockpit the pilot needs to know what the OP asked..how many feet per minute is going maintain the GP. Dividing by 2 and adding a zero is just easier than multiplying by 5.


By your logic a pilot needs to understand the carbon chain of AvgGas and how it releases energy when ignited in order to be able to add power to take off.

 

[cirrusmx]

It just seems that when you configure the airplane for an approach you almost always end up automatically with the correct VS.