Glideslope Computations

troopernflight

troopernflight

Well-Known Member
Does anyone have any easy tricks to solve these sorts of problems:
3 percent glide slope, figure required fpm rate of descent at 90 knots, from OM to .....where ever.
or what airspeed would be required to descend 500 fpm on 3 percent glideslope from OM to DH.
So forth and so on. I would appreciate it.
 
Take your speed and drop a zero. Divide by two. Multiply by 100. That is your rate of descent in fpm.

90 kts. 9/2 = 4.5. 4.5 * 100 = 450fpm.
 
B767Driver said:
Take your speed and drop a zero. Divide by two. Multiply by 100. That is your rate of descent in fpm.

90 kts. 9/2 = 4.5. 4.5 * 100 = 450fpm.
Click to expand...

Why not just take the speed, divide by two, multiply by ten?

90 kts.
90/2 = 45
45 X 10 = 450 fpm.

:confused:
 
Easy enough, thanks. Are there any other comps that would be useful to know for the instrument test? I don't know if I've run into all possible questions on the practice tests I've taken. Can you tell I have a math anxiety issue? :)
 
Here's some good ones:

VDP=HAT/GS (Gus wears a HAT).

Miles/minute =Groundspeed/60 (so a 90 knot groundspeed is 1.5 Miles/minute)

VVI= Glideslope*miles/minute*100 (So from the example above- 3 degrees *1.5*100=450 feet/minute)

This last one may seem like more trouble than it's worth in light of the formulas that B767 and SteveC posted, but the formulas they posted only work for a 3 degree glideslope.. The one above will work for any glideslope. In addition, it's the same formula for rate of climb. Let me demonstrate:

Suppose you are flying at 120 knots, and you want to maintain a climb rate of 1000 fpm. How much pitch change do you need, from level flight?

You are going 2 miles/minute, so by rearranging the formula you get Glideslope=VVI/(miles/minute *100). VVI we want is 1000. Divided by 200 gives us 5. So if we increase pitch by 5 degrees from level flight (and add sufficient power to maintain the current airspeed) we will get a 1000 fpm climb.

Another application. You are planning on doing a 5 degree visual descent for an assualt landing to an austere field in a C-130, surrounded by insurgents. You've got to get the large airplane stopped in a minimal amount of distance, which is why the steep angle of descent for the assualt landing. We'll assume that 90 knots is the approach speed. So for a 5 degree descent at 90 knots, what is the VVI you can expect?

90 knots=1.5 miles/minute.
1.5*100= 150

5*150= 750 fpm.

But wait, 90 knots is way too slow. You determine the approach speed should be 150 knots.

150 knots=2.5 miles/minute
2.5*100=250
5*250=1250 fpm.
 
fish314 said:
Here's some good ones:

VDP=HAT/GS (Gus wears a HAT).

Miles/minute =Groundspeed/60 (so a 90 knot groundspeed is 1.5 Miles/minute)

VVI= Glideslope*miles/minute*100 (So from the example above- 3 degrees *1.5*100=450 feet/minute)

This last one may seem like more trouble than it's worth in light of the formulas that B767 and SteveC posted, but the formulas they posted only work for a 3 degree glideslope.. The one above will work for any glideslope. In addition, it's the same formula for rate of climb. Let me demonstrate:

Suppose you are flying at 120 knots, and you want to maintain a climb rate of 1000 fpm. How much pitch change do you need, from level flight?

You are going 2 miles/minute, so by rearranging the formula you get Glideslope=VVI/(miles/minute *100). VVI we want is 1000. Divided by 200 gives us 5. So if we increase pitch by 5 degrees from level flight (and add sufficient power to maintain the current airspeed) we will get a 1000 fpm climb.

Another application. You are planning on doing a 5 degree visual descent for an assualt landing to an austere field in a C-130, surrounded by insurgents. You've got to get the large airplane stopped in a minimal amount of distance, which is why the steep angle of descent for the assualt landing. We'll assume that 90 knots is the approach speed. So for a 5 degree descent at 90 knots, what is the VVI you can expect?

90 knots=1.5 miles/minute.
1.5*100= 150

5*150= 750 fpm.

But wait, 90 knots is way too slow. You determine the approach speed should be 150 knots.

150 knots=2.5 miles/minute
2.5*100=250
5*250=1250 fpm.
Click to expand...

Cool stuff, Fish. Thanks for sharing that!
 
Here's a formula that I learned a couple of years ago that I use probably more than any other (except for crossing restrictions)...it pertains to calculating the crosswing component.

1. Take the degrees of crosswind and divide by 100. (just put a decimal in front of the number.
2. and add .2.
3. now multiply this number by the total wind velocity.

Example....40 degree crosswind with total wind velocity of 20kts. How much direct crosswind component?

.4 + .2 = .6 *20 = 12 kts of direct crosswind.

It's not 100% accurate but is within a knot.
 
B767Driver said:
Here's a formula that I learned a couple of years ago that I use probably more than any other (except for crossing restrictions)...it pertains to calculating the crosswing component.

1. Take the degrees of crosswind and divide by 100. (just put a decimal in front of the number.
2. and add .2.
3. now multiply this number by the total wind velocity.

Example....40 degree crosswind with total wind velocity of 20kts. How much direct crosswind component?

.4 + .2 = .6 *20 = 12 kts of direct crosswind.

It's not 100% accurate but is within a knot.
Click to expand...

That's a great one too! It's the "20 percent rule". Basically if you add 20 to the degrees off, and use that as a percentage, you get an estimate of crosswind.

So 40 degrees off, plus 20 is 60. 60 percent of the total velocity is a guesstimate of the crosswind component. (which is exactly what B767 did).


Incidently, if you don't like that one, there is also the "clock method" for crosswind. Basically you imagine where on a clock the degrees off would fall, then take that percentage of the total velocity as the crosswind velocity.

So for example 40 degrees off, 20 knot wind works out like this:

From the top of the clock to 40 (minutes) is 2/3rds of the clock face. 2/3rds of 20 knots is 13.3 knots of crosswind.
 
Stuff like this is why I love JC.

Thanks guys...I've been making little notes and stuff.

I'm one of the dorks with a kneeboard (I know, I know) but it's just handy reference material in-flight. I keep adding pages to it, including checklists for the different airplanes I fly, and now I'm going to add a page of little equations...
 
killbilly said:
Stuff like this is why I love JC.

Thanks guys...I've been making little notes and stuff.

I'm one of the dorks with a kneeboard (I know, I know) but it's just handy reference material in-flight. I keep adding pages to it, including checklists for the different airplanes I fly, and now I'm going to add a page of little equations...
Click to expand...

Not dorky at all.....its called being a professional aviator :rawk:
 
killbilly said:
Stuff like this is why I love JC.

Thanks guys...I've been making little notes and stuff.

I'm one of the dorks with a kneeboard (I know, I know) but it's just handy reference material in-flight. I keep adding pages to it, including checklists for the different airplanes I fly, and now I'm going to add a page of little equations...
Click to expand...

All the guys I fly with, and myself, have some sort of cheat sheet taped to the clipboards we carry...
 
I generally fly with two boards, or at least one (my trusty 9G) and a leg strap for plates. I keep a pad of blank paper for notes on one, and it has ringed plastic inserts where I put all my checklist sheets and "blue brains" (local/useful freqs, course rules, diagrams, etc). On the other side I keep approach plates, charts, or if flying VFR I will make up a little mission card with turn points, useful frequencies and other little notes. Local area flights I normally stow most of this stuff and just have the one notepad and checklists. If you haven't realized it yet, you will soon realize how key cockpit organization is when things get busy. Find something that works for you and stick to it every time.
 
troopernflight said:
I know it's probably a little advanced for most of us, but would you mind sharing some stuff you have on your clipboard? Thanks.
Click to expand...

I don't think it's advanced for you. I think you could glean some good ideas to help you sift through what's important to you.

On the classic, I just carried the Feet to Meters conversion chart since we did short routes.

For the new whip, I'll have to go out and do it to see what I need gouges for. I'm thinking Oceanic WX diversion and Driftdown gouges, since those are new. As for what else, I'll prolly look at what my compadres have on theirs. I focus on stuff that I would use alot and want to cross check a reference, and stuff that's in the book, you don't do alot but would like the info right close.
 
Some more "quickies"

Your TAS or GS is your mach multiplied by 6 (ie .80 = 480 knots TAS/GS)
Every .1 mach is 60 knots or every .01 mach is 6 knots.
For a 3 degree descent multiply your mach by 3 and that is your vertical speed, correct for headwind/tailwind (.75 mach with a 30 knot tailwind = .81 mach * 3 ~ 2400 fpm)

So on and so forth.
 
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