Instrument pilot math....

[guardguy]

Can you guys share some of your techniques on quick IFR math... such as descent gradients or losing "x" amount of altitude in "x" miles. Or leading a DME arc, etc. I'm just looking for some basic ones that get you a ballpark figure and what different techniques are out there. Any quick IFR math would be appreciated, thanks!

[Pietrantonio]

Originally Posted by guardguy

Can you guys share some of your techniques on quick IFR math... such as descent gradients or losing "x" amount of altitude in "x" miles. Or leading a DME arc, etc. I'm just looking for some basic ones that get you a ballpark figure and what different techniques are out there. Any quick IFR math would be appreciated, thanks!

I know Jepp Charts have a decent rate to shoot for at specific Ground speeds while flying the published approaches.

Leading DME arcs...Say its a 10 DME Arc...I would lead it either by 9.5 DME FROM the VOR, or about 10.5 DME TO the VOR, etc...This usually works well, and then you can adjust your correction angle for wind.

Other than that, it just takes practice doing the math in your head. Or, if you have a E-6B, give that a try. It should help a lot with timing your routes, decents, etc..

Hope this helped a little.

[Micaoct]

For the decent one I just use the 3/6 rule and it works pretty well. Altitude you have to lose * 3, that gives your distance that you have to descend from and ground speed * 6 gives you the FPM that you have to maintain.

As for the DME Arc it really depends on the winds. During normal conditions leading it by half a mile works good but if you have a crazy wind you have to adjust. Yesterday we had to lead the arc by 1.5 miles, and this was in a PA28. We had 40 degrees of wind correction in holding, wind shear on final and moderate turbulence from surface to 3000. Fun day.

I think experience is the main factor that will help you figure stuff out quicker. Once you repeat the same thing a bazillion times then you start to get a pretty good idea and you know what you have to do.

[ppragman]

Originally Posted by guardguy

Can you guys share some of your techniques on quick IFR math... such as descent gradients or losing "x" amount of altitude in "x" miles. Or leading a DME arc, etc. I'm just looking for some basic ones that get you a ballpark figure and what different techniques are out there. Any quick IFR math would be appreciated, thanks!

IFR Math!! Yeah!!!

"x" altitude in "x" time?

simple 3 to 1, lets say that I'm at 10000 feet and I need to get down to 5000 feetI need to lose 5 grand! multiply that by 3:

5 x 3 = 15

It will take you 15 miles to get down if you use the right descent rate. How do you figure that out? Ground Speed. For a rough estimate, take your ground speed and multiply it by 10.

So...

150Kts x 10 = 1500

Now devide this by two:

1500 / 2 = 750

Descend at 750fpm should work great! A lot of people also use groundspeed times 7 to calculate , or

150kts x 7 = 1050fpm

which is a little steeper, which isn't a bad thing if you want to guarantee you make the restriction. Regardless, just make sure you make the crossing restriction.

As for leading a DME arc, what I use, and seems to work really well is groundspeed/100.

For a 172, with a ground speed of 100, use a mile, for a beech 1900 @ 250 knots, 2.5miles, works great!

For calculating your distance on the arc (for descent purposes) use the following:

2 x (radius of DME arc) x 3

This gives you a really rough circumference of the arc.

Divide this number by how much of the arc you're going arround, e.g. a fourth, sixth, half of the arc.

Check it out:

We're on the 12 DME arc into Bethel Airport. We've joined the arc at the 090 radial and we're flying north to the 360 degree radial to join the ILS for 180.

2 x (12DME) x 3 = 72 Knots all the way around, pretty simple

Now 090 to 360 is exactly 1 quarter of the arc. so, we devide 72 by 4 that's 18 nautical miles. (If you rounded up to 80NM, no problem you get 20NM on the arc)

How is this practical? Welllllllllll simple. Let's say that you're coming into bethel. On the ARC you can descend to 1800', but who wants to get down that low and battle the ice for 18 miles, when you could bomb out of the flight levels closer in and be descending on the arc?

Well, let's say you're at FL240, and you want to be at 1800' at the 360 radial on the arc. Lets round up 1800 to 2000, so you have to lose 22K feet. Multiply that by 3 you get 66, now, subtract 18 equals 48, if you want to hit two thousand at the top of the arc, start your descent about 48 miles out of the VOR. Perfecto, you'll roll wings level on the localizer at glide scope intercept and be ready to slow for approach if you're not a freight dawg.

That's just a hypothetical situation, and coming out of the flight levels may be more than you were interested in hearing, but that method works pretty dammed will in most scenarios, you've just got to vary the numbers for the situation. Also, what I said about trying not to fly the arc at the minimum is important. If you fly at the min in the winter months and you don't think you're going to break out, you'll ice up, if you wait and bomb it over, you'll have altitude and airspeed to play with if you need it, and you'll be in the ice less if you're on top at cruise. Just something to think about.

[guardguy]

Originally Posted by ppragman

IFR Math!! Yeah!!!

"x" altitude in "x" time?

simple 3 to 1, lets say that I'm at 10000 feet and I need to get down to 5000 feetI need to lose 5 grand! multiply that by 3:

5 x 3 = 15

It will take you 15 miles to get down if you use the right descent rate. How do you figure that out? Ground Speed. For a rough estimate, take your ground speed and multiply it by 10.

So...

150Kts x 10 = 1500

Now devide this by two:

1500 / 2 = 750

Descend at 750fpm should work great! A lot of people also use groundspeed times 7 to calculate , or

150kts x 7 = 1050fpm

which is a little steeper, which isn't a bad thing if you want to guarantee you make the restriction. Regardless, just make sure you make the crossing restriction.

As for leading a DME arc, what I use, and seems to work really well is groundspeed/100.

For a 172, with a ground speed of 100, use a mile, for a beech 1900 @ 250 knots, 2.5miles, works great!

For calculating your distance on the arc (for descent purposes) use the following:

2 x (radius of DME arc) x 3

This gives you a really rough circumference of the arc.

Divide this number by how much of the arc you're going arround, e.g. a fourth, sixth, half of the arc.

Check it out:

We're on the 12 DME arc into Bethel Airport. We've joined the arc at the 090 radial and we're flying north to the 360 degree radial to join the ILS for 180.

2 x (12DME) x 3 = 72 Knots all the way around, pretty simple

Now 090 to 360 is exactly 1 quarter of the arc. so, we devide 72 by 4 that's 18 nautical miles. (If you rounded up to 80NM, no problem you get 20NM on the arc)

How is this practical? Welllllllllll simple. Let's say that you're coming into bethel. On the ARC you can descend to 1800', but who wants to get down that low and battle the ice for 18 miles, when you could bomb out of the flight levels closer in and be descending on the arc?

Well, let's say you're at FL240, and you want to be at 1800' at the 360 radial on the arc. Lets round up 1800 to 2000, so you have to lose 22K feet. Multiply that by 3 you get 66, now, subtract 18 equals 48, if you want to hit two thousand at the top of the arc, start your descent about 48 miles out of the VOR. Perfecto, you'll roll wings level on the localizer at glide scope intercept and be ready to slow for approach if you're not a freight dawg.

That's just a hypothetical situation, and coming out of the flight levels may be more than you were interested in hearing, but that method works pretty dammed will in most scenarios, you've just got to vary the numbers for the situation. Also, what I said about trying not to fly the arc at the minimum is important. If you fly at the min in the winter months and you don't think you're going to break out, you'll ice up, if you wait and bomb it over, you'll have altitude and airspeed to play with if you need it, and you'll be in the ice less if you're on top at cruise. Just something to think about.

Thanks..... That's exactly what I was looking for actually. I'm currently flying a King Air 90, so the generic formulas that work for all planes is perfect! I know some more complex ways to get similar answers, but I knew there were some basic math formulas.

Thanks everyone!

[EvilGN]

2 x dist on arc x 3 is the same thing as dist on arc x 6

keep it simple!

as far as turning onto an arc, 1/2 of 1% of Ground speed is what we teach in the navy. at 120 kts GS, .6 dme, the faster you go, you need more lead time to turn, works for turning over VOR's as well, but dont forget to add altitude for that. ie at 15000 ft at 240 kts, you would use 1.2 DME plus 3 DME (altitude) for a total of 4.2 DME for a lead turn onto another radial.

[ppragman]

Originally Posted by EvilGN

2 x dist on arc x 3 is the same thing as dist on arc x 6

keep it simple!

as far as turning onto an arc, 1/2 of 1% of Ground speed is what we teach in the navy. at 120 kts GS, .6 dme, the faster you go, you need more lead time to turn, works for turning over VOR's as well, but dont forget to add altitude for that. ie at 15000 ft at 240 kts, you would use 1.2 DME plus 3 DME (altitude) for a total of 4.2 DME for a lead turn onto another radial.

I actually tried the navy way because I'd heard of this method. I found that at higfh airspeeds the angle of bank was too extreme sometimes. However that method does work well at slower airspeeds or in certain circumstances, but by all means try it out, there are so many ways to do ifr stuff.

As for why I split up the numbers I use that to remind my self that I'm using 3 as an approximation of pi. That way I remember to add a little fudge factor in, however, if u don't care about that, do what ever u like, just remember these are approximations and it will almost always be necessary to recompute and apply corrections as u go, nothing in aviation is gospel.

[EvilGN]

ah I see where you were going with that....and agreed, many ways to skin this cat.