calculating distance along a DME arc

[TonyC]

I should probably know this, but why would anybody care what the distance they've gone around a DME arc is? I mean, is there any practical application for this?

Take a look at this approach to Casper, WY (Natrona County International): http://204.108.4.16/d-tpp/0602/00072VD3.PDFYou begin the approach at HETTY (DDY 137/29).First question: Suppose your airplane has a turn radius of 2 miles. When should you begin your right turn to intercept the DDY 204 Radial inbound in order to roll out on centerline, no wind? (How many radials along the 29 DME arc constitutes 2 miles?)Second question: You want to be at 8400 feet when you begin your turn to intercept the DDY 204 Radial, but you have to be at or above 10,300 feet until you pass NUNTE. What descent rate should you use to arrive at 8,400 feet at the point where you plan to begin the turn inbound? (Again, how much distance between two radials along the arc?)Third question: If you failed to plan your descent properly, and you can't land because you were too high when you broke out of the weather, do you have enough gas to try it again? .
I don't know what's going on here, but I've tried to use nice paragraphs and all, but it keeps cramming it all together in one little nugget. My apologies for it being difficult to read....

 

[ricecakecm]

Take a look at this approach to Casper, WY (Natrona County International): http://204.108.4.16/d-tpp/0602/00072VD3.PDFYou begin the approach at HETTY (DDY 137/29).First question: Suppose your airplane has a turn radius of 2 miles. When should you begin your right turn to intercept the DDY 204 Radial inbound in order to roll out on centerline, no wind? (How many radials along the 29 DME arc constitutes 2 miles?)Second question: You want to be at 8400 feet when you begin your turn to intercept the DDY 204 Radial, but you have to be at or above 10,300 feet until you pass NUNTE. What descent rate should you use to arrive at 8,400 feet at the point where you plan to begin the turn inbound? (Again, how much distance between two radials along the arc?)Third question: If you failed to plan your descent properly, and you can't land because you were too high when you broke out of the weather, do you have enough gas to try it again? .
I don't know what's going on here, but I've tried to use nice paragraphs and all, but it keeps cramming it all together in one little nugget. My apologies for it being difficult to read....

Apology accepted

I just plug it all into the magic boxes and do what the little green things tell me to

 

[B767Driver]

Take a look at this approach to Casper, WY (Natrona County International): http://204.108.4.16/d-tpp/0602/00072VD3.PDFYou begin the approach at HETTY (DDY 137/29).First question: Suppose your airplane has a turn radius of 2 miles. When should you begin your right turn to intercept the DDY 204 Radial inbound in order to roll out on centerline, no wind? (How many radials along the 29 DME arc constitutes 2 miles?)Second question: You want to be at 8400 feet when you begin your turn to intercept the DDY 204 Radial, but you have to be at or above 10,300 feet until you pass NUNTE. What descent rate should you use to arrive at 8,400 feet at the point where you plan to begin the turn inbound? (Again, how much distance between two radials along the arc?)Third question: If you failed to plan your descent properly, and you can't land because you were too high when you broke out of the weather, do you have enough gas to try it again? .
I don't know what's going on here, but I've tried to use nice paragraphs and all, but it keeps cramming it all together in one little nugget. My apologies for it being difficult to read....

That's good advice TonyC is giving. I don't know what other guys do...but on an instrument arrival...I always look at the distance between stepdown fixes and plan the rate of descent I will need to comply with each one. Most of the time the distance between fixes are right on the 3:1 'typical' descent profile. Sometimes they are not.

FWIW, on a dme arc...a procedure i've not flown in ages by the way...I don't use the turn radius formula...but use a rule of thumb. Lead your turn on the arc to intercept the final approach course by a dme that is 10% of your groundspeed. If you're doing 180kts...lead the turn by 1.8 dme. Has always worked for me...and lets me keep as much of my brain power where it needs to be...on flying the gages. If you're like me...where brain power available is close to brain power required...this gets important!

 

[aloft]

Lead your turn on the arc to intercept the final approach course by a dme that is 10% of your groundspeed. If you're doing 180kts...lead the turn by 1.8 dme.

Of course, this is only useful for intercepting the final approach course if you're flying with GPS and there's a waypoint where the arc and the final approach course meet. For those of us without all the gadgetry, you need to guesstimate a lead radial at which to start your turn. Just how many degrees prior to the final approach course that lead radial needs to be is why you need to be able to calculate distance around an arc. If your turn radius at your approach speed is .75 nm (4500 ft) and you're on a 10 nm arc, using the 100' per degree per mile rule of thumb, you should start your turn to intercept the final approach course when you reach the radial 4.5 degrees prior to the radial defining the final approach course. (4500' turn radius divided by 100 divided by 10 = 4.5 degrees)

Most modern GPS systems like Ricecake's dual 530s will anticipate the turn and tell him when to begin it, which is why his brain has turned to mush.

 

[Alchemy]

Even in the RJ we still use the lead in radial when turning to the final approach course off an arc. Most of our Mexican plates have published lead ins about 10 degrees offset from the final approach course. They work pretty well.....the accuracy of Mexican VOR's on the other hand, well that's a different story.

 

[B767Driver]

Of course, this is only useful for intercepting the final approach course if you're flying with GPS and there's a waypoint where the arc and the final approach course meet. .

You can't fly a dme arc without dme. It's perfectly useful for getting established on the arc...and intercepting final you'd need to use the lead radial.

 

[bluelake]

I should probably know this, but why would anybody care what the distance they've gone around a DME arc is? I mean, is there any practical application for this?

TonyC's practical application seems to have answered this (and Tony.. apoligies for not seeing your clear reply in the earlier thread..) and as well, it is a question that has been recently asked by an interview board for which I get to be in front of in two weeks. Is that practical enough???

 

[B767Driver]

As a matter of practical application...I use the 60:1 rule most often when using the weather radar. The weather radar beam is 3 degrees wide...so a weather return 60 miles ahead is at least 3 miles or 18000' tall. If you can find the zero tilt position for your altitude...then note the change in upward radar tilt...you can estimate how high above you the thunderstorm tops are.

 

[ricecakecm]

TonyC's practical application seems to have answered this (and Tony.. apoligies for not seeing your clear reply in the earlier thread..) and as well, it is a question that has been recently asked by an interview board for which I get to be in front of in two weeks. Is that practical enough???

I guess that's practical enough. I've gone through 4 pilot interviews and gotten hired 4 times, a bunch of checkrides, and a bunch of writtens, and never been asked to figure out how far I've gone around an arc (or at least I don't remember being asked).

Good luck on your interview.

 

[bluelake]

I guess that's practical enough. I've gone through 4 pilot interviews and gotten hired 4 times, a bunch of checkrides, and a bunch of writtens, and never been asked to figure out how far I've gone around an arc (or at least I don't remember being asked).

Good luck on your interview.

thanks. hopefully go well. I will be the DME EXPERT...