DME arcs and 3:1

[RichardRambone]

Could someone explain to me the 3:1 rule and how to figure distance between points on a DME arc. Did a search with no luck and I have an interview coming up. Also whats difference b/w FAF and FAP? Any info would be fantastic.

 

[BobbyBiplane]

Could someone explain to me the 3:1 rule and how to figure distance between points on a DME arc. Did a search with no luck and I have an interview coming up. Also whats difference b/w FAF and FAP? Any info would be fantastic.

What part of Pi did you not learn in high school?

 

[Amish RakeFight]

DME / 60 X degrees


For instance:

12 DME Arc

between radials 010 and 090

12/60 = .02 X 80 = 16 miles


So if you've maintainedthe 12 DME arc from the 010 radial to the 090 radial, you travelled 16 miles.

 

[ohplease!]

Could someone explain to me the 3:1 rule and how to figure distance between points on a DME arc. Did a search with no luck and I have an interview coming up. Also whats difference b/w FAF and FAP? Any info would be fantastic.

one word: FMS (maybe thats three words).... sorry, wish I could help.

 

[crxpilot]

Could someone explain to me the 3:1 rule and how to figure distance between points on a DME arc. Did a search with no luck and I have an interview coming up. Also whats difference b/w FAF and FAP? Any info would be fantastic.

Google 101:

dme arcs: http://stoenworks.com/Tutorials/IFR, VOR-DME Approaches.html

FAF and FAP: http://www.flightsimaviation.com/aviation_theory_15_Instrument_Approach_Procedures_part_1.html

3:1 rule: http://www.avtrial.com/descent-planning.html

 

[Amish RakeFight]

3:1 Rule

Basically, you multiply the altitude you need to lose by 3.

20,000' down to 5,000' = 15,000' to lose.

3 X 15 = 45

45 miles out you begin a descent.
(It's best to add a few miles as a fudge factor and for any level off at 10K for 250 below)

For descent rate, cut your ground speed by 2
400 knots / 2 = 200 or 2000 FPM

 

[RichardRambone]

Thanks guys. I'll add it to the file.

 

[Brett Hull]

...how to figure distance between points on a DME arc.

And you would need to know this in the real world because???

 

[Americam]

And you would need to know this in the real world because???

You don't, but for some reason some FAA inspectors still ask this DME-arc-distance crap on instrument checks (got 'em on both my instrument checkride and my CFI-I checkride.) I can't imagine any airline interviews would ask this stuff.

 

[vectorvictor]

Are you guys talking about airplanes or a recipe for sugar cookies?

 

[Dumb Pilot]

And you would need to know this in the real world because???

Not knowing this is what created the Helena debacle for PCL a few years ago. HLN doesn't have radar coverage and the northern arc is not on the FMS so folks had to shoot this arc on green needles. Lack of very basic instrument knowledge like this almost created two real world holes on the ground.
Don't loose your basic instrument skills folks, never know when you will need them in the real world

 

[flyinghunter]

Not knowing this is what created the Helena debacle for PCL a few years ago. HLN doesn't have radar coverage and the northern arc is not on the FMS so folks had to shoot this arc on green needles. Lack of very basic instrument knowledge like this almost created two real world holes on the ground.
Don't loose your basic instrument skills folks, never know when you will need them in the real world

Flying an arc and knowing how long it is are two seperate things.

But to answer the question about FAF and FAP, the fix is noted by the 'x' or whatever it's called on the plate and applies to nonprecision approaches. FAP is whenever you're established inbound within the required distance on a terminal VOR/NDB approach. It also marks the beginning of the final approach segment on an ILS. It's where the lowest glide slope intercept altitude and the glideslope intercept, not the same as the 'x' as that applies to the localizer. In other words, if you're established inbound on a terminal approach, you've intercepted the glideslope at the lowest published altitude or passing an 'x' on any nonprecision approach and the weather goest to crap you can still continue.

 

[CrewDawg]

60/Distance from station = Radials per NM. So on a 10 DME you will travel 6 radials per mile. Multiply that by your turn radius and you get a lead radial for a 90 degree intercept. 1/2 of that for a 60 degree and 1/3 for a 45 degree intercept.

Real world - Most likely never. Flying the Tweet with avionics from 1955 I'll take every bit of help I can get!

 

[Dumb Pilot]

Flying an arc and knowing how long it is are two seperate things.

Knowing how long you will be on the arc (IE knowing how long it is)to determine time available for step down to the procedure altitude is very much part of flying the arc.

 

[FmrFreightDog]

Knowing how long you will be on the arc (IE knowing how long it is)to determine time available for step down to the procedure altitude is very much part of flying the arc.

You are so correct. But, then again, in the age of FMS and reset jets, airmanship is, unfortunately, becoming a lost art. I have a rant about this entitled "RJs hitting mountains, lakes, and cornfields at alarming rates in about 3 years" but nobody seems to want to listen to me........

 

[MusicMan]

Very simply, at 60 miles from the dme fix, each degree is equal to 1 mile. Using simple ratios, at 30 miles, each degree is equal to .5 miles. At 15 miles, each degree is worth .25 miles. Use these ballparks and you should be alright. The FAA likes to ask these questions, but they aren't known for asking the distance on a 17 mile dme arc. It should be a nice round number which is divisible by 60. Keep in mind, they are still FAA. They aren't geniouses by anybody's account!

 

[91100 100 set]

groundspeed x descent angle x 1.77 will give you the exact descent rate in fpm. Just remember your GS will change in a long descent as your TAS and winds change in the descent.

 

[mhermann596]

Not knowing this is what created the Helena debacle for PCL a few years ago. HLN doesn't have radar coverage and the northern arc is not on the FMS so folks had to shoot this arc on green needles. Lack of very basic instrument knowledge like this almost created two real world holes on the ground.
Don't loose your basic instrument skills folks, never know when you will need them in the real world

How about the fact they were flying them at over 300 knts. 250 above 10 right. Even if you are on a segment of the approach.

 

[Amish RakeFight]

Real world or not, every instrument pilot, CFII and especially those who fly professionally in FMS equiped aircraft should be able to do these basic calculations. It's part of being a competent professional pilot. Don't skip steps in learning your trade whether you'll utilize them or not.

 

[JohnnyP]

......

And you would need to know this in the real world because???

Go check out the government/DOD plate for Delhi, India (VIDP), ILS/DME or VOR/DME ARC RWY 28.

What was that about dme arcs?.....once you get outside the lower 48, you might have to fly one (not nessecarily in Delhi, but the plate looks intimidating nevertheless).