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-   -   what is the accuracy reqd in DME arc (http://www.pprune.org/tech-log/455255-what-accuracy-reqd-dme-arc.html)

stubby1 21st Jun 2011 12:32

what is the accuracy reqd in DME arc
 
hi friends,

while doing a, say, 12DME arc approach, what are the range of DMEs authorized? and where is it mentioned ?

thanks:ugh:

PT6A 21st Jun 2011 12:55

When I did my training I had to do it within .5nm I think the IR pass/fail is 1nm.

Checkboard 21st Jun 2011 13:15

Instrument approach tolerances are recommended in ICAO PANS-OPS, and are specified for each country in their respective AIP.

http://www.pprune.org/tech-log/76882...tolerance.html
http://www.pprune.org/tech-log/22428...s-dme-arc.html

The UK-AIP is silent on the matter of DME arcs, and only specifies tolerances for ILS, VOR and NDB tracking:

UK AIP ENR 1.5 HOLDING, APPROACH AND DEPARTURE PROCEDURES
Quote:

3.15 Established

3.15.1 Aircraft are considered to be 'established' when they are within half full scale deflection for the ILS and VOR, or within 5
of the required bearing for NDB(L).
... compared to the Australian AIP ENR, which states:
Quote:

1.20.2 ... "Established" means being within half full scale deflection for the ILS, VOR and GPS, within 5 of the required bearing for the NDB, or within 2NM of the DME arc.

aterpster 21st Jun 2011 16:18

The general testing standard is +/- 0.5 (one-half) nautical mile. +/- 2.0 miles is unsatisfactory in my view. The PANS-OPS arc primary containment area is +/- 3 miles and the DME can have a total error of 0.25 n.m.

Checkboard 21st Jun 2011 17:15

I don't think the Australian standard is about maintaining the arc - that's pretty easy, I think it's about "As I am approaching the arc, when can I call my self "established" and thus descend in accordance with the procedure?"

So, with a 90 intercept onto the arc, you would start the turn at 220 odd knots at about 1.3 miles or so - and may commence descent (as you are within 2 miles).

aterpster 21st Jun 2011 17:43

Checkboard:

Quote:

So, with a 90 intercept onto the arc, you would start the turn at 220 odd knots at about 1.3 miles or so - and may commence descent (as you are within 2 miles).
Given that is the case, it is nonetheless poorly thought out and poorly worded. It should state "...within 2 miles of the arc..." not "+/- 2 miles."

And, two miles from the arc centerline is too far out to begin descent in any case. Descent should commence at the bisector of the turn onto the arc, just as in RNAV.

capt. solipsist 22nd Jun 2011 03:53

And remember the lead distance to turn into the arc at RATE 1: 10% of IAS

mig3 22nd Jun 2011 04:45

Quote:

Originally Posted by capt. solipsist
And remember the lead distance to turn into the arc at RATE 1: 10% of IAS

IAS? Ground speed is more appropriate. 10%? More like 1% I think.

Slasher 22nd Jun 2011 05:07

Lead dist = 1% GS. So 200kt = 2nm. Works too.

I've always flown a DME yark within +/- 0.5nm, at which
point to adjust the heading towards the station.

Checkboard 22nd Jun 2011 12:03

Actually,

The Radius of Turn = Vsquared / [g.tanθ]

Where θ is the angle of bank, and V the TAS, you would then need to take the time of the turn to account for the wind effect.

For a 30 AoB turn, this equates fairly accurately to:

Radius = [V/200]squared

Using the circular slide rule (which I carry in the aircraft):

---Radius-------GS
-----+-------------+----
-----GS-----------400

So, for;
  • 100 knots, Radius = 0.25 nm
  • 200 knots, radius = 1 nm
  • 250 knots, radius = 1.6 nm

... which is why I would start my turn at 1.3 miles or so at 220 knots .. but that's just me ;)

aterpster 22nd Jun 2011 13:09

Where to start the turn onto the arc and where to start the descent are two different, but related, issues.

theficklefinger 22nd Jun 2011 18:18

Gee whiz, it's a good thing you guys don't teach this stuff..

It's 1 mile inside or outside of the arc..there is no speed requirement other then airspace rules...so if under 10k, then 250kts would apply.

ATP standards, or professional standards may apply that once established, pro level would be to hold inside of a half mile in or out of the arc or better, depends on who's holding your feet to the fire.

aterpster 22nd Jun 2011 19:26

theficklefinger:

Quote:

It's 1 mile inside or outside of the arc..
Reference?

172_driver 22nd Jun 2011 19:45

FAA PTS says 1 nm wiggle room. I believe JAA ride was the same.

In my opinion 1 % of GS is too early, at my speeds around 140 kts. I start 1 nm from the arc, slightly adjust the turn rate slightly as appropriate.

Alteburger 22nd Jun 2011 20:07

Protected range is published in PANS-OPS. 2.5nm from DME ARC centreline gives full clearance (Primary Area). For the next 2.5nm, terrain clearance linearly tapers to zero (Secondary area).

Speed limits are subject to procedure limit speeds (or more stringent if published). Generally 240kts for initial approach and reducing for final approach. Limiting speeds for each flight phase are published in PANS-OPS.

Checkboard 22nd Jun 2011 20:12

Quote:

It's 1 mile inside or outside of the arc..there is no speed requirement other then airspace rules...so if under 10k, then 250kts would apply.
DME Arc procedures are almost invariably part of an instrument approach, so ICAO instrument approach limits after the IAF (and in the UK, the UK AIP limit of 180 knots) apply.

Please tell me YOU don't teach "this stuff". :ugh:

stubby1 23rd Jun 2011 05:27

still awaiting figure with legal / doc backing
 
Hi
I am still not sure what are the allowed limits while executing a DME arc.
Particularly interested for india.
thanks in advance

172_driver 23rd Jun 2011 06:27

What limit are you asking for then?

FAA Practical Test Standards: +/- 1 nm
PANS-OPS protected area: +/- 2,5 nm

So if you drift 1,5 nm off the arc your flying may be sloppy, but you are still within the protected area for the procedure.

mig3 23rd Jun 2011 08:10

Quote:

Originally Posted by Checkboard
The Radius of Turn = Vsquared / [g.tanθ]

[bows down]

Checkboard 23rd Jun 2011 10:40

Quote:

Originally Posted by stubby1
Particularly interested for india.

You can have a look at The AIP India - my own quick look couldn't find a reference. The next step would be to look at an Instrument Rating test form (an Indian one, obviously) which should specify the testing standard for the officer conducting the test.


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