what is the accuracy reqd in DME arc

[stubby1]

hi friends,

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

thanks

[PT6A]

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

[Checkboard]

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

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:

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

[aterpster]

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]

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 90º 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]

Checkboard:

So, with a 90º 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]

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

[Slasher]

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]

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 30º 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]

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

[theficklefinger]

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]

theficklefinger:

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

Reference?

[172_driver]

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]

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]

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".

[stubby1]

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]

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.

[Checkboard]

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.

[Matt101]

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

I found 0.5% of GS +0.1 always worked in practical and gives answers very close to the above formula in most normal speeds. 220x 0.005 +0.1 = 1.2 and your answer was 1.3 I believe. Just for speed on the go. Most instructors taught it.

Our JAA test tolerance was 1 nm though even the worst of students never let it much outside of 0.2 on a light aircraft. DME arc in a c172 is a lovely lazy way onto the ILS I thought...

[parabellum]

Used to do these regularly at Pago Pago as part of a base check, 2nm seemed to do the trick in a B737-200 at around 170-180kts. (Gravel Protect system fitted to u/c with speed limitation).