DME for ILS Approach

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mikecolin
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DME for ILS Approach

Unread post by mikecolin » Tue May 07, 2019 7:36 am

Morning Guys,

a quick question hoping some of you experienced IF guys can answer.

We are looking to purchase a new Aircraft (Cirrus SR22) when going through the factory fitted extras it seems the DME is not included as standard, i assume this is because in the USA they typically fly the RNAV approach as a standard.

Here in SA many fields still only have the ILS approach and when looking at if it is worth having a DME i see it shows the a DME is a requirement to fly an ILS in IMC conditions.

I am not instrument rated yet but i know you can use the GPS distances on the ILS approach but is it legal, what would insurance say if an accident happening during an ILS approach in IMC and you did not have a DME but using the GPS for distance.

so my question is although you can fly an ILS using GPS is it legal according to the CAA?

its almost impossible to get hold of someone at the CAA to ask this question so turning to trusty AVCOM :-)

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Re: DME for ILS Approach

Unread post by Multirotordronepilot » Tue May 07, 2019 8:46 am

mikecolin wrote:
Tue May 07, 2019 7:36 am
Morning Guys,

a quick question hoping some of you experienced IF guys can answer.

We are looking to purchase a new Aircraft (Cirrus SR22) when going through the factory fitted extras it seems the DME is not included as standard, i assume this is because in the USA they typically fly the RNAV approach as a standard.

Here in SA many fields still only have the ILS approach and when looking at if it is worth having a DME i see it shows the a DME is a requirement to fly an ILS in IMC conditions.

I am not instrument rated yet but i know you can use the GPS distances on the ILS approach but is it legal, what would insurance say if an accident happening during an ILS approach in IMC and you did not have a DME but using the GPS for distance.

so my question is although you can fly an ILS using GPS is it legal according to the CAA?

its almost impossible to get hold of someone at the CAA to ask this question so turning to trusty AVCOM :-)

Mike

Hi Mike, I'm not 100% sure exactly what you are asking or rather why to be more accurate. Most aerodromes have GNSS approaches which can be flown on GPS. Or as long as the RNP is within limits this is what will be used down to DA. If the RNP is below 0,3NM accuracy then I suppose your option comes into play.

But many operators have been flying ILS approaches (VOR/DME) without a DME for a fairly long period already. Is it legal, well I'm not sure about that. Let me find out .
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Re: DME for ILS Approach

Unread post by Only » Tue May 07, 2019 10:51 am

I do not think it is legal because it says DME not DME/GPS.

That is my short answer to you and I think that is what CAA or your insurance will tell you if something happens.

Now the long answer to explain why it is required and why there is a difference between the two.

When on the ILS at a certain given DME(from the ILS/VOR) you need to check that your altitude is the same as that on the plate for that point. This is to check that your Altimeter is reading correct and you QNH are set correctly.

Instrument rated pilot should know that DME measures slant range and GPS your distance along the ground so there is a slight distance between the two. Now if the plate states that you must use the DME from the ILS(which most do) you can not set that point accurately on your GPS. This means with this two discrepancies you will not be able to check your altitude accurate which is quite unsafe that close to the ground.

Just to add on to the use of DME. If you fly through someone's airspace that does not have radar(like FAKM, FAUP, FABL, FAPP, ect) they will ask you your radial/DME to keep you clear of traffic and now if you GPS is set to you destination you will have to reset that tot he VOR every time they ask.

I hope this help.
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Re: DME for ILS Approach

Unread post by Multirotordronepilot » Tue May 07, 2019 12:01 pm

Only wrote:
Tue May 07, 2019 10:51 am
I do not think it is legal because it says DME not DME/GPS.

That is my short answer to you and I think that is what CAA or your insurance will tell you if something happens.

Now the long answer to explain why it is required and why there is a difference between the two.

When on the ILS at a certain given DME(from the ILS/VOR) you need to check that your altitude is the same as that on the plate for that point. This is to check that your Altimeter is reading correct and you QNH are set correctly.

Instrument rated pilot should know that DME measures slant range and GPS your distance along the ground so there is a slight distance between the two. Now if the plate states that you must use the DME from the ILS(which most do) you can not set that point accurately on your GPS. This means with this two discrepancies you will not be able to check your altitude accurate which is quite unsafe that close to the ground.

Just to add on to the use of DME. If you fly through someone's airspace that does not have radar(like FAKM, FAUP, FABL, FAPP, ect) they will ask you your radial/DME to keep you clear of traffic and now if you GPS is set to you destination you will have to reset that tot he VOR every time they ask.

I hope this help.

I came to the same conclusion as you with regards to legal to the requirements. Although the GNS approach will indicate an altitude at a point you are correct in that accuracy between points would be questionable.
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Re: DME for ILS Approach

Unread post by fstols » Wed May 08, 2019 7:30 am

In addition to the slant distance error already mentioned, a more significant consideration is that the GPS distance may be referenced to the aerodrome reference point whereas the DME is the the actual distance from the DME station. And this difference may be quite significant in some cases.
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Re: DME for ILS Approach

Unread post by Baggie » Wed May 08, 2019 3:39 pm

Just had a quick look at some of the ILS approach plates in SA. The majority of them require either VOR/DME or DME in order to fly the approach (probably something to do with the withdrawal of the NDB’s, no more marker beacons).

Practically, using the RNAV/GNSS approaches results in a higher DA/H or MDA/H. Generally around 150’-250’ higher than the ILS minima. You can get lower minima but that requires special aircraft and aircrew certification. Depending on what you intend using the aircraft for will determine the level of instrumentation you will require.


SACAA recently withdrew all the RNAV approaches for Cape Town (not sure if they have been reinstated yet)
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Re: DME for ILS Approach

Unread post by Iceberg » Thu May 09, 2019 2:52 pm

I agree that when the plate states 'DME required' - that is what is legally required.

Lets look at the math though:

First the tolerances of DME and other navaids: (Credit to the airline pilots forum)

Fix Tolerance Factors

If no radio nav aid is available or other factors constrain the use of such aids, positions and turning points may be referenced by other means. e.g.

i) When leaving an airway, an aircraft may be given radar vectors to a point marking the beginning of the arrival route.

ii) The start of the final segment of a CATI ILS may be marked by a 75 Mhz marker beacon.

The fix tolerance factors of such aids are:

• Surveillance Radar. Radar fix accuracies are based on radar mapping accuracies, azimuth resolution, flight technical tolerance, controller technical tolerance and the speed of the aircraft.

• Terminal area radar (TAR) within 20 NM: Fix tolerance is ± 0.8 NM.

• En-route surveillance radar (RSR) within 40 NM: Fix tolerance is ± 1.7 NM.

• DME Fix tolerance is ± 0.25 NM + 1.25% of distance to the antenna.

• 75 MHz Marker Beacons: Typically fix tolerance is +/- 0.45 nm at 6000 ft and 0.2 nm at 1000 ft.

• VOR Overhead: Fix tolerance overhead a VOR is based upon a cone of confusion 50º from the vertical.
At 3000 ft the accuracy of the "on top" is given by:

2 x tan 50° x 3000 = 2 x 1.19 x 3000 = 7140 ft or 1.17 nm

• NDB Overhead: Fix tolerance overhead an NDB is based upon an inverted cone of ambiguity extending at an angle of 40º either side of the facility.
At 3000 ft the accuracy of the "on top" is given by:

2 x tan 40° x 3000 = 2 x 0.84 x 3000 = 5040 ft or 0.83 nm

Lets assume you are 6NM horizontally from the DME fix flying the ILS.
At a slant angle of 3 degrees the slant distance is 6.0082 NM - this is the DME distance
That is only 0.0082NM difference - 15.25 m. A DME only outputs distance to 0.1NM - 185m accuracy, and then the tolerance is +/- 0.25NM
or +/- 463 m!. GPS is accurate to 30m or less.

I always make my final fix the VOR if the DME is co-located and cross check the distances to go. If RAIM checks out, I tend to trust the GPS a lot more than the DME receiver! As for checking altitude at a certain 'DME', the difference in altitude would be less than 3 feet when checking between DME and GPS distance, assuming both were 100% accurate.

Edit: I also found this reference to DME tolerances:
DME systems that comply with the TSO requirements meet the RTCA DO-189 standard. It has an accuracy requirement of +/- 0.17 NM
So still +/- 315m!

Edit: Using GPS in lieu if DME has been legal for a long time in the USA - t.'s and c's apply:
https://www.aopa.org/advocacy/advocacy- ... of-dme-adf

I guess we are just behind the trends - no surprise there....
Last edited by Iceberg on Fri May 10, 2019 8:28 am, edited 1 time in total.
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Re: DME for ILS Approach

Unread post by Multirotordronepilot » Thu May 09, 2019 3:15 pm

My word Karl..... do you know how to put a point across the table or what =D> .

In a sense we agree, not legal but acceptable and practiced very often.

However Only, Fstlos and Baggie have valid points in their view as well.

I can only think Mike must be even more conflicted now than when he first asked the question.

Mike you asked for Avcom.... you got Avcom :lol:
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Re: DME for ILS Approach

Unread post by Jayson v Schalkwyk » Fri May 10, 2019 1:52 pm

When looking at a specific approach designation (Jepp, top right corner), your aircraft is required to have all required nav equipment and must be functional.
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Re: DME for ILS Approach

Unread post by MemyselfandI » Sat May 11, 2019 9:51 am

so my question is although you can fly an ILS using GPS is it legal according to the CAA?
To answer your question, yes it is. BUT, there's always a but...if the ILS requires a DME then it is not legal because you will not have any protection from the legally promulgated approach, ie your insurance will not pay in the event of a mishap. So read the fine print on the plate.
Get the DME 8)
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Re: DME for ILS Approach

Unread post by Hop Harrigan » Tue May 14, 2019 6:36 pm

It gets worse...some RNAV approaches (such as FALA) can’t even be legally flown without DME as the missed approach requires a turn at a fixed DME distance from the MAP.
Time SA got into the 21st Century Mr CAA?
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Re: DME for ILS Approach

Unread post by tansg » Wed May 15, 2019 7:44 am

Hop Harrigan wrote:
Tue May 14, 2019 6:36 pm
It gets worse...some RNAV approaches (such as FALA) can’t even be legally flown without DME as the missed approach requires a turn at a fixed DME distance from the MAP.
Time SA got into the 21st Century Mr CAA?
Hop
A common misconception obvious here. Instrument procedures are NOT designed and owned by the CAA. They are the property of the airport operator and their appointed service providers or by the airline that commissioned them. The CAA's role is to validate the compliance to the CARs, CATs, ICAO Doc 8168 and safety protocols. Flight procedures are designed by ATNS Flight Procedure Department and independent contractors for customers. The design process and oversight is governed by CAR/CATS Part 173 and the Manual of Procedures and Standards. CAR/CATS 177 additionally deals with the Aeronautical Charts, which is still being performed by CAA despite ATNS previously agreeing to take over this function (about 10 years ago) as it is a service provider function.

So the charts for Lanseria were designed by ATNS and are owned by Lanseria. when these particular procedures were submitted to CAA, this dependency on an "outside" navigation source was pointed out to the designer. The procedures however complied with the CAR/CATS, ICAO Doc 8168 and the safety protocols but were definitely not best practice. There were reasons why the procedure was designed as such but in my opinion did not warrant the use of another dependent navigation system but it could not be rejected as the procedure had complied with all regulatory requirements. Is it a good design, my opinion no. So bottom line go complain to Lanseria and ATNS this is not CAA's fault. CAA was actually accused of being obstructive when this concern was raised with the designer. Go figure.

As to the use of the GNSS distance in place of the DME, the distance measured will have to be measured from the location of the DME antenna so the DME would have to be a waypoint on the system, how will this distance be displayed on the instruments whilst it is set up for the ILS, is it easily visible on your primary display, the difference in slant range vs GNSS distance shouldn't have a significant impact except at larger distances (haven't done the math to see what distance it becomes relevant), further there are other GNSS operating factors to take into account of. So you can see this is not as straight forward as Aunty Jane at SAA thought when she previously brought up this subject of replacing DME with GNSS distance. Further the fundamentals of the design of RNAV and conventional procedures are so different in nature in order to authorise the use of GNSS in place of DME or vice versa requires a full safety assessment including the redesign of the procedure fixes to ensure safety tolerances of the design are adhered to. About 7 years ago it was common practice to certify aircraft to fly conventional procedures with GNSS as it was more accurate. This went on until Norwegian had a major incident where one of their aircraft using GNSS navigation went outside the design safety parameters of a VOR/DME procedure (if I remember right) due to this fundamental design philosophy difference and this problem became obvious. Now in order to be able to fly a particular procedure with GNSS instead of the conventional equipment requires a procedure specific approval.

Hope this clarifies some things.
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