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Overlay Approaches

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Overlay Approaches

Old 9th Oct 2014, 14:10
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Overlay Approaches

Flying into an airfield in Africa the other day

Was a DME Arc onto the ILS

Approaching the field I noticed the DME was U/S. Pointed this out to the captain, he acknowledged it, mentioned it to ATC, no further discussion.

GPS was Primary and the aircraft few the arc perfectly (as it was programmed to do) intercepted the ILS landed, went to the hotel. Didn't think twice about what we had just done.

A while later I realized that we had just done a DME Arc with no DME! Which led me to a discussion with another captain; our reliance and expectation of our FMS is such these days that we forget that except for RNAV approaches all other NPA are not meant to be flown by the FMS alone. So long as the chart says, VOR, DME, or NDB the said ground aid should be fully serviceable, even if we are going to let the box manage. Technically and legally speaking that is. I think.

Would anyone like to comment on this, i.e. the permissibility or impermisibility of flying approaches, or parts of approaches without the required ground aids working. Pros and Cons, the reality vs. what the book says.
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Old 9th Oct 2014, 14:37
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FMS/FMC fly ARINC 424-XX leg type, it doesnot not use ground stations, the only prerequisite is that the stations (DME,VOR,NDB) have to be in the NavDataBase.
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Old 9th Oct 2014, 14:51
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Yes, I know, that's how we managed to pull off the approach without a hitch, but legally speaking, can we fly the approach if the ground aid is U/S? I don't think we can, but then I'd like someone with a little more experience to comment on this, especially with regards the reality of commercial aviation vs. what the book says.
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Old 9th Oct 2014, 15:35
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Airmann:

Yes, I know, that's how we managed to pull off the approach without a hitch, but legally speaking, can we fly the approach if the ground aid is U/S? I don't think we can, but then I'd like someone with a little more experience to comment on this, especially with regards the reality of commercial aviation vs. what the book says.
If a VOR or NDB approach is not coded in the database as an approach, the FMS will remain in terminal mode sensitivity, alerting, and monitoring. You can force the sensitivity to 0.30, but you will still be in terminal mode.
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Old 9th Oct 2014, 15:52
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Didn't we just have a nice, exhaustive thread on overlay approaches? What happened to it?
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Old 9th Oct 2014, 16:09
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I don't think we can, but then I'd like someone with a little more experience to comment on this, especially with regards the reality of commercial aviation vs. what the book says.
Reality should match the book but it doesn't always.

I'll give an example of an NDB approach in Canada that is NOT approved for a GPS overlay.

Example approach is the NDB RWY 01 into CZTM. For final approach track guidance you're required to use the NDB raw data as your primary navigation source. This airport is located where there are magnetic disturbances in the area. If you relied entirely on your RMI, you could be way off your inbound track which is not a problem from an obstacle standpoint because they've taken that into account and given you a very high MDA. It can throw you off though when you're expecting the runway to be a certain way in the windscreen when you break out and it's not.

Anyways... I would fly that approach using GPS as my primary navigation and have the NDB as a back up. It's silly to think that the NDB is going to provide you better navigational accuracy... it will most certainly not!! I'll OBS the final approach course on the NDB and use that to fly the approach.

If the NDB is NOTAMd to be u/s then I won't do the approach. Following the rules makes your life a lot simpler. There is no decision making here - you're not doing the approach. Even though we all know that it's much safer to fly it based off the GPS anyways - even with en route RAIM and CDI sensitivity!

I should clarify that I do still monitor the morse code and do timing for approaches where an overlay is not approved.

As far as why you can't do an overlay approach without it being published has to do with the approach needing to be in the database.

To add to what aterpster said:

If a VOR or NDB approach is not coded in the database as an approach, the FMS will remain in terminal mode sensitivity, alerting, and monitoring. You can force the sensitivity to 0.30, but you will still be in terminal mode.
You can force the CDI to 0.3 but you can't force the RAIM to go to 0.3 which is what you require to descend to altitudes in the final approach segment.

When an overlay approach is made, sometimes they'll add a GPS waypoint to act as a FAF when there originally wasn't one.

Example approach is the NDB RWY 31 (GNSS) at CYAV. There is no intermediate segment and no FAF. MUVRI has been added so that the approach will work on the GPS as it requires a FAF. Within two miles of MUVRI, the approach will go ACTV when the automatic check of approach-level RAIM (0.3) at the FAF and MAP says approach-level RAIM will be available. This is where CDI will transition from 1.0 to 0.3 at the FAF so that you fly the entire final segment with 0.3.

Technically, if you're doing the overlay, you can't descend to the MDA until past MUVRI as you require the CDI and RAIM to be at 0.3 and the CDI won't reach 0.3 until at the FAF. In that case you could look at your RMI and see that you're within 5 degrees of your final approach course and descend to the MDA. It's a bit awkward when there isn't a FAF to begin with.

Specifically as far as your approach goes, it sounds like you should have discontinued the approach and flown a different one. Possibly request vectors to intercept the ILS.

Didn't we just have a nice, exhaustive thread on overlay approaches? What happened to it?
There was one. Dunno what happened to it. There was a lot of good discussion in it.
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Old 9th Oct 2014, 16:39
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Surely an overlay approach without the raw data becomes an RNAV approach?
My understanding of an overlay is simply that you are saying to the AC that you can't be bothered to manage it yourself (selected/selected) so please will you do it. But the primary source of NAV data is the raw stuff. So if it's not there, you can't do it.
Unless you know different!!
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Old 9th Oct 2014, 16:45
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I know it was Egypt, but ATC clearing you for a DME procedure with no DME....? I recall Tripoli giving me an NDB09 with no NDB once ---yes, we cracked it
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Old 9th Oct 2014, 17:33
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Surely an overlay approach without the raw data becomes an RNAV approach?
No, an overlay approach generally doesn't require the underlying NAVAID (NDB or VOR) to be serviceable. That's the case in Canada. But we're talking about a lot of countries here so you'd have to check your country's regulations.

My understanding of an overlay is simply that you are saying to the AC that you can't be bothered to manage it yourself (selected/selected) so please will you do it.
You're thinking of an FMS doing the overlay approach. And possibly calculating the 3 degree path for you. You only need a GPS to do a GPS overlay approach... no FMS required.

But the primary source of NAV data is the raw stuff. So if it's not there, you can't do it.
Not true! Again, speaking from Canada but I think most countries have the same rules. When an overlay approach is allowed, you do not require the underlying NAVAID (raw stuff) to be operational. The whole point of an overlay is to make the primary source of navigation a GPS instead of a VOR or NDB!

When there is no overlay published then your primary source is the raw data and you must follow it even though you really should be setting it up on the GPS or FMS, as I detailed in my post above.
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Old 10th Oct 2014, 11:51
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There's a bit of confusion here regarding the term overlay. In my case I am using it to refer to the fact that the FMS is flying what is coded in the box, and that is the DME arc in my case or whatever other procedure you are supposed to be flying. In reality my chart simply says ILS VOR DME it is a traditional approach. I did my flight training Canada so I know that there are certain procedures known as overlays. I am not referring to that here. What I mean is that we are essentially flying our own overlay, and we have become so confident in the FMS and with GPS navigation that we happily let the computer fly the procedure oblivious to the NAVAIDS we should really be monitoring.

The airfield we were going into had no radar. They could not have given us radar vectors, all their approaches are based off using the VOR and the DME to set yourself up for final. In reality the airport should really not have been accepting any IFR approaches. When we informed approach of the issue their response was just OK, we'll let someone know about it and we just happily continued along, confident that the FMS would get us to the IAF.

The other issue is that a DME arc is a curve, and an FMS is not meant to fly a curved path based on its own internal position estimates except during RNP-AR (SAAAR) approaches, all other RNAV legs whether terminal or enroute are straight lines.

Ultimately I believe that legally speaking what we did was wrong, and that it speaks of our age that we continued oblivious to the obvious.
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Old 10th Oct 2014, 12:15
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For us if you are flying a conventional non preciccion approach you can allow the FMS to guide the airplane but you must have the raw data displayed to confirm that a valid path is being flown.

In your case the legality would depend on wether the dme arc was flown in IMC or VMC and wether it was above or below the MSA.

What you must ask your self is what would you do if while flying below the MSA you got a "position disagree" type message. RNAV approaches take this into account while a conventinal non precission approach doesn't. For expample it may ask you to get to a dme point before turning on the missed approach. If the DME is not working and your FMS spits the dummy you are left with limited options.
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Old 10th Oct 2014, 17:24
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Hello all,

If the navaid on which the letdown procedure is based on is not working, then it is illegal to perform the approach even if the app is coded in the nav db.

It is ironic to say that performing the app with the ref navaid bieng u/s or deselected is, sometimes, safer than having the navaids selected and monitored.
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Old 10th Oct 2014, 20:16
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Airmann,

If the traditional NAVAID is down for the approach, what data is your FMS using to continue providing guidance to you?

The FMS has multiple inputs including INS, GPS, WAAS, VOR, NDB, etc. It processes all inputs through a Kalman filter to provide one position solution. It uses this to plot your position relative to the FMS database points. That's the basics of an FMS.

So when you lose your traditional (VOR or NDB) then your FMS continues to provide a position solution to you using the other inputs from the system. However, if you are required to use the VOR or NDB for "final approach course guidance" then you must discontinue the approach - regardless of how accurate your position solution from the FMS currently is.

Now, with regard to your original question, I don't believe you did anything illegal or unsafe - provided the FMS was giving a position solution for the loaded DME arc without any warnings, and you used the ILS once you intercepted to fly to the runway.

The ILS RWY 08 approach into CYBR I think is a good example of the approach that you are asking about.

http://imageserver.fltplan.com/merge...ILS_RWY_08.pdf

The title of "ILS RWY 08" indicates that you only require the ILS to fly the final approach course. Prior to the final approach course you may use another means of navigation (provided it's set up correctly and you adhere to the limitations for that navigation source [FMS]) to fly the procedure. In this case, I could fly FMS direct to LISTU and then start the DMC arc as programmed by the FMS all the way to the final approach course, at which time I'd intercept and fly the ILS. Or I could go FMS direct to KESVA for the straight-in ILS if I was coming from the west. Prior to flying the 085 degree course inside of KESVA, you may use another nav source to navigate... which seems quite obvious in this case since the ILS only works when lined up on the final approach course!
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Old 11th Oct 2014, 02:26
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There's a bit of confusion here regarding the term overlay.
In the FMS/FMC world, Overlay means that the procedure legs are copied from an NPA approach, with the same bearing, distances, height as the original NPA.
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Old 12th Oct 2014, 05:14
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Didn't we just have a nice, exhaustive thread on overlay approaches? What happened to it?
Yes, what did happen to that thread?
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Old 12th Oct 2014, 08:52
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This one? http://www.pprune.org/questions/5432...rocedures.html
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Old 12th Oct 2014, 13:05
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No, it was the thread where we were all talking about overlays, overlays by Country, et al....
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Old 13th Oct 2014, 16:38
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The ILS RWY 08 approach into CYBR I think is a good example of the approach that you are asking about.
you may use another nav source to navigate... which seems quite obvious in this case since the ILS only works when lined up on the final approach course!
I cannot say that I agree. At least where I am we are not allowed to fly below the MSA unless under radar control or visual with the ground or flying an approved instrument procedure. The OP said there was no radar so the only way to fly the approach , starting at the dme arc, is either if you are visual or you just keep above the msa (4300 in your plate) and then intercept the localiser at that altitude and descend with the GS. A RNAV approach would specify the required navigation (GNSS, IRS...) and might also specify a minimum temperature. An RNAV approach also has specific procedures in case of a nav accuracy downgrade. You cannot invent a RNAV appoach out of a normal approach just because one of the aids is not working.

All an overlay apprach does is allow the FMS to guide the airplane while you check it complies with the raw data. You cannot fly an overlay approach if the raw data is missing. In that case what you need is a RNAV approach and that is a slightly different animal for the reasons mentioned.

The only reason your plate does not say ILS DME is because arriving from the west you dont need the DME. That does not mean that you can fly a DME arc below the MSA without the DME.
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Old 13th Oct 2014, 18:41
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Elephant..

At least where I am we are not allowed to fly below the MSA unless under radar control or visual with the ground or flying an approved instrument procedure.
I agree. And if you are visual with the ground, you need to request either a contact or visual approach before you're allowed to deviate from the instrument approach that you've been previously cleared for.

When you are on the DME arc, you are flying an approved instrument procedure so you may descend to the altitude depicted on the arc, and any other altitude for the procedure. The key is that you must have your avionics correctly setup for the phase of flight you're in.

I'm going to specifically talk about GPS use in Canada, however, these procedures are exactly the same or very similar for FMS or RNP use around the world.

When you're within 30 NM of an airport and have an approach procedure loaded, the GPS will automatically change to terminal mode RAIM and CDI sensitivity - both being 1.0 NM. You are required to be in terminal mode when flying the initial segment, intermediate segment, and missed approach segment. The only time you are required to have approach mode RAIM and CDI sensitivity is during the final segment - between the FAF and MAP.

So as long as you are in terminal mode (1.0 NM), you may fly the entire approach up to the FAF. From the FAF to the MAP you must use the equipment specified as the title of the approach plate for lateral guidance. See this: http://i.imgur.com/r4lyK1f.png

As long as you load the approach from the database, everything else will be done automatically. If there is any issue, you will get a warning from the GPS telling you something along the lines of "RAIM NOT AVAILABLE" and that means that you need to discontinue using the GPS for your primary means of navigation for that phase of flight. The FMS should tell you the same thing but I won't guarantee it - you need to read your FMS's manual or AFM supplement for your airplane and equipment. Nothing you ever read on the internet will ever supersede what's written in your AFM!

So, as long as the FMS Airmann was using had the approach loaded from the database and it was not giving any warnings as to its performance, then they could use it to fly the approach all the way up to the final approach segment - transition to flying the ILS from that point (in real world you intercept and track the ILS before the final approach segment) - then transition back to the FMS navigation for the missed approach if they went missed. Most FMSs will automatically transition to using the ILS without you having to push any buttons. So, after loading the approach into the FMS you'd only have to press one button for the entire procedure - (the G/A button) to get it to sequence to the missed approach.

Considering Airmann said this:

GPS was Primary and the aircraft few the arc perfectly (as it was programmed to do) intercepted the ILS landed, went to the hotel. Didn't think twice about what we had just done.
I don't believe he violated any rules. If the GPS didn't have the proper integrity for the airspace he was in it would have given him a message.

Last edited by italia458; 13th Oct 2014 at 19:03.
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Old 13th Oct 2014, 23:22
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I cannot speak for Canada. In the UK that is not allowed.
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