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Understanding FLTA alerts

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brou0040 View Drop Down
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    Posted: 02 Aug 2020 at 4:13pm
I've already submitted my suggestion to address FLTA alerts in the 10.3 thread so I thought it would be better to break this into a separate thread.

Instead of discussing what the thresholds should be, I'm trying to understand if what I'm seeing is what is published and what it is supposed to be, to make sure I'm not doing something wrong or not understanding what its supposed to do.

I forgot to grab my logs so I'll try to remember to grab them next time I'm out at the airport, but I did some stuff with the sim and I think I have enough to ask some questions.

From the manual for reference:
There is a rectangular suppression area for all runways in the nav database. Terrain and obstacles inside this suppression area will not generate any FLTA alerts. Dimensions of the FLTA runway suppression area are 2.0 nm laterally and 4.0 nm off each runway end when runway orientations are known or a circle with a radius of 1 nm centered on the Airfield Reference Point (ARP) when the on-board databases do not know the runway orientations. Note that for airfields with multiple runways, this may have the effect of looking like several irregular shapes around the airfield. If the aircraft position or the aircraft flight path projected impact point is anywhere within that runway suppression area, no FLTA alert will be generated. Obstacle threat bubbles can still appear inside the suppression area.

To me, this tells me that for L05, the exclusion zone should extend 4 miles off the end of each runway, meaning not a box that is 2x4, but that the opposite ends each have their own 4 miles, 17 has 4 miles, and 35 has its own 4 miles.  So the white exclusion zone will be 8 miles plus something along the lines of the length of the runway by either 2 or 4 miles wide (the manual doesn't say 2 miles laterally from each side).  When doing the sim into L05, which shows the runway orientation so I assume this satisfies being in the database, when entering the area from the south, the airplane enters the white exclusion zone when the distance to the airport shows 1.5 miles or about a mile from the end of the runway.  What am I doing or thinking about wrong, or is this not operating according to the design or publication?  I was going to try to do the same thing into KTSP, but the sim keeps crashing, but I think this is enough to start the conversation.






Edited by brou0040 - 02 Aug 2020 at 4:19pm
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dmtidler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmtidler Quote  Post ReplyReply Direct Link To This Post Posted: 02 Aug 2020 at 5:19pm
I took a look at several airports in the IFD Trainer and agree that the FLTA exclusion areas depicted in the IFD Trainer appear much smaller than advertised above in the PG. I saw the same 1 NM radius for airports where runway orientation is not in the database and approximately 1NM off each end and approximately 0.75 NM laterally each side of runway centerline for those airports whose runway orientations are in the database. 

My IFD Trainer uses the demo database; I'm interested in verifying these results with the current databases in the airplane. 
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Bob H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 02 Aug 2020 at 6:31pm
I've always assumed that the software only knows the runway orientation if there is an instrument approach at the airport.  A pictorial view of the airport as on a sectional may not mean the orientation is known.  Steve will need to confirm if this is indeed the case.

The blank spot around L05 is certainly an odd shape, but It looks more like a void rather than a exclusion area.  I also note that there is no “flashlight” graphic associated with the FLTA.  It would be interesting to see what SVS shows for this situation.  It would also be interesting to see if the warnings are suppressed within one mile of the airport.  I don't have that database so can't test it myself.  So, my suspicion is that L05 has the one nm exclusion area.

As you mentioned, warnings will be suppressed for terrain within the exclusion area.  But they will also be suppressed for ALL terrain when the AIRCRAFT is within the exclusion area.

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Aug 2020 at 7:18pm
Originally posted by Bob H Bob H wrote:

I've always assumed that the software only knows the runway orientation if there is an instrument approach at the airport.  A pictorial view of the airport as on a sectional may not mean the orientation is known.

I was wondering the same thing, but I'd think that having the runways in the drop down in the flight plan would be sufficient.  The exclusion area is also rectangular rather than circular making it appear that it is recognizing the runway orientation.

Below shows L71 with an approach loaded and still a relatively short exclusion zone compared to what I expected to see.




Edited by brou0040 - 02 Aug 2020 at 7:22pm
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Bob H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 02 Aug 2020 at 8:08pm
A little bit confusing about how all this ties together, but that latest picture looks like a Terrain Awareness suppression zone which is defined as follows:
A rectangular suppression area exists for all runways in the FMS database and user waypoints that have been designated to be an airfield (see page 3-50). Terrain inside this suppression area will not generate any TA indication. Dimensions of the TA runway suppression area are 0.25 nm laterally and 0.5 nm off each runway.

So, that is different than an FLTA warning.
Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dmtidler Quote  Post ReplyReply Direct Link To This Post Posted: 02 Aug 2020 at 9:30pm
Originally posted by Bob H Bob H wrote:

A little bit confusing about how all this ties together, but that latest picture looks like a Terrain Awareness suppression zone which is defined as follows:
A rectangular suppression area exists for all runways in the FMS database and user waypoints that have been designated to be an airfield (see page 3-50). Terrain inside this suppression area will not generate any TA indication. Dimensions of the TA runway suppression area are 0.25 nm laterally and 0.5 nm off each runway.

So, that is different than an FLTA warning.

Very good info....so my take away is that TA runway suppression areas are visually depicted on the map, whereas FLTA runway suppression areas are not. 

Circular TA and FLTA runway suppression areas are identical for airports in the database with no known runway orientation; therefore, the depicted circular TA runway suppression area also happens to have the same lateral limits as the FLTA runway suppression area for those airports.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Aug 2020 at 10:10pm
So maybe the FLTA exclusion isn't shown and that is always the TA suppression zone, good to know.

Here are some additional pictures of FLTA warnings with different geometries to the airport.

A little west of the extended centerline, but the airplane should still be within the FLTA area.



Is the width of the "flashlight" extending past the exclusion zone?  Even if it is, the airplane still should be within the zone preventing the alert.


Not sure why the sim stopped following the flight plan, but this should still be within the FLTA exclusion area.


Settings in case I'm setting something up wrong.  Airspace alerts never shut up around here...



Edited by brou0040 - 02 Aug 2020 at 10:15pm
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Bob H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 02 Aug 2020 at 11:20pm
In the first picture of the latest post, the aircraft is 2.3 nm from L05, so it is not within the FLTA exclusion zone if the non IFR airport has a 1nm exclusion zone radius.  Likewise in the second picture, the aircraft is outside a 1nm radius from L05.  The same with the third picture.  These tests need to be done within a 1nm radius of the airport.
Here is a summary from the manual:

TERRAIN AWARENESS (TA)

TA is for general situational awareness purposes and consists of hatched terrain on any map page. TA will not generate a CAS message or aural alert.  TA can be triggered by either terrain or obstacles (e.g. towers > 200’ AGL tall buildings, etc). Any terrain that is within a 10nm radius of aircraft position and between 100’ (100’ for helicopters) below aircraft altitude and 1000’ (500’ for helicopters) below aircraft altitude will generate yellow hatched indications on the map pages. 

Any terrain that is within a 10nm radius of the aircraft position and is 100’ (50’ for helicopters) below aircraft to anything above aircraft altitude will generate red hatched indications on the map pages. 

A rectangular suppression area exists for all runways in the FMS database and user waypoints that have been designated to be an airfield (see page 3-50). Terrain inside this suppression area will not generate any TA indication. Dimensions of the TA runway suppression area are 0.25 nm laterally and 0.5 nm off each runway end. Airports with no known runway orientation use a 1nm circular suppression area. 

Terrain Awareness designated by hatched terrain, yellow or red, do not generate aural warnings. 

FORWARD LOOKING TERRAIN ALERTING (FLTA)

The FLTA function looks ahead of the airplane projecting the airplane’s lateral and vertical flight path and provides suitable alerts if a potential terrain or obstacle threat exists. FLTA alerting is triggered by either a projected imminent impact with terrain or obstacle or reduced terrain and obstacle clearance. 

There is a rectangular suppression area for all runways in the nav database. Terrain and obstacles inside this suppression area will not generate any FLTA alerts. Dimensions of the FLTA runway suppression area are 2.0 nm laterally and 4.0 nm off each runway end when runway orientations are known or a circle with a radius of 1 nm centered on the Airfield Reference Point (ARP) when the onboard databases do not know the runway orientations. 

Either terrain or obstacles can trigger FLTA alerts and they are distinguished via the CAS messaging and aural alerting. 

FLTA alerts are inhibited if any of the following conditions are met:

·       FLTA is manually turned off via the pilot-accessible “User Options” LSK on the “SETUP” tab of the AUX page (generates a “FLTA OFF” CAS Advisory message);

·       The ownship position is inside the rectangular (or circular, in the case of no known runway orientation) FLTA Inhibit zone depicted on the 2D map (generates a “FLTA INHIBITED” CAS Advisory message);

·       The projected ground impact point along the current ground track intersects the terrain inside that FLTA Inhibit rectangle/circle (generates a “FLTA INHIBITED” CAS Advisory message);

·       The Reduced Terrain Clearance (RTC) violation is projected to happen inside that FLTA Inhibit rectangle/circle (generates a “FLTA INHIBITED” CAS Advisory message).


Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 02 Aug 2020 at 11:46pm
Originally posted by Bob H Bob H wrote:

In the first picture of the latest post, the aircraft is 2.3 nm from L05, so it is not within the FLTA exclusion zone if the non IFR airport has a 1nm exclusion zone radius.  Likewise in the second picture, the aircraft is outside a 1nm radius from L05.  The same with the third picture.  These tests need to be done within a 1nm radius of the airport.

I'll wait for Avidyne to chime in at this point.  It seems like the FLTA suppression area isn't accounting for the runway orientation even though the TA suppression area does.  I'd be curious to know why there'd be a difference.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 03 Aug 2020 at 1:13am
I don't have the L05 database, and now I see that what I thought was a void in the map is a lake, which seems seriously misplaced.  It will be interesting to see if that is interfering with showing the terrain exclusion area, as I don't see either a rectangular or circular exclusion area depicted on the map.

Be interesting to see what Avidyne has to say.
Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 03 Aug 2020 at 10:18am
If you look carefully at my first pic above, you can see white dots forming a rectangle over the yellow background that is aligned with the runway, it's kind of hard to see the white on yellow, but it is there.

The airport is not in the lake, it's on the east side, but the shoreline varies with how full the lake is.  The dam is currently under repair so they have the lake really low.  I doubt the database takes into account how full the lake is and has some default level that is slightly too drowning the airport.  What is the fidelity of the height of the terrain in the database?  Is that different than for the height of the water?  I don't know that the runway is misplaced, but if the water was lowered, it would show a thin river to the west and the airport on the land to the east, which is the way it actually is.  I don't recall the IFD in the real airplane showing the airport in the water like this, but I don't remember specifically.


Edited by brou0040 - 03 Aug 2020 at 10:20am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 03 Aug 2020 at 10:35am
I can see the rectangular exclusion around L71.  I can't see it around L05.  However, I do see the runway numbers on the map for L05, so it appears that the runway orientation is known and that the exclusion zone should be the larger rectangle of 2.0 nm laterally and 4.0 nm off each runway end.  If that is the case, than certainly your examples are well within the exclusion zone and the FLTA alerts shouldn't be going off.  Something certainly seems "off".  I wonder if the sim is not quite true to life.
Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 03 Aug 2020 at 9:52pm
Originally posted by Bob H Bob H wrote:

I wonder if the sim is not quite true to life.
  The alerts go off like crazy flying into L05 in real life...  I know to use the inhibit switch there by default.

Below are some pics going into TSP, this was the alert that initiated my post to look into the FLTA alerts in the 10.3 suggestion thread.

Here you can see the numbers and rectangular TA suppression area for TSP, and the FLTA alerts still going off while on the extended centerline 2.1 TSP.


Long final with the flashlight highlighting the runway...

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 03 Aug 2020 at 11:32pm
I agree.  Those latest pics look pretty definitive of an issue not consistent with the IFD manual.  It would be good if Avidyne could weigh in.  Did you download the logs?
Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 04 Aug 2020 at 10:25am
I had the wife with me last time so I wasn't planning on doing any projects while I was out there so I forgot about downloading the logs.  I should be heading out there Friday and should be able to grab them then.  Here's to hoping I remember the situation correctly!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSteve Quote  Post ReplyReply Direct Link To This Post Posted: 04 Aug 2020 at 11:35am
FYI, we're looking into this.  Stand by.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSteve Quote  Post ReplyReply Direct Link To This Post Posted: 05 Aug 2020 at 1:08pm
First and foremost... the Pilot's Guide is in error.  We looked back through old versions and, apparently, it's always been in error.  Apologies for that, and we'll get it corrected in the next version.

The exclusion zone for each runway at an airport with defined runways is a rectangle extending 1 nautical mile from the threshold and 3/4 nautical mile on each side of the centerline.  If the nav database contains no runway information at an airport, then the exclusion zone is a 1 nautical mile circle around the airport reference point.

On the map, you'll see white hatching defining the exclusion zone where FLTA is effectively turned off.  The interior of that zone (i.e. right on top of the runways or airport) will actually have no hatching indicating the area in which TA is also turned off.

It's worth noting that the algorithms have changed over the years since the IFD was first released, due to TSO revisions being updated.  So, to do a rigorous analysis of this exact scenario, we need to know which software version you're running on the IFD.  Since you used the sim for some of these shots, we'd also like to know which version of that you are running (hopefully the latest).  Terrain database should all be the same, but if you could report the date/version of that, it would also eliminate any variability there.

We did our best to recreate your situation using the latest released IFD software and trainer app.  Some of it matched up, but other parts not so much.  Some of that could be getting the simulator set up the same and to accurately reflect the real flight.  That's why logs would result in a better analysis (I know you said you were going to get that next time you're at the airplane).

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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 05 Aug 2020 at 11:53pm
All of the screenshots above were from the sim, version 10.2.3.1, Terrain is Aug 5, 2015.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brou0040 Quote  Post ReplyReply Direct Link To This Post Posted: 07 Aug 2020 at 2:40pm
Originally posted by AviSteve AviSteve wrote:

That's why logs would result in a better analysis (I know you said you were going to get that next time you're at the airplane).

Got my logs, but forgot to write down the SW version and terrain date.  Hopefully that's in the logs, but I'm heading back to the airport this afternoon and will grab that.  Where do you want me to send the logs and which ones do you want?

Update on airplane IFD540:
SW 10.2.3.1
Terrain 2/3/2012
Obstacles 2/2/2017
L05 still looks like it's in the water. 


Edited by brou0040 - 07 Aug 2020 at 6:52pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AviSteve Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug 2020 at 9:30am
Send flight, system, and event logs to me.  slindsley at avidyne dot com
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