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Garmin GPSmap 496 False Terrain Alert



 
 
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  #11  
Old January 9th 08, 07:59 PM posted to rec.aviation.ifr
Mxsmanic
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Posts: 9,169
Default Garmin GPSmap 496 False Terrain Alert

Peter writes:

But this is a recent development; more than 10-20 years ago "GPWS" was
basically just a radar altimeter. I've read a fair few crash reports;
the one on Mt Erebus comes to mind in this context, c. 1985.


GPWS was not at fault in that crash, as I recall.
  #12  
Old January 10th 08, 05:30 AM posted to rec.aviation.ifr
Ron Gordon
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Posts: 12
Default Update - Garmin GPSmap 496 False Terrain Alert

Here's the latest information on the issue with the Garmin GPSmap 496
reporting a false terrain alert when flying into KADS along the runway 15
ILS:



Garmin provided me with a current terrain database update, in an effort to
see if corrections that have been made in that database since June 2006
would correct the problem. It did not, though I certainly appreciated their
efforts.



Another pilot who occassionally flies into ADS who also utilizes his 496 for
improved positional awareness and has his 496 audio connected to his
intercom for terrain /obstacle warnings (not IFR operations) has reported to
me that the problem also occurs with his unit when he lands on runway 15 at
Addison. As with my 496, the problem is easily reproduced and occurs 100% of
the time. (Thanks! This was an important point of information for Garmin
technical support.) Another ADS pilot/owner has offered to plug his 496 into
my panel and permit me to fly with him and his unit to confirm that the
problem occurs with his unit in my installation. I'll do that on Saturday,
but we have every expectation that this will repeat, given the prior report.
It will provide a second confirmation. (In his installation, the audio alert
feature is not utilized.)



Garmin agrees that this evidently points to an errant data point within the
terrain database close to the approach end of runway 15. They admit that the
elevation data has errors which they continue to correct. No other airport
approach has been reported by anyone on the Internet as having this problem,
in response to my queries. (If you have experienced such a problem in a
repeatable fashion, Garmin is interested in the details.)



Garmin have asked me to download the recent tracks that demonstrate the
problem from my 496 and email them to them. They will use this information
as they document the problem to the FAA subcontractor who produces the
database that they then license, reformat, and resell for use in the GPSmap
units. I've done that, yesterday.



Thanks to those of you who assisted with the analysis and troubleshooting of
this problem thus far. The rest of you can continue chatting



-Ron Gordon


  #13  
Old January 10th 08, 11:46 AM posted to rec.aviation.ifr
Mxsmanic
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Posts: 9,169
Default Garmin GPSmap 496 False Terrain Alert

Peter writes:

True, but they had one and it gave them no warning until way too late.
It was just a radalt in that case. Went off at about 200ft - no good
for ~ 250kt into rising snow covered ground.


Exactly. As I recall, the real problem was that the routing had been changed
and the crew was not made aware of this. They thought they were flying the
prior route which always took them well clear of the mountain, but in fact
they were moving directly towards it. Optical illusions and visibility issues
made it difficult to recognize the mountain, and by the time the warning
sounded, they were too fast and too close. See:

http://en.wikipedia.org/wiki/Mount_Erebus_disaster
  #14  
Old January 11th 08, 04:22 PM posted to rec.aviation.ifr
Marco Leon[_4_]
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Posts: 46
Default Update - Garmin GPSmap 496 False Terrain Alert

"Ron Gordon" wrote in message
t...
Here's the latest information on the issue with the Garmin GPSmap 496
reporting a false terrain alert when flying into KADS along the runway 15
ILS:
Garmin provided me with a current terrain database update, in an effort to
see if corrections that have been made in that database since June 2006
would correct the problem. It did not, though I certainly appreciated
their efforts.


Ron, have you asked Garmin if they're willing to give you a copy of the raw
datapoints of the terrain elevation around ADS? I'm thinking the erroneous
datapoint(s) will be terrain data since an obstacle should be easily
displayed as such on the 496 map. Since you know the area, you might be able
to spot the errant datapoint and help them out.

Marco


  #15  
Old January 13th 08, 08:08 PM posted to rec.aviation.ifr
Ron Gordon
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Posts: 12
Default Update - Garmin GPSmap 496 False Terrain Alert

That's a very good suggestion. I will make the request on Monday. Since I
can carry the 496 to the elevation data points leading to and around the
approach path, these should be easy to confirm. And as you point out, the
terrain around KADS is very flat. It is likely that an erroneous data point
may be self evident. Thanks! -Ron Gordon

Ron, have you asked Garmin if they're willing to give you a copy of the
raw datapoints of the terrain elevation around ADS? I'm thinking the
erroneous datapoint(s) will be terrain data since an obstacle should be
easily displayed as such on the 496 map. Since you know the area, you
might be able to spot the errant datapoint and help them out.

Marco




  #16  
Old January 15th 08, 05:27 AM posted to rec.aviation.ifr
Ron Gordon
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Posts: 12
Default Update - Garmin GPSmap 496 False Terrain Alert

Garmin have pointed out how I can read the terrain data from the 496. On the
Terrain page use the cursor to point to a location. The elevation MSL is
shown on line two in the black band above the map.

The south end of runway 15 has a terrain elevation of 715 ft msl in the
database. In fact, it has an actual elevation of only 639 ft msl using the
Jeppesen chart for the airport.

In addition, the terrain just north of the airport is also shown as 715 ft
msl. Using the GPS I've gotten readings that this terrain is actually more
like 650 ft msl.

That may be part of this problem.

Thanks,
-Ron


"Ron Gordon" wrote in message
. net...
That's a very good suggestion. I will make the request on Monday. Since I
can carry the 496 to the elevation data points leading to and around the
approach path, these should be easy to confirm. And as you point out, the
terrain around KADS is very flat. It is likely that an erroneous data
point may be self evident. Thanks! -Ron Gordon

Ron, have you asked Garmin if they're willing to give you a copy of the
raw datapoints of the terrain elevation around ADS? I'm thinking the
erroneous datapoint(s) will be terrain data since an obstacle should be
easily displayed as such on the 496 map. Since you know the area, you
might be able to spot the errant datapoint and help them out.

Marco






  #17  
Old January 15th 08, 05:05 PM posted to rec.aviation.ifr
Marco Leon[_4_]
external usenet poster
 
Posts: 46
Default Update - Garmin GPSmap 496 False Terrain Alert

"Ron Gordon" wrote in message
et...
Garmin have pointed out how I can read the terrain data from the 496. On
the Terrain page use the cursor to point to a location. The elevation MSL
is shown on line two in the black band above the map.

The south end of runway 15 has a terrain elevation of 715 ft msl in the
database. In fact, it has an actual elevation of only 639 ft msl using the
Jeppesen chart for the airport.

In addition, the terrain just north of the airport is also shown as 715 ft
msl. Using the GPS I've gotten readings that this terrain is actually more
like 650 ft msl.

That may be part of this problem.


That 80' difference could be the culprit but I don't know what the
formula(s) is for triggering a "pull-up" alert. I guess they would have to
program *some* value as a cutoff and maybe that 80' exceed that threshhold.

Have you told them about your observation and if so, are they willing to
tweek the database? If I was Garmin, I'd tweek it and give you a test
database to test out.

Marco


 




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