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Garmin GPSmap 496 False Terrain Alert



 
 
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  #1  
Old January 6th 08, 08:04 AM posted to rec.aviation.ifr
Ron Gordon
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Posts: 12
Default Garmin GPSmap 496 False Terrain Alert

Do any of you fly into KADS with a Garmin GPSmap 496? I'm getting a false
terrain alert when flying the Rwy 15 ILS and am wondering if anyone else has
this problem. I'm working with Garmin to resolve this, but so far what they
have suggested has not solved the problem.



When flying the approach the 496 makes the "500 feet" announcement just as
expected.



Next, an obstacle warning occurs which is OK, because of the upcoming
Addison water tower that is just to the east of the runway. The 496
announces "Warning, Obstacle ahead." Notice that this is just a warning.



Next, a Terrain warning occurs. The unit announces "Warning, Terrain ahead."
Then it alerts "Pull up. Pull up!" This should not occur. We're exactly on
the glide slope, with our normal approach speed of 120 MPH, and a 500 to 600
foot rate of descent. We're at an altitude of approximately 300 feet when
the Terrain alert occurs. This is very disconcerting to the passengers, and
if I was really flying the approach in IMC, you can be assured that this
would be extremely disconcerting to me! At no time during the approach have
we gone below the glide slope. Glide slope intercept at JERIT, the FAF at
2000 feet, confirms that our glide slope receiver is OK along with the
proper VASI lights as we get closer to the runway during the approach.



The 496 is fully updated with the latest software (V3.40 released Jan08) and
with the current Jeppesen database and the current Obstacle database. The
problem also occurred with the previous software version 3.30 and the prior
terrain database. The "nearest airport" criteria is set to the default, "All
airports" and the alert look ahead is set to 90 seconds. Everything else is
set to the default.



The unit does not give a Terrain alert when landing on the opposite runway,
33. Nor does it give a Terrain alert when landing at any other airport. This
Terrain alert including the command "Pull up, Pull up" only occurs when
flying the ILS 15 approach into KADS during short final.



Thus far the support fellow at Garmin (Todd) has verified the pertinent
options and settings in my unit, asked that I update to the latest software,
and update to the latest terrain database. This hasn't helped, and after
checking with the other support technicians, they are stumped.
Unfortunately, neither Garmin nor I have been able to get the unit to give a
terrain alert in simulator mode, but it does it 100% of the time in the air.



Anyone have any ideas? Has anyone flown with the GPSmap 496 into KADS with
audio alerts enabled and gotten the same terrain alert? Has anyone
experienced a similar problem at any other airport?



Thanks,

-Ron Gordon

Ron "at" GordonFamily.com

1968 V35A Bonanza N8485N


  #2  
Old January 6th 08, 08:12 AM posted to rec.aviation.ifr
Bob Noel
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Posts: 1,374
Default Garmin GPSmap 496 False Terrain Alert

In article ,
"Ron Gordon" wrote:

Anyone have any ideas? Has anyone flown with the GPSmap 496 into KADS with
audio alerts enabled and gotten the same terrain alert? Has anyone
experienced a similar problem at any other airport?


Have you been able to use a different 496 and see if it gives the same
false alert?

--
Bob Noel
(goodness, please trim replies!!!)

  #3  
Old January 6th 08, 10:25 AM posted to rec.aviation.ifr
Thomas Borchert
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Posts: 1,749
Default Garmin GPSmap 496 False Terrain Alert

Ron,

This is very disconcerting to the passengers, and
if I was really flying the approach in IMC, you can be assured that this
would be extremely disconcerting to me!


I wish you the best for resolving the problem, but as an aside, you're
putting way more trust in a non-certified handheld device than I ever
would. What you pay for is what you get. The ILS you're following down
there IS certified, as is your receiver. I don't think the 496 nor its
terrain database were ever designed to do close range avoidance. Trust it
when avoind terrain by miles, not tens of feet.

--
Thomas Borchert (EDDH)

  #4  
Old January 6th 08, 10:52 AM posted to rec.aviation.ifr
Sam Spade
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Posts: 1,326
Default Garmin GPSmap 496 False Terrain Alert

Thomas Borchert wrote:
Ron,


This is very disconcerting to the passengers, and
if I was really flying the approach in IMC, you can be assured that this
would be extremely disconcerting to me!



I wish you the best for resolving the problem, but as an aside, you're
putting way more trust in a non-certified handheld device than I ever
would. What you pay for is what you get. The ILS you're following down
there IS certified, as is your receiver. I don't think the 496 nor its
terrain database were ever designed to do close range avoidance. Trust it
when avoind terrain by miles, not tens of feet.

Very well stated.

Garmin has two divisions in its product line:

1. IFR avionics

2. Consumer products.

The 496 is made by the latter division. It is no different than an
automobile navigator; i.e., a nice aid, a toy of sorts, but seriously
not a certified IFR tool.
  #5  
Old January 6th 08, 07:03 PM posted to rec.aviation.ifr
Ron Gordon
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Posts: 12
Default Garmin GPSmap 496 False Terrain Alert

Nope. I'm not following the ILS using the 496. I'm following the glide slope
receiver and localizer. The 496 is sitting there as a toy, and complaining
that we're about to hit terrain. Even as a non-IFR certified device, Garmin
confirms that the unit should not do this. Nor does it do this at any other
airport, nor on the flip side runway. This may be a software bug or an error
in the terrain database.

Will continue to troubleshoot.

Thanks,
-Ron


"Thomas Borchert" wrote in message
...
Ron,

This is very disconcerting to the passengers, and
if I was really flying the approach in IMC, you can be assured that this
would be extremely disconcerting to me!


I wish you the best for resolving the problem, but as an aside, you're
putting way more trust in a non-certified handheld device than I ever
would. What you pay for is what you get. The ILS you're following down
there IS certified, as is your receiver. I don't think the 496 nor its
terrain database were ever designed to do close range avoidance. Trust it
when avoind terrain by miles, not tens of feet.

--
Thomas Borchert (EDDH)



  #6  
Old January 7th 08, 07:37 AM posted to rec.aviation.ifr
John Godwin[_2_]
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Posts: 36
Default Garmin GPSmap 496 False Terrain Alert

"Ron Gordon" wrote in
. net:

Do any of you fly into KADS with a Garmin GPSmap 496? I'm getting a
false terrain alert when flying the Rwy 15 ILS and am wondering if
anyone else has this problem. I'm working with Garmin to resolve this,
but so far what they have suggested has not solved the problem.


It might be the calculation used to determine the terrain. All it has is
the distance from the satellite and formulae to determine terrain distance.

  #7  
Old January 7th 08, 11:16 PM posted to rec.aviation.ifr
Sam Spade
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Posts: 1,326
Default Garmin GPSmap 496 False Terrain Alert

Peter wrote:


If I got the terrain warning in IMC I would go missed immediately even
if on an ILS. Anything is possible in terms of conventional instrument
failure (I am an electronics engineer and know how avionics work) and
would never bet my life on the warning coming from a software bug.


Thus, you have created your own emergency based on a piece of consumer
electronics.
  #8  
Old January 8th 08, 06:12 PM posted to rec.aviation.ifr
Ron Lee[_2_]
external usenet poster
 
Posts: 233
Default Garmin GPSmap 496 False Terrain Alert

Do any of you fly into KADS with a Garmin GPSmap 496? I'm getting a
false terrain alert when flying the Rwy 15 ILS and am wondering if
anyone else has this problem. I'm working with Garmin to resolve this,
but so far what they have suggested has not solved the problem.


It might be the calculation used to determine the terrain. All it has is
the distance from the satellite and formulae to determine terrain distance.


Actually the GPS receiver should have terrain (assume in MSL feet) in
the database (unknown accuracy of data). The GPS receiver as part of
its position determination can calculate the aircraft altitude in the
same units and coordinate system and then compare using whatever
method the receiver programmers decided upon.

Ron Lee

  #9  
Old January 8th 08, 09:02 PM posted to rec.aviation.ifr
Sam Spade
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Posts: 1,326
Default Garmin GPSmap 496 False Terrain Alert

Peter wrote:


Think of all those airliners that flew into mountains, killing
everybody on board. You can bet that - within the areas contained in
the G496 terrain database - every one of those accidents would have
been avoided. All of them involved a "very simple" case of loss of SA.


Today they have EGPWS, which is a whole lot better than a Garmin handheld.

Having said that, I know of at least one airline CFIT that would have
been avoided with a Garmin 296/496, that is the TWA crash 21 miles NW of
IAD in 1974.

But, the bulk of airline CFITs were well inside the FAF and the 296/496
is just not nearly good enough for the resolution required close in.
Keep in mind that EGPWS in airliners is augmented by radar altimetes,
which become very important inside the FAF.
  #10  
Old January 8th 08, 11:56 PM posted to rec.aviation.ifr
Sam Spade
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Posts: 1,326
Default Garmin GPSmap 496 False Terrain Alert

Peter wrote:

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 mandated in 1975, shortly after the TWA 514 crash.

It was installed within a couple of years, so it became common 30 years ago.

EGPWS became effective about 10 years ago.
 




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