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

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!!!)

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)

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.

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)
>

"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.

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.

>> 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

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.

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.

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.

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

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

"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

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
>
>

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
>>
>>
>
>

"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