GPS396 terrain warnings near airports

When I fly with my GPS396, I usually don't put in a flight plan or any
direct-tos; I use the panel mounted KLN94 for that. I use the 396 just
for weather, terrain, and additional situational awareness. I had
never heard the spurious terrain warnings near airports that Dave
Butler reported in another thread. Btw last week I updated the
software in the 396, but I never heard the warnings before that either.


Well today I heard them. And the difference was, I had a direct-to
waypoint which was NOT the airport programmed in and active. I was
safety pilot and we were doing hood work with VFR flight following. I
had the IAF for the GPS approach set as the direct-to waypoint, but for
various reasons including conflicting traffic and an incoming weather
front, we decided to break it off and just do a normal VFR pattern and
land. From the base-to-final turn all the way in, the GPS 396 gave
terrain warnings. We had done two approaches earlier in the flight
without a direct-to active in the GPS 396, and it stayed quiet all the
way down to either minimums or the runway. But as soon as we landed
with a direct-to (which was NOT the landing airport) active, it
squawked all the way down from 500 AGL.

So in my experience, having a direct-to waypoint and landing somewhere
else must confuse the GPS396 into giving terrain warnings as you come
in to land.

I dont own one, so I didn't follow the copious discussion of this
problem on CPA (Cessna Pilots Association). What I did notice, though,
is that this problem is well known, and that there are workarounds.

In article . com,
"xyzzy" wrote:
So in my experience, having a direct-to waypoint and landing somewhere
else must confuse the GPS396 into giving terrain warnings as you come
in to land.

I guess the assumption would be that you're navigating direct to the
airport if you end up there. However, the terrain warnings do not sound
if there is no flight plan, nor do they sound if you are direct-to the
airport, so I'm not sure why they would sound if you're direct-to
somewhere else and just happen to end up at the airport.

What I noticed is that obstacle alerts still DO sound, which is very
nice. For example, my field has close-in surrounding terrain and
obstacles, and since v2.80 I get the obstacle alerts but not the terrain
alerts.



JKG

xyzzy wrote:
When I fly with my GPS396, I usually don't put in a flight plan or any
direct-tos; I use the panel mounted KLN94 for that. I use the 396 just
for weather, terrain, and additional situational awareness. I had
never heard the spurious terrain warnings near airports that Dave
Butler reported in another thread. Btw last week I updated the
software in the 396, but I never heard the warnings before that either.

good stuff snipped

After reporting my complaints about the terrain warnings here, I got a surprise
phone call from a Garmin Engineer. They do read these newsgroups! He offered a
beta-version of what I assume is the fix that eventually went into version 2.80
of the software, in return for which I was to provide some testing, which I
happily did.

My observations:
- before the fix, I got the terrain warnings on every landing, whether the
airport was in the flight plan as the destination or not.

- with the beta version of the fix, no terrain warnings when landing except
during one circling approach at TTA. I haven't had a chance to try this again
with the 2.80 software, but I assume it's the same code.

- with the 2.80 software, no terrain warnings except one time last week when
approaching runway 32 at RDU. I was following the GNS480 glideslope to the
runway, so no high descent rates were involved.

In my discussion with the Garmin engineer about the beta code, he attempted to
explain the algorithm on the phone... a difficult task without a picture phone.
What I did get out of that explanation was that the protected area around an
airport is shaped differently in the direction of that airport's longest runway.
Since runway 32 is perpendicular to the main runways at RDU, that could be part
of the explanation for receiving that warning.

Dave

Dave Butler wrote:

- with the beta version of the fix, no terrain warnings when landing except
during one circling approach at TTA. I haven't had a chance to try this again
with the 2.80 software, but I assume it's the same code.

Very interesting. The one time I got them was when doing a standard
VFR pattern to rwy 21 at TTA, which would approximate a circling
approach. Is that the runway you were circling to? Except for that
one time I've never had a terrain warning while approaching an airport
with the GPS396. (but that was also the only time I landed when I had
a flight plan that wasn't to the airport I landed at -- maybe that's
the cause or maybe there's just something funny with TTA rwy 21?)

xyzzy wrote:
Dave Butler wrote:

- with the beta version of the fix, no terrain warnings when landing except
during one circling approach at TTA. I haven't had a chance to try this again
with the 2.80 software, but I assume it's the same code.


Very interesting. The one time I got them was when doing a standard
VFR pattern to rwy 21 at TTA, which would approximate a circling
approach. Is that the runway you were circling to?

Yes.

Except for that
one time I've never had a terrain warning while approaching an airport
with the GPS396. (but that was also the only time I landed when I had
a flight plan that wasn't to the airport I landed at -- maybe that's
the cause or maybe there's just something funny with TTA rwy 21?)