KLN94 GPS and flying "overlay" GPS approaches

On 06/14/06 13:32, Peter wrote:
Mark Hansen wrote

Do you know whether or not you're allowed to use the GPS as primary
navigation to fly the GPS overlay approaches in your country?

Since the overlay approaches don't work properly within the GPS (no
automatic sequencing into APR mode, etc) I can't believe the answer is
Yes.

That's how it sounds, but I wonder if this is the case with all
GPS Overlay approaches, or just the ones you've noticed?


In practice, of course you can. You self-position a bit outside the
FAF (in fact you fly the proper procedure to get there) then track
across the FAF nicely established on the final approach track, check
the inbound on the navaid (VOR/ADF) just to make sure, and follow the
line on the GPS on the way down.

Well, if you're using the ground-based nav aid for primary navigation,
and just using the GPS for situational awareness, then sure. However,
when the approach is approved for GPS use and the GPS meets all of
its requirements for flying the approach, we can fly the approach
without the VOR being tuned in at all.




What puzzles me is whether the KLN94 actually fails to do anything
with the HSI, when on a non-"GPS" approach. That sounds almost too
"vindictive"

Well, if the GPS cannot switch to Approach (Active) mode, then it
will still drive the CDI, but not at 0.3sm sensitivity. The manual
states that it will remain at the 1.0sm sensitivity.

I didn't see that in the manual.

Yes.

Look in the sections on what is required to switch from Term (Arm) mode
to Approach (Active) mode. There is a list that includes RAIM, 2 miles
from the FAF inbound, etc.

It says what it will do if it can't switch.


I do see

"When a non-approved
approach is selected the unit will
remain in approach arm mode (or
terminal mode) and will not transition
to the approach active mode."

That's one of the things that will keep it from switching to Approach (Active)
mode.


but it also says, page 6-36, section 6.2.12,

"Since another navigation source is providing primary navigation
guidance, the KLN 94 may not be driving the CDI or HSI."

This assumes you've switched the GPS/VOR button to VOR (or whatever
it is that switches the CDI on the external VOR head from indicating
the GPS to indicating the VOR).

I think by "may not be driving", they are leaving open the possibility
that you haven't changed this switch as appropriate.


I was taught that you cannot legally fly the approach with the GPS
unless it switches to Approach (Active) mode. Maybe that's a U.S.
only rule (I doubt it).

That makes sense, though of course you can legally fly it using the
normal navaid equipment and use the GPS for monitoring only, and the
end result is about the same.

By "fly the approach with the GPS", I mean using the GPS as the
primary navigational source - where you could have your VOR/ILS/
NDB receivers switched off (not that you would do that).

If you still use the GPS to drive the CDI when it is not in Approach
(Active) mode, then you're not getting the sensitivity that is
required for this purpose. That is a difference, IMHO.

--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA