Instrument Question

[Dick]

Can a GPS with compass, heading & groundspeed capability be used legally in
lieu of a separate compass and airspeed instruments on an experimental
homebuilt airplane? Or at least in lieu of the separate compass?
Thanks

[Dave S]

You can use whatever you want on a homebuilt. Day, VFR is pretty much up
to what the builder wants to install

However, I would recommend against using GPS as a sole source for the
uses you are envisioning.

The heading information provided by GPS is track along the ground, not
the magnetic heading that you are pointing. Your ground speed and
airspeed are two separate and distinct animals. Your airplane will fly
with respect to indicated airspeed... regardless of what the groundspeed
reads. Winds aloft will cause differences between indicated airspeed and
what the GPS groundspeed is.

Dave

Dick wrote:
> Can a GPS with compass, heading & groundspeed capability be used legally in
> lieu of a separate compass and airspeed instruments on an experimental
> homebuilt airplane? Or at least in lieu of the separate compass?
> Thanks
>
>

[nauga]

Dick wrote...
> Can a GPS with compass, heading & groundspeed capability be used legally
in
> lieu of a separate compass and airspeed instruments on an experimental
> homebuilt airplane? Or at least in lieu of the separate compass?

Yes to all, *if* the inspector signs it off. Whether or
not it's a smart thing to do will probably be the subject
of some debate. I vote no, mostly because I think you
ought to have at least *one* onboard nav gadget that
will keep working when you lose electrical power.
Groundspeed is also practically useless in terms of
flight safety.

Dave 'wind in your hair' Hyde

[Bill Daniels]

"Dick" > wrote in message
...
> Can a GPS with compass, heading & groundspeed capability be used legally
in
> lieu of a separate compass and airspeed instruments on an experimental
> homebuilt airplane? Or at least in lieu of the separate compass?
> Thanks
>
>

Short answer, no.

The GPS is showing ground track, not heading, even though this information
is displayed on the GPS as if it were a compass. That said, once I knew
where I was going, which way I was pointing lost a lot of significance. The
GPS "compass' shows where you are actually going.

For the most part, ATC personnel are used to fast jets where the heading and
track are essentially the same. Give them a Cub and a strong 90 degree
crosswind and it gets interesting. They will say, "turn left, heading 270"
clearly expecting the blip on their radar will move westward even though the
Cub is fighting a stiff North wind while heading West and tracking
Southwest.

I fly gliders cross country and sometimes it is very important to track
precisely toward a known safe landing spot. I use the GPS "compass" for
this since it will steer me exactly over the intended point without any wind
calculations at all. Compared to GPS, a magnetic compass is nearly useless.

Bill Daniels

[Richard Lamb]

Golly!

You guys are all real nice.

I'd have told him to go learn to fly...



Troll O Meter

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[Dick]

Thanks to everyone else.

BTW Richard, I do know how to fly and was addressing a "available space"
problem on the panel. This led me to think about "glass" cockpits and
redundancy on a Day/VFR mission; hence the question..

Dick

[Dave S]

Buy a dynon.. 1800 bux and does it all..

Dave

Dick wrote:
> Thanks to everyone else.
>
> BTW Richard, I do know how to fly and was addressing a "available space"
> problem on the panel. This led me to think about "glass" cockpits and
> redundancy on a Day/VFR mission; hence the question..
>
> Dick
>
>

[Richard Lamb]

Dick wrote:
>
> Thanks to everyone else.
>
> BTW Richard, I do know how to fly and was addressing a "available space"
> problem on the panel. This led me to think about "glass" cockpits and
> redundancy on a Day/VFR mission; hence the question..
>
> Dick

Then you already knew the answer.

GPS track information is NOT the same as heading info.

As a suggestion I'd trade off the gyro compass for a vertical card
compass.

That takes out most of the "whisky compass" foibles and is quite usable.


Richard

[nauga]

Dave S wrote:

> Buy a dynon.. 1800 bux and does it all..

Then take a week off and go put some hours on it :-)

Seriously, My horizon gyro was DOA and out of warranty
from first flight on. Rather than replace it, I'm saving
up for a Dynon and will chuck the horizon, VG, vacuum
gauge, and vacuum pump. The compass stays <g>.

BTW, who's got them for $1800?

Dave 'MFD' Hyde

[Brian Whatcott]

On Wed, 26 May 2004 22:57:55 GMT, "Dick" > wrote:

>Can a GPS with compass, heading & groundspeed capability be used legally in
>lieu of a separate compass and airspeed instruments on an experimental
>homebuilt airplane? Or at least in lieu of the separate compass?
>Thanks
>


Hmmmm... the level of supercilious on the list is still as high as I
recall.

Anyway: a GPS with a built in flux-gate compass would be approvable,
I'd think. A GPS that deduces heading from displacement would not be.

My opinion

Brian W

[Ed Wischmeyer]

> Can a GPS with compass, heading & groundspeed capability be used legally
in
> lieu of a separate compass and airspeed instruments on an experimental
> homebuilt airplane? Or at least in lieu of the separate compass?

You definitely need the airspeed!! Winds can make a GPS groundspeed pretty
far off and useless for airspeed deductions that you'd really want on
takeoff and landing.

As for compass, that's got to be in the FARs somewhere, too, likely 91.xxx.
On the other hand, given how good GPS course readouts are, I'd love it if
ATC would let you fly an assigned course, not just an assigned heading.

Ed Wischmeyer
ATP/CFII