Here's my layman's understanding of this. I hope someone will jump
in and correct me if I've got something wrong,

If you're IFR en-route, ATC has three ways of directing you::

a) via an airway;
b) direct;
c) vectors;

ATC needs to have you on radar to issue (b) or (c).

You should only accept a direct clearance if you're confident you can
navigate direct.
It doesn't matter what method you plan to use to navigate direct -- it
can be dead reckoning, celestial navigation, visual (if you're VMC),
VOR, hand-held GPS, panel-mount GPS, etc.

If you're not confident you can navigate direct, then you should
refuse the direct clearance. ATC already has you on radar, so it's
typically no big deal for them to issue a radar vector instead.

What happens if you've accepted a direct clearance, and your hand-held
GPS's battereis die?
It's just the same as any other reason for being unable to navigate
direct (clouds cover the stars, you can no longer see your
destination, your ham sandwich falls to the floor...). You inform
ATC, eg:
"Cessna 12345 is no longer able to navigate direct xyz. Request
radar vectors."
(Remember, they've already got you on radar to issue the direct
clearance.)

So, it's OK to accept a direct clearance, even if you're /U, as long
as you're confident you can do the navigation.

It's also good practice to always cross-check your navigation using
some independent means.

Once again, if any of this is wrong, stupid, or illegal, please
correct me. I'm here to learn!

Cheers,

Tim.


On 2 May 2006 21:23:33 -0700, wrote:

I've seen other discussions get off topic and go on about handheld GPS
use under IFR. And people get all worked up about it. I know there is
a lot of history on this newsgroup, but I haven't followed most of it
(so don't lynch me, please).

It seems that a lot of pilots believe a handheld GPS shouldn't be used
while IFR. And a lot more believe that it can't be the "primary"
navigation method. Sometimes that makes sense, but other times it
doesn't (to me). So here are a few questions I have that work up to
GPS:

1) Can you use celestial navigation while IFR? And does your sextant
have to be "approved" in some way?

2) Can you use dead reckoning for IFR navigation? If so, can you use
your wrist watch as your "primary" timer? Or does it have to be an
"installed" clock? And to do dead reckoning calculations, can you use
a drugstore-bought calculator? An abacus? Or how about a handy "dead
reckoning computer" that calculates ground speed and track?

I can understand that a handheld GPS is not supposed to be used as a
substitute for VOR or DME or ADF (say for navigating along airways or
for shooting instrument approaches). But can you not use one for
flying off-airway routes without playing tricks (like pretending to
double-check position with VOR/DME or asking ATC for a vector and then
ignoring the heading they issue)?