Just to interject a comment here. Use of a handheld to supplement other
instruments, or even to supplement radar vectors, under IFR flight
rules, is what I would call a gray area of FAA rules. On the one hand
it's not really expressly PROHIBITED. But it's not expressly permitted
either. The OFFICIAL word, because of liability and the FAA's history
of wanting things to be certified, is you can't do it. But because
handhelds do function as useful navigation devices quite well, the
UNOFFICIAL rule is, if it works, you can do it.

It's not much different than using a handheld VOR (which exist and do
work) for VOR navigation. But since GPS works so much better, and
almost all aircraft flown IFR have VORs , the subject doesn't get
questioned.

What ATC wants is for you to be able to fly your clearance. Its really
up to you how you do it.

Things would start falling apart though, if there were an accident, and
the accident could be atrributed to incorrect navigation on the part of
the pilot using the handheld device. I am sure the FAA would see a
violation on that. I've seen a few FAA violations after accidents, and
they can violate the pilot, oh yes they can, and do. And they just do
it and you don't have much recourse. They will find some reason.

But so long as you keep your mouth shut, fly your clearance, and don't
cause problems, there is no citation.

The FAA used to publicize that pilots were PROHIBITIED from using
handhelds for navigation. But that has stopped. There seems to a quiet
acceptance that pilots can use them, so long as it doesn't cause
problems. Probably legal for VFR flight, where your primary navigation
is by eyesight anyway. Probably not strictly legal for IFR flight, but
can be used safely if you have the required navigation systems in the
airplane. Probably not a good idea for IFR flight if you don't have the
required, certified navigation system in the airplane.

IFR GPS's are getting inexpensive enough so that it's becoming a moot
point anyway. Why not just get a used King KLN89 and get it certified
and you will have all you legally need to file /G, no questions asked.
Then use your handheld as a backup to that unit, if it fails. And as a
backup to that unit if there is any ambiguity on the integrety of the
signal. If both units don't agree, there is a problem. If one unit
fails and the other doesn't that tells you its the unit and not the
satellites etc. For full safety you need two units anyway. The reason
why most IFR aircraft have two VOR's, not one. Safe IFR flight is all
about having redundancy. A handheld gives good redundancy to a built in
unit.

Stay safe!