At some point, you're 5 miles from SAX and 30 miles from CMK,
and it obviously doesn't make any sense to double back any more.

I'm not sure it ever made sense to double back to CMK. That's not
really the point. The regulation is clear - your next fix is CMK. I'm
not saying that going to CMK is the right thing to do, especially if
you are 5 miles from SAX - merely that it's what the regulation calls
for. Sometimes the regulation is just wrong.

I don't think there's any right answer to this, it's just an
interesting exercise in PIC decision making ability to figure out what
the right thing to do is.

Well, I sort of agree. There really isn't one exact right answer, but
there are reasonable answers and unreasonable ones. One of those
unresonable answers involves actually following the rules when you're 5
miles from SAX. Part of PIC decision making in the real world is
knowing when to bend the rules in a non-emergency situation, and
knowing what you can get away with.

If you talk to an actual practicing center or approach controller, he
will tell you that in the event of lost comm, the best thing for
everyone is for you to get on the ground by the most expeditious route,
since they can't count on you doing anything in particular (more likely
than not your lost comm is associated with a more serious emergency)
and are going to sterilize the airspace around you anyway. In the age
of RADAR capable of seeing primary targets, that's the sensible course
of action. 91.185 hasn't kept pace. It doesn't need to. Lost comm
due to equipment failure is a rare event these days, and lost comm due
to equipment failure not associated with another emergency in IMC has
got to be so rare it's not worth the effort of rewriting the rules. We
torment our instrument students with the minutiae of lost comm rules,
but in reality that's strictly a checkride exercise.

So forget about following the letter of the rules - do something
sensible. Remember - no action is foolproof. A fed who wants to get
you will get you. One who isn't after your hide will accept that you
did something reasonable. Backtracking 40 miles and tying up the
system with your NORDO self for an extra 45 minutes isn't reasonable,
even if that's what the rule calls for (and clearly it does).

Let's say you're changing radios on your audio panel, and due to crappy
soldering and decades of vibration, the switch comes off in your hand.
Everything else still works, but with no audio panel you can't use
either comm. No real emergency - the fan is still turning, the juice
is still flowing, and the nav is still pointing.

If you've established comm with ATC, climbed to your final altitude,
and were left on your DP heading, there's very little chance the
controller had any intention of ever sending you back to CMK. He's
either going to send you direct by RADAR vectors, or have you join the
route somewhere in the middle (or reroute you). I would suggest that a
reasonable thing to do is look at your position relative to your
cleared route, and draw a reasonable intercept course from your present
position to your route. We generally consider 30 degrees to be a
reasonable intercept. If you can make that work, that's what makes
sense. If you're too close to SAX to make that work, you need to go
direct SAX. That's how the controller would put you back on the route
if he had to (due to transponder problems, for example) and that's what
you ought to do.

However, if there's a flight restriction area that prompted the
circuitous clearance, be damn sure your course for getting back on the
cleared route avoids it. If the only way you can assure this is by
backtracking to CMK, well, that's what you do.

It's not an exact right answer, but it does give you some reasonable
guidance on what to do. It's what makes sense in the real world. It
may not play so well on an IFR oral.

Michael