TD,

I agree with N5, it's good that you ask questions (and maybe we can even answer one or two correctly).

But Dhc8Fo has a point too...this particular question goes beyond the usual internet forum debates. The situation that occured had the potential for SERIOUS trouble if the geometry had been different. There are several problems here:

1) You didn't know how to make your airplane do something it's capable of.
2) You filed a flight plan that indicated that you COULD use that capability.
3) When asked to do something that you didn't know how to do, you tried to wing it.
4) You did not even know that what you were doing was not in compliance with IFR rules.


This is one of those chain-of-events you've probably hear people talk about. Fortunately the end result was a technical foul that nobody noticed.

Another factor was the other pilot (I assume you were the FO?) He probably filed the flight plan, and you relied on his "superior" experience. Another lesson to be learned here.

Suggestions:

1) Review the 91 and the AIM on IFR ops, especially RNAV & GPS
2) Review the equipment codes that you use or might want to use. They changed recently due to RVSM and might be a little confusing.
3) Read your Garmin supplement.
4) Don't let your captain or boss or whatever put you in this kind of predeciment...eventually it will cost you. I only trust people that I have a lot of positive experience with. And even then I double check things if I have time.